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Title: Aeroplane construction
        A handbook on the various methods and details of construction employed in the building of aeroplanes

Author: Sydney Camm

Release date: May 8, 2024 [eBook #73566]

Language: English

Original publication: London: Crosby Lockwood and son, 1919

Credits: Aaron Adrignola and the Online Distributed Proofreading Team at https://www.pgdp.net (This file was produced from images generously made available by The Internet Archive)


*** START OF THE PROJECT GUTENBERG EBOOK AEROPLANE CONSTRUCTION ***





                               AEROPLANE
                              CONSTRUCTION

                 _A Handbook on the various Methods and
                  Details of Construction employed in
                      the Building of Aeroplanes_

                                   BY
                              SYDNEY CAMM
              ASSOCIATE FELLOW ROYAL AERONAUTICAL SOCIETY

                             [Illustration]

                                 LONDON
                        CROSBY LOCKWOOD AND SON
                7, STATIONERS’ HALL COURT, LUDGATE HILL

                                  1919




                               PRINTED BY
                   WILLIAM CLOWES AND SONS, LIMITED,
                          LONDON AND BECCLES.




PREFACE


The articles embodied with other matter in this book, were intended
as a broad survey of the principles and details of modern aeroplane
construction, concerning which there is a noticeable deficit amongst
existing aeronautical literature.

They were written at a time when specific references to modern British
aircraft were forbidden, and although from a comparative point of
view this is to be regretted, the details and methods dealt with are,
in the author’s opinion, representative of those most generally used
in machines of present-day design. It is hoped that the book will
appeal not only to those engaged on the manufacture, but also to those
concerned with the uses of aircraft.

                                                               S. C.




TABLE OF CONTENTS


  CHAPTER I.
                                          PAGE
  INTRODUCTION                               1

  CHAPTER II.

  MATERIALS                                  6

  CHAPTER III.

  SPARS AND STRUTS                          18

  CHAPTER IV.

  PLANE CONSTRUCTION                        30

  CHAPTER V.

  DETAILS OF PLANE CONSTRUCTION             40

  CHAPTER VI.

  INTERPLANE STRUT CONNECTIONS              51

  CHAPTER VII.

  WING-TRUSSING SYSTEMS                     59

  CHAPTER VIII.

  FUSELAGE CONSTRUCTION                     67

  CHAPTER IX.

  FUSELAGE FITTINGS                         77

  CHAPTER X.

  UNDERCARRIAGE TYPES                       86

  CHAPTER XI.

  UNDERCARRIAGE DETAILS                     93

  CHAPTER XII.

  CONTROL SYSTEMS                          101

  CHAPTER XIII.

  WIRES AND CONNECTIONS                    108

  CHAPTER XIV.

  ENGINE MOUNTINGS                         116

  CHAPTER XV.

  ERECTION AND ALIGNMENT                   123

  INDEX                                    135




AEROPLANE CONSTRUCTION




CHAPTER I.

INTRODUCTION.


The purpose of this book is to give some indication of the principles
and methods of construction of modern aeroplanes, as distinct from
those considerations pertaining purely to design, although occasional
references to various elementary principles of aerodynamics have
been found necessary to illustrate the why and wherefore of certain
constructional details.

To many the aeroplane is a structure of appalling flimsiness, yet the
principle which it exemplifies, that of obtaining the maximum strength
for a minimum of weight, constitutes a problem of which the solving
is not only an unceasing labour, but one demanding the observance
of the best engineering procedure. The whole future of aviation,
commercially or otherwise, may be said to be indissolubly bound up with
the development of efficiency; and whether this is to be attained in
improvements in aerodynamical qualities, by the discovery of a material
giving a greatly enhanced strength to weight ratio, or by progress in
the arrangement of the various members of the complete structure of the
aeroplane, is a matter upon which some diversity of opinion exists.
However, it is certain that the very great developments of the last few
years are due more to refinements in design rather than construction;
and it is questionable whether the constructional work of the modern
aeroplane has developed equally with design, so that, even taking for
granted the oft-repeated, but very doubtful, statement that we are
approaching the limitations of design, there is certainly plenty of
scope for experiment and improvement in the constructional principles
of the modern aeroplane.


Standardization of Details.

Whatever may be said for the standardization of aeroplane types,
a scheme which should effect a considerable saving in labour and
material, and which offers chances of success, would consist in the
standardization of metal fittings and wood components generally, for in
this direction there is certainly great need for improvement. Taking
as a hypothesis the various makes of scouting machines, we find hardly
any two details the same. This means that if in this country there are
six firms producing machines of their own design (these figures, of
course, being purely suppositionary), there will be six sets of detail
drawings, six sets of jigs, templates, and press tools, and sundry
special machine tools. There seems no valid reason why many of the
fittings for all machines within certain dimensions should not be of
standard design, and a brief review of the various details which could
be standardized without detracting in the least from aerodynamical
efficiency will indicate the extent to which the conserving of labour
could be carried. In the construction of the fuselage, the clips
fastening the longerons and cross struts could easily be of one design,
whatever the make of the machine. At present we find some clips are
bent up from a stamping and attached to the longeron without the
drilling of the latter; some built up from various parts, such as
washer-plates, duralumin pressings, and bolted through the longeron;
while some combine advantages and others the disadvantages of both. In
some cases the longerons of spruce are spindled out for lightening;
in others no spindling occurs; while in a few instances hickory or
ash, with or without channelling, is used. There are the interplane
strut attachments, stern-post fittings, control-surface hinges, and
undercarriage attachments, all showing great variations, and in all
of which the design could be brought within reasonable limits. As
indicating how unnecessary a good deal of the variation is, one may
instance the fact that for the swaged streamline, or R.A.F. wires,
there are at least three different terminals in use. Although more
difficult of achievement, there is scope for improvement in the
different arrangements for the fixed gun mounting, while a standard
instrument board would benefit the pilot.


Methods of Manufacture.

It is fairly well known that the output of some firms is considerably
better than others, although the machines are of the same design.
Although a good many factors may contribute to this result, it
seems fairly certain that in some cases the methods of manufacture
must be superior, which calls for some system of standardizing the
broad principles pertaining to manufacturing procedure. Under this
arrangement a much better estimate of probable output could be made.
It is also necessary by the fact that some firms have been developed
through the exigencies of war, and not as a result of any great
manufacturing ability, whereas in peace time the spur of competition
would force the adoption of the most rapid methods of production.
The creation of a central or universal office for the design of the
various jigs used in the manufacture of aircraft, with power to
decide the process of manufacture, although a somewhat far-reaching
reform, would certainly eliminate a number of useless experiments made
by the individual constructors, and would also greatly improve the
interchangeability of the various components. In addition, fresh firms
to the aviation industry would be at once acquainted with the general
methods of manufacture, which should be of considerable assistance in
expediting initial output. Of course, this system would tend rather to
destroy individual initiative, in that much that is now left to the
skill and experience of the workman would be predetermined, although
this would be more than compensated for by the increased benefits
accruing to the State. Jigs designed to produce the same work in
different works often differ in detail considerably, and this, of
course, often influences the rate of production. As an instance, in
some works elaborate benches are considered necessary for the erection
of fuselages, while in others a pair of trestles suffices. With this
system of unified manufacturing procedure extreme regard would have to
be paid, in the design of various jigs and fixtures, to adaptability
for modifications in design. Otherwise the various alterations which
are bound to occur would result in an unnecessary expenditure on
fresh jigs. It is somewhat unfortunate that in the general design
of an aeroplane, in numerous cases, far too little regard is paid
to considerations of ease of manufacture, and this is frequently
responsible for the many changes in design after a contract has been
started. Under an ideal system of standardization, the requirements
of manufacture would necessitate consideration in the design of the
constructional details.


Metal Construction.

The question of the aircraft materials of the future is not so much
a problem as a matter of gradual evolution. In view of the dwindling
supplies of suitable timber, it certainly seems more than probable that
some form of metal construction will one day constitute the structure
of the aeroplane. The manufacture of the various components in wood
does not necessitate an extensive plant, the labour necessary is
comparatively cheap and easily available, and moreover the transitory
nature of the whole business, and the ease with which essential changes
in type can be made without the wholesale scrapping of the expensive
jigs associated with the use of steel, all strengthen the case in
favour of wood. The conclusion of hostilities would introduce another
state of affairs, and it is conceivable that the various types will
then be standardized for different purposes, which may necessitate
the greater use of steel. Certainly the advantage of steel would be
better realized under some system of standardized design, but this
unfortunately is not possible while present conditions obtain. The
advantages of metal as a material considered briefly, are that it
permits of design to close limits without the allowance of so-called
factors of safety, which are now necessary through the great variation
in the strengths of wood, manufacturing procedure would be expedited,
while one can reasonably expect a greater degree of precision in the
finished machine, due largely to the increased facilities for accurate
manufacture of components which metal affords. It is quite possible, of
course, given a uniform grade of steel, to design to extremely close
limits without fear of collapse; but the human factor in the shape of
fitter, welder, or operator introduces the unknown element, and one
for which some allowance must always be made. One cannot assert that
any very decided indication exists of a trend in modern design towards
metal construction, and it is quite possible that this will not arrive
until it is rendered imperative by reason of the scarcity of timber.
The precise composition of the metal is rather a controversial matter,
some authorities favouring steel, and others some alloy of aluminium,
such as, for instance, “duralumin.” The production of a suitable alloy
constitutes a real problem and one upon which the Advisory Committee
for Aeronautics have already made investigations and experiments. A
disadvantage with steel is that, although it is quite possible to
produce, say, a fuselage entirely of this material to withstand easily
the greatest stress encountered in flying, such a structure, owing to
the thin nature of the various components, would suffer damage through
shocks induced by rolling over rough ground, and also by handling. In
addition the effects of crystallization would require some considerable
study. These and other reasons indicate that an alloy of aluminium,
which for a given weight would be considerably more rigid than steel,
offers possibilities as a material. It might prove advantageous to
combine both metals, using steel for the more highly stressed parts,
such as, for instance, wing spar attachments, interplane bracing lugs,
and indeed any part where the load to be carried is one induced by
tension.

The foregoing is indicative of some of the more important directions
in which improvement and development are possible, and certainly ample
scope yet exists for the attention of the student, or indeed any one
interested in the future of the aviation industry.




CHAPTER II.

MATERIALS.


Seeing that wood constitutes the material for the greater part of
the structure of the aeroplane, that is with very few exceptions,
some notes on the characteristics and qualities of those woods most
commonly used may prove of interest. The choice of a suitable wood
for aircraft construction is a matter of some difficulty, engendered
by the variety of considerations of which at least some observance is
essential. The fundamental principle of aircraft construction, that of
obtaining the maximum strength for a minimum of weight, affords one
standpoint from which a particular wood may be regarded, but this does
not constitute in itself a sufficient reason for its choice. Of almost
equal importance are such considerations as the length and size of the
balks obtainable from the log, the total stock available, the relative
straightness of grain and freedom from knots as well as the durability
of the wood.


Variable Qualities of Wood.

The choice is additionally complicated by the very great variation
found in the strength and characteristics of trees of exactly the
same species, and also of different portions cut from the same tree.
The nature of the site upon which a tree is grown exercises a marked
influence upon its properties, while as a general rule, it may be taken
that the greater number of annual growth rings per inch, the greater
the strength. It is also a general rule that up to certain diameters,
the timber contained in that part of the tree the greatest distance
from the pith, or centre, is the stronger.

The wood obtained from the base of a tree is heavier than that at the
top, and one finds the influence of this in the necessity for balancing
and alternating the different laminæ of air-screws before gluing.


Shrinkage.

Another point, and one which is intimately concerned with the proper
seasoning of timber, is the amount of moisture contained in a specimen,
and this latter point is of some considerable importance, as not only
is a large amount of moisture detrimental to the strength values of
the timber, but it also renders useless any attempt at precision of
workmanship. It is this very point of shrinkage, which constitutes
the greatest bar to the achievement of a measure of component
standardization, and it is also one of the most serious disabilities of
wood as a material for aircraft construction. It is now necessary in
the production of finished parts to make some allowance for resultant
shrinkage, which is a matter of guesswork, and only practicable where
some time will elapse between the finishing of the part and its
erection in the complete machine. Under present conditions, more often
than not the parts are assembled almost immediately they are made,
which means that no allowance over the actual size is possible, this
being due to the various fittings which in the majority of machines are
of set dimensions and clip or surround the material.

As a natural sequence shrinkage occurs subsequent to the attachment
of the fitting, followed by looseness and loss of alignment in the
structure. Until the proper period for seasoning can elapse, between
the cutting of the tree and its conversion into aeroplane parts, it is
difficult to see how this disability can be obviated, although latterly
some considerable advances have been made with artificial methods of
seasoning. The prejudice against kiln drying is founded on the belief
that the strength of the timber is reduced, and that extraneous defects
are induced. A method which is a distinct improvement on those systems,
using superheated steam and hot air, is now being used with apparently
good results. In this system, steam under very low compression is
constantly circulated through the timber, drying being effected by a
gradual reduction in the humidity of the atmosphere.


Unreliability of Tabulated Tests.

The various tables which exist indicating the strength, weight, and
characteristics of various woods are of very doubtful utility, in
some cases fallacious, and in nearly all cases far too specific. The
foregoing enumeration of some of the variations existing with wood will
indicate the enormous difficulty of obtaining with any exactitude a
result representative of the species of wood tested, and which could
be regarded as reliable data for the calculation of stresses, or for
general design. The moisture content of timber, an extremely variable
quantity, greatly affects the figures relating to the strength and
weight of timber, so that tables indicating the properties of woods
should include the percentage of moisture contained in the examples
tested. Again, certain woods possessing relatively high strength
values, are frequently short-grained and brittle, and therefore not
so suitable as other woods of lower strength values, but of greater
elasticity and resiliency.


WOODS IN USE.


Silver Spruce.

The wood most extensively used for the main items of construction is
silver spruce, or Sitka spruce, found in great quantities in British
Columbia. Experience has proved this wood pre-eminently suitable for
aeroplane construction, its strength-weight ratio is particularly
good, it can be (at least until recently) obtained in long lengths up
to 30 ft., and, moreover, is particularly straight grained and free
from knots and other defects. There are other woods possessing higher
strength qualities, but in most cases their value is greatly diminished
by reason of the greater weight, and that only a limited portion
straight of grain and free from knots is obtainable. The weight of
Sitka spruce varies from 26 to 33 lbs. per cubic foot, and although it
is difficult to give a precise figure, a good average specimen fairly
dry would weigh about 28 lbs. per cubic foot. Some impression of the
extent to which it enters into the construction of the aeroplane will
be gathered if the components usually of spruce are detailed. For the
main spars of the planes spruce is almost universally used, as here
great strength for the least weight is of extreme importance, while a
consideration almost as important is the necessity of a good average
length, straight grained and free from defects. It is also used for
the webs and flanges of the wing ribs, the leading and trailing edges
and wing structure generally. The longerons or rails of the fuselage
of many machines are spruce, although in this instance ash and hickory
are used to a moderate extent. The growing practice is to make the
front portion of the fuselage of ash, as this is subject to the greater
stress, while the tail portion is of spruce; but in a number of cases
the latter material is used throughout. The cross struts of the
fuselage are invariably of spruce, as well as such items as interplane
and undercarriage struts and streamline fairings.


Virginia Spruce.

This is of a lower weight per cubic foot than Sitka spruce, but does
not possess such a good strength value, cannot be obtained in such
large pieces, and is generally subject to small knots, which limit the
straight-grained lengths procurable.

It is distinguishable from Sitka spruce by its whiteness of colour and
general closeness of grain.


Norwegian Spruce.

This wood is also known as spruce fir and white deal, and is grown
principally in North Europe. Selected balks can be obtained to weigh no
more than 30 lbs. per cubic foot, which compares very favourably with
silver spruce. It can be obtained in average lengths, but it is subject
to the presence of small hard knots and streaks of resin, although the
writer has seen consignments with very few knots. A material known
as Baltic yellow deal and Northern pine is procured from the same
source, and is more durable than Norwegian spruce. It is inclined to
brittleness when dry, and is heavier than white deal, weighing about
36 lbs. per cubic foot. The recent shortage of silver spruce has led
to the employment of Norwegian spruce for items such as fuselage
struts, hollow fairings to tubular struts, the webs and flanges of
the plane ribs, and generally for those components for which long
straight-grained lengths are not absolutely essential.

For fuselage struts, where the chief consideration is stiffness, to
resist the bending strain produced by inequalities of wiring, fittings,
etc., it may actually give better results, being slightly more rigid
than silver spruce--at least that is the writer’s experience of it. In
addition, very little increase in weight would result, as this wood
can be obtained of almost the same weight per cubic foot as silver
spruce. The defect usually met with in this wood, of knots occurring
at intervals, would be of no great detriment, the lengths needed for
the fuselage struts being approximately 3 feet and less, and it would
therefore be easily possible to procure wood of this length free from
knots. The other items enumerated are of varying lengths, which, with
care in selection and conversion, could be arranged for. The practical
application of this would be the increased amount of silver spruce
available for such highly stressed items as wing spars, interplane
struts, and longerons.


Ash.

This wood is one of the most valuable of those employed, being
extremely tough and resilient. There are two varieties in use, English
and American, the former being considered the better material. It is
used mainly for longerons, undercarriage struts, and for all kinds
of bent work. It possesses the quality of being readily steamed to
comparatively sharp curves, and will retain the bend for a considerable
period. The strength and characteristics of ash vary greatly with the
climate under which it is grown, and it is also much heavier than
spruce, the weight per cubic foot ranging between 40 and 50 lbs.
Difficulty is also experienced in obtaining lengths greater than 20
ft., and even in lengths up to that figure, continuity of grain is
somewhat rare. It is notable that on various German machines, ash in
conjunction with a species of mahogany is used for the laminæ of the
air-screw.


Hickory.

Hickory, a species of walnut, is imported from New Zealand and America,
and possesses characteristics similar to those of ash. It is obtainable
in about the same lengths as ash, but in the writer’s experience is
of greater weight. Its chief property is extreme resiliency, which
makes it especially suitable for skids, and it has also been used to
a limited extent for longerons. It is subject to excessive warping in
drying, is not so durable as ash, and the great difficulty experienced
in obtaining straight-grained lengths is responsible for its waning
popularity.


Walnut.

This wood is almost entirely devoted to the making of air-screws,
although the dwindling supplies and the very short lengths obtainable
has practically enforced the employment of other woods for this purpose.


Mahogany.

The term “mahogany” covers an infinite variety of woods, possessing
widely different characteristics, many of the species being quite
unsuitable for the requirements of aircraft work. That known as
Honduras mahogany possesses the best strength values, is of medium
weight, about 35 lbs. per cubic foot, and is in general use for
airs-crews and seaplane floats. It has been used on some German
machines for such parts as rib webs, but is not really suitable for
parts of comparatively small section, such as longerons, as it is
inclined to brittleness. It is of particular value for seaplane floats
and the hulls of the flying-boat type of machine, as it is not affected
by water. A defect peculiar to Honduras mahogany is the occurrence of
irregular fractures across the grain known as thunder shakes. Although
other so-called mahoganies are similar in appearance to the Honduras
variety, a species quite distinct in appearance is that known as Cuban
or Spanish mahogany, which is of darker colour, and much heavier in
weight, averaging about 50 lbs. per cubic foot, which latter factor
almost precludes its use for aeroplane construction.


Birch.

One finds very few instances of the use of this wood for aeroplane
details, although it is used fairly extensively in America for
air-screw construction, for which it is only moderately suited. It
possesses a high value of compressive strength across the grain, but is
much affected by climatic changes, and does not take glue well. It is
useful for bent work, and might conceivably be used instead of ash for
small bent work details. Its weight is about 44 lbs. per cubic foot.


Poplar.

Under this name is included such woods as American whitewood, cotton
wood, bass wood, etc. The wood sold under one or other of these names
is generally very soft and brittle, and although of a light nature,
weighing about 30 lbs. per cubic foot and less, it is of very little
utility for the work under discussion. It has been used for minor parts
such as rib webs, and fairings to tubular struts.


Oregon Pine.

The scarcity of silver spruce has led to the adoption of the wood known
as Oregon pine for most of the components for which the former wood has
hitherto been used. The term “Oregon pine” is applied to the Douglas
fir, one of the largest of the fir species, a length of 200 ft. being
an average. It is altogether heavier than silver spruce, weighing
about 34 lbs. per cubic foot, and also differs greatly in appearance,
possessing a reddish-brown grain, with very distinct annual rings. Its
strength to weight ratios are practically equal to those of silver
spruce, although in the writer’s experience it has a tendency towards
brittleness, and is not so suitable as Sitka spruce for components of
small scantling. With some specimens of this wood it is noticeable
that the effect of drying on freshly sawn lengths for longerons, etc.,
is the appearance of “shakes” or cracks, not previously discernible.
Its appearance generally is reminiscent of pitch pine, for which wood
it is sometimes substituted in connection with building.


Other Woods.

The foregoing constitute woods which are in fairly general use for one
purpose or another, there being, of course, very many other varieties,
some of which may be called into use with the progress of the industry.
Of the conifer species, silver spruce is easily the most suitable
timber for aeroplane construction, and one realizes this more as the
various substitutes are tried. As an instance, cypress is straight of
grain with no very great increase over the weight of spruce, being also
well up the table of strengths. It is, however, much too brittle for
the various members of small section of which an aeroplane is composed,
and does not seem to have any extensive future for aircraft work.
Another, at one time much-advertised wood, is Parang, a species of
mahogany. It has been reputed to bend well, but it certainly does not
enter into the construction of modern aeroplanes. A consignment handled
by the writer some years ago and intended for bending, was found to
be exceedingly brittle, and although standing a good load, fractured
almost square across the grain, in a manner known colloquially in
the workshop as “carrot-like.” The latter term is indicative of a
characteristic which precludes the use of many woods possessing other
physical properties especially suitable for aircraft work.


Multi-ply Wood.

This term is applied to the sheets of wood composed of a number of
thin layers glued together with the grain reversed. As the layers are
obtained by rotating the tree against cutters in such a manner that a
continuous cut is taken from the outside almost to the centre, it is
possible to get very great widths, which makes it particularly suitable
for aircraft work. It is made in varying widths up to 4 ft., and in
thickness from 1/20 in. up to ½ in., consisting of three, five, and
seven layers, although the three-ply variety in thicknesses up to 3/16
in. is more commonly used. It is made up in nearly all woods, but those
mostly utilized in the aeroplane industry are birch, ash, poplar, and
satin-walnut, birch being superior by reason of its closeness of grain.
Ash ply-wood in some instances tends towards brittleness, while poplar,
although exceptionally light, is very soft and only used for minor
parts. Satin-walnut is very even in quality but is apt to warp.


Defects in Timber.

[Illustration: FIG. 1.--Heart shake.]

[Illustration: FIG. 2.--Star shake.]

[Illustration: FIG. 3.--Cup shake.]

Perhaps the most common and prolific defect encountered with the
use of timber is the presence of cracks or shakes of different
character, which are due to different causes. Fig. 1 indicates
a very common form, known as a “heart shake,” dividing the
timber at the centre; while Fig. 2, a “star shake,” is really a
number of heart shakes diverging from the centre. The process of
seasoning sometimes results in the separation of the annual rings,
forming cup shakes, as shown in Fig. 3. It should be understood
that the presence of shakes may render useless an otherwise
perfect specimen of timber, as it frequently happens that in the
conversion of timber so affected the usable portions do not permit
of the sizes necessary for such items as wing spars and struts. The

[Illustration: FIG. 4.--Twisted grain.]

defect of twisted grain (Fig. 4) is often found in ash, and is caused
by the action of the wind when the tree is growing, and renders such
wood of limited utility. Shrinkage affects all timber in varying
degrees, and its effect on boards due to their position in the log
is shown by Fig. 5, while Fig. 6 indicates the effect of drying on a
squared-up section. Incidentally one may point out that the annual
rings, viewed from the end of the section, should be as straight as
possible, which would obviate to an extent the distortion due to drying
in a component subsequent to its finishing. Another defect, and one
somewhat difficult to detect, is the presence of a brownish speckled
tint in the grain. Any evidence of this in a specimen indicates the
beginning of decay, and is caused by insufficient seasoning and lengthy
exposure in a stagnant situation.

[Illustration: FIG. 5.--Shrinkage of boards due to position in log.]

[Illustration: FIG. 6.--Effect of drying on a squared-up section.]


Steel.

The greater proportion of the various fittings employed in the
construction of the aeroplane are built up from sheet nickel steel,
usually of a low tensile strength, to permit of working in a cold
state, as, with a higher grade steel, the process of bending to
template by hand, in many cases a none too careful procedure, would
result in a considerable weakening of the material at the bend.
In addition, the operation of welding, which now enters into the
construction of a number of fittings, also necessitates a moderate
grade of steel. A higher class of sheet steel, from 35 to 50 tons
tensile, is used for parts subject to stress, such as interplane
strut-fittings, wiring-lugs, etc. As a higher grade of steel is better
from a strength-for-weight point of view, its employment for bent-up
clips is desirable, although where such a steel is used it is almost
necessary, if the original strength of the material is to be retained
in the finished fitting, to effect the various bends in a machine, in
conjunction with bending jigs. Careful heat-treatment after bending
to shape is an important factor in removing the stresses set up by
working, and in rendering the structure of the material more uniform.


Steel Tube.

Steel, in the form of tubing of various sections, enters largely into
aeroplane construction, and may be said to contribute largely to the
efficiency of the structure. It is now being used for the different
items of the undercarriage, for struts in the fuselage, interplane
struts, and in many cases control surfaces, such as the ailerons,
elevators, and rudder, are being built of this material entirely. In
the early days of aviation steel tubing attained some considerable
popularity, many machines being built almost entirely of tubing; but
difficulties in its manipulation, and the fact that very often the
methods of attachment reduced its strength considerably, gradually
led to the general employment of wood. The great advances lately made
in the production of a high-grade nickel-chrome steel, with a high
ultimate tensile stress, are responsible for its present increasing use.


Aluminium.

The present use of aluminium is restricted to the cowling of the
engine, and occasionally as a body covering. Although it is light in
weight, its extremely low strength values render it of very little
use for other purposes. It attained some measure of popularity in
the early days of aviation, particularly for the manufacture of
different strut-sockets, which were cast from aluminium; but the
general bulkiness of the fittings, in addition to the fact that it
was generally necessary to incorporate a steel lug to form the wire
anchorage, caused it to gradually fall into disuse. The tendency of
aluminium to flake and corrode, which is intensified by the action
of salt water, also limits its use for seaplane construction. Many
attempts have been made through various alloys to impart greater
strength to the material, and although progress has resulted, the
characteristics of most of the products are unreliable.


Duralumin.

Of the different alloys, duralumin is probably the best, although one
believes that its qualities are principally the result of special
heat treatment. Its use is at present restricted to those parts not
subjected to any great tensile strain. It is considerably less than
half the weight of steel, bulk for bulk, and, properly used, may effect
a considerable saving in weight. The fact that it has not achieved the
popularity it deserves may be ascribed to the difficulties experienced
in working it, especially for such parts as body clips, where several
bends are necessary, and to the rather arbitrary methods in use. If
properly annealed, no difficulty should occur in obtaining a reasonably
sharp bend. The process recommended by the makers consists in heating
the metal in a muffled furnace to a temperature of approximately 350°
C., and the necessary work done as soon as possible after cooling. The
importance of this is due to the fact that the process of annealing
imparts to the metal a tendency to become brittle with time. The writer
has often contended that, where duralumin is used, it should be with
a real desire to reduce weight. Too often one sees a fitting of such
lavish dimensions as to entirely nullify the advantage of the lighter
metal.




CHAPTER III.

SPARS AND STRUTS.


Having thus considered generally the chief materials of aircraft
construction, we will proceed to examine the various types of spars
and struts in present use. The main spars of the wings are by far the
most important items of the complete structure, and very great care is
always taken to ensure that only the best of materials and workmanship
are concerned with their manufacture. Looking back at the days one
usually associates with the aero shows at Olympia, multitudinous
methods of building wing spars can be recalled. Some composed of
three-ply and ash; others, less common, of channel steel; and a few of
steel tubing, either plain or wood filled. Various reasons and causes
have combined to eliminate these methods of construction. For instance,
the spar of channel steel proved much too flexible, although this
characteristic was no great disadvantage in those machines employing
wing-warping for lateral control, for with this arrangement a certain
amount of flexibility in the wing structure is essential. While steel
tubing is excellent for many details it can hardly be said to be really
suitable for wing spars, which are stressed essentially as beams.
Now, the strength of a beam varies as the square of the depth of the
beam, and it is obvious that in the case of a circular steel tube the
material is evenly distributed about the neutral axis, and therefore
its strength in both horizontal and vertical directions is equal;
although employed as a strut, this feature becomes of real value. One,
however, still encounters its use on modern machines; indeed, it must
not be supposed that the progress made in construction generally since
1914 has tended greatly towards a reduction in the number of different
methods employed, and this will be realized from a consideration of
the accompanying spar sections which are in use to-day on one make of
machine or another.


Spar Sections.

[Illustration: FIG. 7.--Solid spar.]

The I section form of wing spar, shown by Fig. 7, is in general
use, being spindled from the solid. It is comparatively easy to
produce, which in a measure explains its popularity, and it also
disposes the material in probably the best manner for the stresses
involved. The laminated spar, Fig. 8, is an improvement on the
solid channelled spar; it is stronger, will withstand distortion
to a greater degree without injury, and the strength is also more
uniform than with the solid spar. An additional point in its
favour is that it is much easier to procure three pieces of small
section timber free from defects than one large piece, which, in
view of the increasing scarcity of perfect timber, is an important
consideration. In order to minimize the risk of the glue between

[Illustration: FIG. 8.--Laminated spar.]

the laminations failing, the usual practice is to copper rivet or bolt
the flange portion, while both spars are left solid at the point of
attachment of the interplane strut fittings and wire anchorages. The
spar shown by Fig. 9 is of the hollow box variety, chiefly used for
machines of large wing surface, where weight reduction is an important
factor. The two halves of channel section are spindled from the solid
and glued together. The joint is strengthened by the provision of small

[Illustration: FIG. 9.--Hollow box-spar.]

fillets or tongues of hard wood, and in some instances the complete
spar is bound with glued fabric. Comparing the hollow spar with
the solid, and neglecting the cost factor, the writer contends that
the advantage is indisputably with the former. The tendency of the
I-section spar to buckle laterally is of much lesser moment in a hollow
spar of the type shown by Fig. 9, while for a given weight it shows
an increase in strength, and for equal strength it is much lighter. A
different version of the hollow spar system is that indicated by Fig.
10, consisting of two channelled sections, tongued together at the
joint, the sides being stiffened with three-ply. The disposition of the
joint in a vertical plane is a distinct improvement on the hollow spar
previously considered, mainly in that better resistance to a shearing
stress is afforded.

[Illustration: FIG. 10.--Hollow spar with stiffened sides.]

[Illustration: FIG. 11.--Hollow spar with multi-ply sides.]

The principle underlying the construction of the spar shown by Fig. 11,
is that in its manufacture the lengths of wood necessary are of small
section. The sides of this spar are built up with a centre of spruce
about ⅛ in. thick, to each side of which is glued thin three-ply,
these being glued, screwed, and bradded to the flanges. The wing spar
shown in section by Fig. 12 is unique in that it really constitutes
two spars placed closed together, the connection being formed by the
top and bottom flanges of three-ply. This spar was used in a machine
with planes of small chord, but of very deep section, and in which no
interplane wiring occurred, the wings functioning as cantilevers. Its
chief advantage is great rigidity for a low weight, but such a spar
necessitates a deep wing section, and is not in general use.

[Illustration: FIG. 12.--Twin box spar.]


Hollow Spar Construction.

The advantages of the hollow type of spar summarized are (1) greater
strength for a given weight; (2) it can be produced from wood of small
section, and is therefore a better manufacturing proposition. On
the other hand, the strength of a hollow spar is greatly and almost
entirely dependent on the glue used. Now, however well the joint may be
made, the glue is susceptible to a damp atmosphere, and if so affected
is of greatly reduced strength, while possible depreciation in the glue
due to age renders the life of the spar a problematic quantity. Where
the various fittings occur it is also necessary to place blocks before
the spar is glued up, which is rather an unmechanical job. The practice
of forming vertical sides of a hollow spar from three-ply is not to be
commended, by reason of the doubtful character of the glue used in its
manufacture. However, in spite of these disabilities, there is a future
for hollow spar construction in the manufacture of the big commercial
machines of the future, for with these the question of maximum strength
for minimum weight, to permit the carrying of the greatest possible
useful load, will be a primary consideration. This, of course, assuming
that the era of the all-steel machine has not arrived.


Strut Sections.

In the construction of the interplane and undercarriage struts, one
does not find a very decided preference for any one particular method,
although the interplane strut spindled from the solid to a streamline
section is common to many types of modern aircraft. The strut shown
in section by Fig. 13 is in use for both interplane and undercarriage
struts. This consists of ordinary round section steel tubing, to which
is attached a tail piece or fairing of wood, this being bound to

[Illustration: FIG. 13.--Steel tube strut with fairing bound _on_.]

[Illustration: FIGS. 14, 15.--Interplane struts spindled from the
solid.]

the tube by linen tape or fabric, doped and varnished. This strut is of
practically equal strength in both lateral and longitudinal directions,
and from this point of view is superior to the solid spindled strut,
which is usually of great strength in the fore and aft direction, but
always possesses a tendency to buckle laterally. Fig. 14 indicates a
hollow plane strut, in which the sides of spruce are spindled from the
solid, and glued to a central stiffening piece of ash; while Fig. 15 is
arranged so that a stiffening web is formed in the spindling process.
Owing to the rather extensive nature of the latter operation, one does
not find many instances of its use. Where the hollow wood struts used
are not completely bound with tape or fabric, they should at least be
bound at intervals with tape or fine twine, as there is always the
possibility of the glued joint failing under the combined attentions of
rain and heat.

A type of strut which is now being widely used is that of streamline
section steel tubing, drawn or rolled from the round section. It is
employed for both the interplane and undercarriage struts, but for
the latter has not given entirely satisfactory results, owing to the
tendency to buckle under extra heavy landing shocks. This would be
more pronounced with a tube of fine section than with one possessing a
bluff contour; but in any case, a strut of parallel section, whatever
the material, is not well suited to withstand sudden shocks. This point
is referred to later. Seeing that progress is being made with the
production of a seamless streamline tapered strut this defect should
soon disappear.

[Illustration: FIG. 16.--Interplane support from body.]

[Illustration: FIG. 17.--Section of built-up strut.]

In some machines the top plane is supported from the fuselage by struts
which are formed integrally with a horizontal compression member, as
in Fig. 16; the section of the vertical struts being shown by Fig. 17.
The ply-wood is cut to the shape of the complete component, and forms
a tie for the spruce layers, which are jointed at the junction of the
vertical and horizontal members.


Strut Materials.

Referring again to the material generally employed for struts, _i.e._
silver spruce, it is perhaps necessary to explain further the reasons
for its predominance over ash, as on a strength-for-weight ratio
the latter wood is slightly the better material. The points already
detailed, indicate that an interplane strut is stressed essentially
in compression, and therefore the chief characteristic of ash, great
tensile strength, is of but secondary importance. There is also
the fact that, for the same weight, spruce would be thicker, and
correspondingly more able to resist collapse. However, in machines of
the flying-boat class, where the engine is invariably mounted between
the four central plane struts, and consequently subjected to an amount
of vibration varying with the type of engine used, ash forms the
material.


Tapering of Interplane Struts.

The correct shaping of struts longitudinally, particularly those for
interplane use, is apparently a rather controversial subject. Taking
the case of an untapered strut, it is evident that the greatest
stress will be located at or near the centre, so that if at this
point the section is strong enough, clearly there must be an amount
of superfluous material at the ends. By suitably reducing or tapering
the strut from the centre one can obtain the same degree of strength
for less weight. Conversely, for the same weight a much stronger
strut is possible. So it has always appeared to the writer. It is,
however, admittedly possible that unless carefully done, the operation
of tapering a strut may actually diminish the strength. One method
of tapering, that of making the maximum cross-section at the centre,
and from this point diminishing in a straight line to the ends, is
undoubtedly open to criticism, and a way more nearly approximating
to the correct method of shaping is to reduce the cross-section at
various points so that the finished contour is curvilinear, as in Fig.
18. In this connection it is pertinent to emphasize the importance of
ensuring that all strut ends are cut to the correct bevels, and this
is particularly applicable to those struts which seat directly in a
socket. The slightest irregularity will cause considerable distortion
when assembled under the tension of the bracing wires, and frequently
the writer has seen an ostensibly perfect strut assume the most
hopeless lines directly the operation of truing up is commenced.

[Illustration: FIG. 18.--Tapering of interplane struts.]


Design of Strut Sections.

Although, strictly speaking, the design of strut shapes is outside
the scope of this book, a few remarks anent the development of
streamline may emphasize the advances made, and also the need
for careful construction. The resistance of a body is generally
considered to increase as the square of the speed, _i.e._ double
the speed and head resistance is doubled, and while this is true
for a moderate range of speeds, experiment has proved that for high
speeds, exceeding say 100 miles per hour, resistance increases at
rather less than as the square of the speed. However, it is certain
that the correct shaping or otherwise of the struts and other
exposed members, affects generally the performance in flight of the
aeroplane. The accepted feature of all streamline forms is an easy
curve, having a fairly bluff entrance and gradually tapering to a
fine edge. The ratio of length to diameter, called the fineness
ratio, varies in modern machines, being in some instances 3 to 1 and
in others 5 to 1, a good average being 4 to 1. Considering only the
point of head resistance, it would be better to choose a section of
high fineness ratio, but constructionally such a strut would buckle
sideways under a moderate load, and therefore the cross section
must be sufficient to resist this. The strut section used on the
earliest aeroplanes, such as the Wright biplane, shown by Fig. 19, is

[Illustration: FIG. 19.

FIG. 20.

FIG. 21.

FIG. 22.

FIGS. 19–22.--Strut sections.]

nothing more than a rectangle with the corners rounded off. Fig. 20
shows a development of Fig. 19 consisting of a semi-circular head with
a cone-shaped tail, which by gradual evolution has resulted in the
section Fig. 21. Some experiments carried out a considerable time ago
by Lieut.-Col. Alec Ogilvy, revealed the rather interesting point
that a strut shaped as in Fig. 22 gave the same results as a similar
strut taken to a fine edge. The reasons for the non-suitability of a
sharp-pointed section are apparent from a consideration of Fig. 23,
showing the action of a side wind with the resultant dead air region.

[Illustration: FIG. 23.--Showing inefficiency of pointed section in a
side wind.]


Fuselage Struts.

[Illustration: FIG. 24.--Channel-section fuselage strut.]

[Illustration: FIG. 25.--T section fuselage strut.]

In the general features of those struts associated with the
construction of the fuselage and nacelle, there is very little
diversity of practice, the majority of constructors favouring a square
spruce strut, Fig. 24, channelled out for lightness. A defect with
this type of strut is the tendency, engendered by irregularities in
the fittings and wiring, to buckle laterally, although this can be
obviated by the provision of a strut of larger section at the centre
and diminishing in width to the ends. A strut not nearly so popular
but nevertheless in use is that indicated by Fig. 25, consisting of
spruce spindled to a T section the web being of considerable width at
the centre. It would seem that the piece of wood necessary to obtain
such a strut is out of proportion to its actual finished dimensions,
and from the standpoint of economy in both labour and material is not
justified. The circular turned and tapered strut noticeable on a number
of machines disposes the material in probably the best manner for the
conditions applicable to this component, although it necessitates the
provision of tubular ferrules in the fuselage clip. On one modern
machine the fuselage struts are circular, but of hollow section, built
up of two pieces glued together. An obsolescent method is that in which
the strut is shaped to something approaching a streamline section, as
the fact that all aeroplane bodies are now fabric covered renders it
unnecessary.




CHAPTER IV.

PLANE CONSTRUCTION.


Of the various components which comprise the complete machine, the
wings, aerofoils, or planes, as these items are variously designated,
may be said to contribute the greater part of the ultimate success of
the complete machine. The aerodynamical properties of a wing are now
fairly well determined, and have been the subject of a great number
of experiments, resulting in the clearing away of many hazy ideas and
notions, so that the actual design of the wing section for machines of
given purpose is almost standardized. From this it might be deduced
that the methods of construction were equally well determined, and
although absolute uniformity of practice does not exist, the wing
construction of most machines is similar, as far as the main assembly
is concerned.


Effects of Standardization.

Incidentally, one may point out the detrimental effects of undue
standardization as applied to an industry in its preliminary stages.
These effects are well exemplified by certain machines, in which
standardization has been studied to an almost meticulous extent, with
the logical result that their performance is considerably inferior to
that of other machines of contemporary design, but in which desirable
improvements are incorporated as they occur. Although at present one
cannot give actual figures, the average performance of modern British
aircraft in range of speeds, rate and extent of climb is superior
to the products of any other country, and one certainly cannot cite
the construction of the average British machine as an example of
standardization. Seeing that, as a typical instance, wing sections
are frequently altered in minor detail, the impracticability of
standardization is apparent, for this would entail, to a firm wishing
to keep pace with developments, a considerable loss, through scrapping
of jigs, etc., consequent upon the new design. When the principles of
aeroplane design are as well defined as those pertaining to internal
combustion engines, one may expect the various manufacturers to produce
one type of machine per year, and the various improvements adduced
from the year’s experience would be incorporated in the type of the
succeeding year.

[Illustration: FIG. 26.--Plan view of wing assembly.]

However, leaving the realms of vaticination for the more prosaic
subject of wing construction, it will be realized that the process
of producing the full-sized wing, accurately conforming to the
measurements, etc., deduced from experiment, and so constructed that
the chief characteristic of the section will permanently remain, is of
importance. As one or two of the spar sections in use were dealt with
in the first chapter, it will be unnecessary again to consider them in
detail.

Fig. 26 shows diagrammatically the plan view of a wing assembly typical
of modern practice, so far as the disposition of the various components
is concerned.


Shaping of Main Spars.

[Illustration: FIG. 27.--Shaping of main spars.]

Taking in greater detail the different parts, it is apparent that
the spars form the nucleus of the general arrangement. There are two
methods of shaping the spar longitudinally, and, as shown by Fig. 27,
the one consists of leaving it parallel for the greater part of its
length, while the end forming the tip of the wing is gradually tapered
to a comparatively fine edge. This may be said to constitute prevailing
practice. The other method which is illustrative of monoplane practice
is not used to anything like the same extent, and differs in that
it is constantly tapering from root to tip. The advantage of this
spar construction is the improved distribution of the material for
the stresses involved, and also that a wing built with this spar
may possibly possess a greater degree of lateral stability owing to
the weight of the complete wing being located nearer the centre of
gravity. Against this one must balance the fact that each rib must
necessarily be different in contour, entailing a greater number of
jigs, an increase in the time taken in building, with a consequent
increase in cost. In addition, all strut fittings would differ in size,
so that, taking all things into consideration, this construction is
hardly justified. It will be noted that at the point of attachment of
the interplane strut fittings, or, in the case of the monoplane wing,
the anchorage for the wires, the spar is left solid. It is possible to
channel the spar right through, from root to tip, and to glue blocks
where fittings occur; and although there is a possible saving of
labour thereby, it hardly conforms to the standards of modern workshop
practice.


Defects of Glue in Wing Spars.

Although gluing is a most necessary operation in modern wing
construction, it is not what one would call an engineering proposition.
It has a tendency to deteriorate with time, especially if exposed to
a humid atmosphere. A great deal depends on the method of making the
joint, and an operation such as gluing a laminated wing spar is usually
carried out in a special room of certain temperature. Such spars are
generally additionally fixed by rivets, bolts, or screws through
the flanges. The material should always be dry, and as straight and
close-grained as can be procured. The straightness and closeness of
grain affect the strength to a remarkable degree; and here it may be
remarked that the use of the best material is a most important factor
for ensuring sound construction, and one that in the end pays. If a
spar should happen to be cut from a wet log, it may in the interval
between its finishing as a part and subsequent assembly in the wing
cast or warp, which may cause trouble in assembling, and is more
likely to result in eventually being sawn up as scrap. The resultant
section of any wing is really dependent upon the spar being of correct
section, and should the spar be out of “truth,” the section will vary
at different points. This may not be eradicated even in the erection
of the machine, so that finally the actual flying properties of the
machine will be affected--another illustration of the importance of
thorough construction in ensuring a good and lasting performance. To
secure uniformity and interchangeability the wing spars are set out for
the wing positions, and the necessary holes for the fittings drilled to
jig, before being handed over to the wing erectors.


Arrangement of Planes.

The usual arrangement on machines of the scout type is for the lower
plane to butt against the lower members of the fuselage, and the top
planes being the same span, the width of the body is made up by a
centre plane. Another method is to make the top plane in two portions
only, thus obviating the centre plane; and occasionally the spars of
the top plane run through, from wing-tip to wing-tip, although this
is only possible in machines of small span. Apart from the fact that
such a wing requires extra room, it is difficult to procure timber
of length exceeding 20 ft. sufficiently straight in the grain; and a
minor detail would be the difficulty of repair, as a damaged wing-tip
would practically entail a new spar, as splicing, although permissible
in some parts of the machine, should not be tolerated as a means of
repairing wing spars.

The difficulty of obtaining timber will necessitate the wings of large
machines being made in sections; and there are several instances where
this form of construction has been adopted, in one case the sections
being only five feet in length. This construction seems eminently
suited to the post-war sporting machine, as chance damage would be
confined to a smaller area, transport simplified, and, providing the
joints are well made, no appreciable loss in efficiency should ensue.


Types of Wing Ribs in Use.

From a survey of the plane diagram, Fig. 26, it will be noticed that
the chief components, in addition to the main spars, are the ribs,
box-ribs, stringers, and leading and trailing edges.

[Illustration: FIG. 28.--Construction of ribs.]

The ribs, which is the term applied to the very light framework built
over the spars to maintain the correct curvature, are variously
constructed; one of the most popular methods in vogue is that shown
by Fig. 28. The central portion, or web, which includes the nose and
trailing edge formers, may be cut from either spruce, whitewood, cotton
wood, which can be bent to a surprising degree without fracture, and
three-ply. Three-ply, while excellent for some items, is hardly suited
for this purpose, as the laminations have a tendency to come apart,
especially in the lower grades, which is aggravated by the screws or
brads necessary for the attachment of the flange. A rib, fretted out
as in Fig. 28, with the web of cotton wood and a spruce flange, can be
made extremely light. A rib for a chord of from 4 ft. 6 in. to 5 ft.
would weigh about 5½ oz. As it is very necessary that every rib should
correspond, these parts should be made to a metal jig, which is about
the only way to ensure exactitude. This should be made from mild sheet
steel, about 16 B.W.G., and need only be shaped to the outer curve,
as the lightening holes are of but secondary importance, these being
usually marked out in the saw mill, and cut to the line with a fine
jig saw. For production in quantity a box jig, between which a dozen
ribs might be clamped and shaped, is preferable. Templates of wood
are of doubtful accuracy, for not only do corners wear, but gradual
shrinkage soon renders them useless. The incorrect shaping of the
most insignificant piece of wood may have far-reaching effects when
assembled, and any extra trouble taken in the preparation of parts is
more than repaid by the subsequent ease and precision of erection.

While the method of rib building previously described constitutes
general practice, there are, of course, other arrangements in vogue.
Fig. 29 illustrates a system in which the front spar forms the leading
edge, a procedure which is somewhat rare now, owing to the features
of modern wing sections, but at one time quite common. In this case
the web is of three-ply lightened with a series of graduated holes,
according to the width of the web, and the flanges of spruce.

The rib assembly, Fig. 30, is extremely simple and light, as in this
case the web proper is superseded by thin strips of three-ply, glued
and bradded each side of the spruce flange. The amount of woodwork
between the spars is reduced to a minimum, although one can hardly
imagine such a system answering for a chord over five feet. Even then
the wing curvature would require to be fairly simple, as a pronounced
curve would flatten out. As a point of fact, this assembly is rarely
used for chords exceeding 4 ft. 6 in. In another arrangement as shown
in Fig. 31, the connection between the top and bottom flanges is
formed by blocks, a method which is certainly economical of material.

[Illustration: FIG. 29.]

[Illustration: FIG. 30.]

[Illustration: FIG. 31.]

[Illustration: A FIG. 32.

FIGS. 29–32.--Construction of ribs.]

An interesting form of rib design is that shown by Fig. 32, and in this
instance the fretting is specially designed to prevent any flattening
out of the camber. The rib section is shown at A, Fig. 32, and it will
be noticed that the flange of chamfered section is grooved to take the
three-ply web. The vertical parts of the web are stiffened by small
semi-circular fillets.


Ribs under Compression.

For those ribs contiguous to the interstrut joints, a different
construction is necessary to withstand the tension of the
cross-bracing of the planes and, to a lesser degree, the internal
plane wiring, so that at this point the rib performs two functions,
that of maintaining the wing curve, and also taking the strains due
to compression. Where such provision is not made, the tension of the
wiring will result in either or possibly both of the following: (1) the
rib will buckle laterally; (2) the camber will increase to an extent
varying with the pressure on the wires, both results being extremely
detrimental to efficiency. In this respect the old box-kites of varying
origin used to offer some interesting studies in variable camber, and
when it is remembered that the wing ribs were commonly composed of a
single ash lath, steamed to shape, and the fabric attached on the top
side only, the wonder is that extended flying was possible at all.
For all that, some comparatively classic cross-country flights were
accomplished. One popular system is to incorporate a box-rib at these
points, sometimes made by placing two ordinary ribs close together and
connecting them with three-ply or thin spruce, so that, although the
overall width of the finished box-rib would be approximately 2 in., it
is exceptionally rigid and withal light.

[Illustration: FIG. 33.--Compression rib.]

Another solution is to use a solid web, lightly channelled out, as in
Fig. 33.

In some wing structures the ribs are uniform throughout, a strut of
either steel tube or wood being inserted and to which the internal
wiring is attached. This latter method is possibly more desirable,
that is, if the joint between the compression strut and spar can be
combined with the interstrut fitting. This may necessitate a little
extra work in the latter, but this is preferable to the use of a
separate fitting, involving additional piercing of the spar.


Importance of Even Contour.

Whilst on the subject of rib building, one cannot over emphasize the
desirability of even contour, and the template, illustrated by Fig.
34, serves as an admirable check. It is cut from very dry material to
the outside curve of the section, and if this is tried on as each rib
is fixed, one may be sure of comparative uniformity. The root rib is
generally of stouter construction, and usually follows the same lines
as the compression ribs. At this point the pull of the fabric has to be
contended with, which is not infrequently a considerable strain. The
same conditions prevail at the wing tip, which is one reason against
excessive reduction of material at this point. Instances occur where
the tension of the fabric after doping has considerably deformed the
tip curve, which is at least unsightly, and may entail reconstruction.

[Illustration: _SHAPED TO UNDER SURFACE_

FIG. 34.--Template for testing rib contours.]


Wing Tip Details.

The actual shape of the wing tip varies with the make of machine, and
forms one of the distinctive features of the complete assembly. There
is a general tendency to rake the ends, making the back spar longer
than the front, on the score that increased efficiency due to reduction
of end losses is attained. While this is somewhat problematic, seeing
that several notable machines have square tips, and some actually
constructed with the longest edge leading, it undoubtedly imparts a
pleasing and distinctive appearance.

The actual construction is largely a matter for individual preference,
as there are several ways of forming it. For instance, a single piece
of ash may be bent to shape, or it may be cut out in sections from
spruce boards and glued together with a long splice, while in another
instance oval steel tube is the material. This small section steel
tubing seems admirably suited for such items as wing tips, trailing
edges, and the various components of the empennage, such as the fixed
stabilizer, elevators, fin, and rudder.

Another method of construction used for the wing tips of some machines
consists of a number of strips, about six for a wing tip 1 in. wide by
¼ in. thick, the joints between which are disposed vertically, forming
a laminated wing tip. In manufacture, each piece is bent round bending
jigs or blocks of the required shape, the edges of the strips having
previously been glued. It is apparent that the smaller the section of
strip used, the easier it can be bent, and with this arrangement quite
sharp bends can be successfully formed in spruce. The alternative
method of steaming a solid piece is often wasteful, apart from the fact
that it enforces the use of ash.




CHAPTER V.

DETAILS OF PLANE CONSTRUCTION.


The tendency to lose lift, pronounced in some machines, hardly
noticeable in others, may be directly traced and attributed to the
manner in which the wings are built, which is largely dependent upon
the design. In the preliminary stages of design it is usual to take
as a basis the figures for lift and drift of a known tested section,
that is if facilities are not available for testing an exact scale
model of the section it is intended to use. Anyway, the whole design
is dependent upon these figures, in respect of both the maximum and
minimum speeds, and also the rate of climb, and the extent to which
the actual performance of the machine complies with these calculations
is determined solely by the exactitude and precision with which the
full-size wing conforms to the scale model. By this means only is it
possible to design with any degree of accuracy.


The Sagging of Fabric.

The sagging of the fabric between the ribs is one of the principal
reasons for the failure of the finished machine to satisfy expectation
and also of the tendency to lose lift. One or two causes contribute to
this result. One is the spacing of the ribs, which in some cases is
not nearly close enough. A rough average spacing is from 10 ins. to
1 ft., but in modern high-speed machines, loaded to anything from 5
lbs. to 8 lbs. per square foot, the spacing should be much closer. In
addition, the ribs near the wing root should be closer than those at
the tip, for at this point the stresses are greater, a certain amount
of vibration from the engine having to be contended with, in addition
to the effects of the slip-stream of the air-screw. Particularly
noticeable is the tendency for the fabric to sag down on the top
surface of the leading edge, a feature which imparts to the machine,
especially when viewed from the front, a not unpleasing corrugated
appearance. At this part of the section the curve is somewhat sharp,
and naturally the fabric tends to conform to the definition of the
shortest distance between two points, a straight line. This, of course,
is aggravated in flight, when the planes are under load, and by far the
greatest amount of pressure is located at the front portion, or leading
edge, of the wing.


False Ribs.

[Illustration: _FALSE RIBS_

FIG. 35.--Arrangement of ribs at leading edge.]

In some wing constructions the forces are minimized by the provision
of subsidiary or false nose-ribs, Fig. 35, which extend usually from
the leading edge as far back as the front spar and occasionally to the
longitudinal stringer. While this prevents, to a certain extent, the
sagging in of the fabric, it does not entirely eradicate it. The only
successful way in which the characteristics of the wing contour may be
preserved is by covering the leading edge with thin veneer, spruce,
or, still better, three-ply, as Fig. 36. Despite the great advantages
attending this constructional feature, its use cannot be said to be
really extended.

[Illustration: FIG. 36.--Three-ply covering for leading edge.]


Pressure at Leading Edge.

The pressure at the leading edge produced by the enormous speed at
which the modern machines fly (and the maximum diving speed of which,
owing to the reduction of resistance, is correspondingly increased)
must be abnormal, and calls for different methods of construction from
those which at present obtain. There is at least one case on record
where the fabric has burst at this point with fatal results. It is
interesting to note that in the report of the N.P.L. for the year
1916–17 mention is made of the deformation of the wing form, due to
the sagging of the fabric, which has been reproduced in model form,
so that the allowances to be made and the resultant effects have been
determined.


Effect of Lateral Control.

The system adopted for the lateral control is a decisive factor in
deciding the general lines of construction. The arrangement of plane
warping, whereby the wing was twisted or warped from root to tip, or
the outer section only, has given place to the almost universal use
of aileron control. With the old warping system the ribs, spars, and
the whole wing collectively was subjected to a torsional strain, which
could only have had a deleterious effect upon it. This fact was almost
entirely responsible for the practice of using steel tube for wing
spars, for by its use it was a fairly easy matter to arrange the ribs
to slide or hinge upon the tube, which, at least, relieved some of the
torsional stress.


Leading and Trailing Edges.

[Illustration: FIG. 37.--Leading edges.]

The average practice concerning the formation of the leading and
trailing edges is shown by Figs. 37 and 38. Where the section in use
requires a bluff entry the spindled-out nose-piece is applicable, while
for a sharp entry a fillet let into the nose-formers suffices. As
previously mentioned, steel tubing makes a satisfactory trailing edge,
although somewhat heavier than the spruce strip, while an extremely
fine leading edge can be formed by steel wire. The edge, under pressure
of the fabric, assumes a variegated shape, a distinctive feature of
some types, but, nevertheless, a wire trailing edge is somewhat flabby
and undulating, and as a method is obsolescent. Longitudinal stringers
are employed to preserve the wing contour and also for a stiffening
medium for the ribs in a lateral direction. About the only variation
of the small spruce strip for the purpose is linen tape, crossed
alternately.

[Illustration: FIG. 38.--Trailing edges.]


Efficiency of the Raked Wing Tip.

In the previous chapter mention was made of the probable gain in
efficiency resulting from the raked wing tip, and that this has some
foundation in fact will be apparent from a consideration of Fig.
39, which illustrates the flow of air across a plane, as generally
accepted. Where the plane surface is continuous from wing tip to wing
tip, the provision of the shaped tip would appear to compensate for any
slight loss, but there are instances where the extent of the pilot’s
range of view is of the utmost importance, and this may necessitate
the cutting away of a portion of the centre section (which sometimes
affords the only means of ingress and egress), or the root of the lower
plane, as in Fig. 40.

[Illustration: FIG. 39.--Diagram showing flow of air across plane.]

[Illustration: CENTRE SECTION CUT AWAY

SPAR ROOTS CUT AWAY

FIG. 40.]


Wing Baffles.

An attempt to prevent air leakage caused by this is occasionally
observed in the employment of vertical vanes, or wing baffles. In the
case of a machine with the lower plane abutting against the side of
the fuselage, these would not be necessary, the fuselage acting in
the same manner. The baffles are usually of three-ply or spruce, and
shaped to project above the top and bottom surfaces, this projection
rarely exceeding six inches. A typical arrangement is illustrated by
Fig. 41, which also shows the exposed spars streamlined with a fairing
of three-ply. It is typical of the varied opinions which still exist,
that on some machines the wing roots are merely washed out somewhat
abruptly. If this air leakage is of any moment, it is apparent that
it must detrimentally affect the lift-drift ratio. As a proof of the
existence of pressure at the openings in the wing, the writer remembers
the case of a well-known seaplane, where the wing baffles on the centre
section were made of somewhat thin three-ply. In flight it was noticed
by the pilot that these were being forced away from the wing, and
subsequently these were replaced by baffles of stouter construction.

[Illustration: FIG. 41.--Wing baffle.]


Metal Wing Construction.

Of two machines, equal in air performance, the one which can be most
easily produced has an obvious and, especially at the present time, a
very important superiority. Rapidity of production is a most cogent
argument in favour of metal construction, for once the necessary
machines are set up, and the jigs and dies made, and given a constant
supply of material, output is only limited by the speed of the machine.
In addition, there are the very exacting demands of interchangeability.
Now, it is infinitely more easy to obtain exactitude in metal than in
wood, and, moreover, assuming that it is possible to produce woodwork
to the nearest ·01 of an inch, what preventive is there against
shrinkage, which occurs even when using the dryest of timber. By the
more extensive use of metal there should be a considerably reduced
proportion of scrapped parts, and erection would be accelerated. It is
significant that the planes of some of the most recent German machines
are constructed largely of steel tubing, which is at present the most
practicable form in which steel can be used. Of course, steel tube spars
are quite an old detail, although the more general English practice is
to core them with spruce or ash, as in Fig. 42. One remembers a

[Illustration: FIG. 42.--Steel tube spar with wood filling.]

monoplane, built some time before the war, in which the spars and ribs
were of steel and the covering of thin aluminium sheet. In flight
this machine was particularly fast, which may be accounted for by the
reduction of skin friction, which a smooth surface such as aluminium
would afford. In addition, the tendency of a fabric covering to sag was
also obviated. Another example of metal construction is afforded by
the Clement-Bayard monoplane, exhibited at Olympia in 1914. The plane
construction of this machine, as shown by Fig. 43, consisted of channel
steel spars, steel leading and trailing edges, and thin steel strips
replacing the usual wooden stringers. However, steel construction in
modern English machines is restricted to the various organs of the
empennage, and occasionally one finds ailerons so built. There seems
no valid reason for the continued use of wood as the material for
the construction of such items as the fin, rudder, and elevators,
as a considerable saving of labour and time can be effected by using
the various forms of steel tubing; moreover, the tendency which most
controlling organs built of wood have to warp and twist with variations
in temperature is prevented by the steel frame. One frequently sees
such items as the ailerons and elevators distorted, which must result
in excessive drift, if not erratic flying. At the present time it is
difficult to obtain aluminium alloy in any large quantity, and this,
in conjunction with the present high prices, precludes its extensive
use. When this material is procurable in quantity, and when design is
reasonably standardized, rolled or lattice spars and stamped ribs may
come into vogue.

[Illustration: STEEL SPARS

FIG. 43.--Rib construction with metal spars.]


Fabric Attachment.

Fabric and its attachment is a matter requiring considerable attention,
with the great pressure to which modern wings are subjected. In the old
days any fabric which was light with a moderate degree of strength was
utilized. Nowadays, it is required to stand a certain strain in warp
and weft, and rightly so, since the bursting of fabric in flight can
only have one result. It is interesting to note that the fabric used
on the Deperdussin hydro-monoplane was specially woven with threads
running at right angles, forming innumerable squares. The purpose of
this was that, should a bullet or any object pierce any one of the
squares, damage would be confined to that square, and thereby prevented
from developing; but the writer cannot recall any instance of its use
to-day.

In covering, the fabric should be tightly and evenly stretched from end
to end of the wing, and only comparatively lightly pulled from leading
to trailing edge. If too much strain is applied to the fabric crosswise
it will result in undulations between each rib. The tendency of fabric
to sag between the ribs is accentuated by this, and, of course, matters
are not improved upon the application of the dope. It should be
remembered that the efficiency of any machine is greatly dependent upon
the tautness of the fabric. It should not be stretched too tightly, as
the application of the specified coats of dope may result in the fibres
or threads of the material being overstrained.

[Illustration: CANE STRIPS SCREWED TO RIBS

FIG. 44.--Attachment of fabric to ribs by cane strips.]

With regard to the actual attachment of the covering to the wing
framework modern practice is restricted to two methods. The older
method is illustrated by Fig. 44, and consists of strips of spruce,
or more usually cane, tacked or screwed to the ribs. It is usual, and
certainly preferable, to affix this beading to every rib of those
sections of the planes adjacent to the fuselage, as the fabric on
these portions is subjected to the slip stream of the propeller,
which meets it in a succession of small blows. The fabric in the
outer sections need only be affixed to alternate ribs. The alternate
method is shown by Fig. 45. In this case the fabric is sewn to the

[Illustration: FIG. 45.--Fabric sewn to ribs.]

ribs with twine or cord, the stitches occurring about every three
inches. It will be noted that every loop or stitch is locked with
a species of half-hitch knot. This stitching is then covered with
bands of fabric, the edges being frayed to ensure perfect adhesion
and doped to the main cover. It is largely a matter of opinion
which system ensures the most even wing contour, although it would
seem that the drift or resistance is slightly lessened by the
sewing method. An obsolete method is that in which the fabric was
tacked to the ribs with brass pins and taped with linen tape. All
sewn joints in wing covers should be, and generally are, of the
double lapped variety (Fig. 46), and arranged to run diagonally
across the wing. A minor and somewhat insignificant detail of wing

[Illustration: FIG. 46.--Double-lapped joint in fabric.]

covering is the provision of small eyelet holes in the under surface
of the trailing edge, allowing water accumulated through condensation
to drain away, and although not general practice, would appear to be
necessary. A refinement which may be necessary on the post-war sporting
machine is the attachment of small blocks, or “domes of silence,” to
the leading edge, as a protection for the fabric against wear. When
planes are dissembled more often than not they are stacked leading edge
downwards on a concrete floor, and any movement or friction is likely
to result in the rubbing away of the fabric, which, if unnoticed, may
result in the bursting of the covering. Such fitments would hardly
constitute an innovation, as the writer has distinct recollections of
seeing such fittings on the D.F.W. biplane at Brooklands just prior to
the outbreak of war. These consisted of brass balls, free to rotate
in a socket, screwed to the leading edge. A narrow strip of aluminium
screwed along the entering edge would be quite sufficient, and would
not add appreciably to the weight.




CHAPTER VI.

INTERPLANE STRUT CONNECTIONS.


It may be taken as fairly conclusive that for war purposes the biplane
has proved its superiority, and it appears also that for the commercial
requirements of the future it is suited still better, and therefore, in
view of the huge possibilities thus opened up, is likely to maintain
this predominance.

As the arrangement of planes in a biplane forms the extremely simple
yet enormously efficient box-girder, it is generally considered
superior in strength to weight requirements, although for monoplanes
of small span it is doubtful if this is so, which affords some
indication of the possibilities of the small monoplane as the sporting
machine of the days to come. Seeing that the principal difference
between the biplane and monoplane consists essentially in the type
of truss employed, the arrangement and attachment of the various
members peculiar to the biplane truss becomes of interest, certainly
of importance. It is intended to deal with the various trusses in a
later chapter, confining the present remarks to the interplane strut
fittings in use, and commencing by detailing the chief requirements
and desirable features. The most desirable requirement is that
the attachment of the fitting to the wing spars does not involve
the drilling of the spar. In practice this is most difficult of
accomplishment, for while no great trouble would be experienced in
making a fitting fulfilling this requirement, it would be quite another
matter to keep it in place under the tension of the bracing wires,
and in the case of the outer strut fitting, to which any strain is
ultimately transmitted, practically impossible. In spite of this, it
must be remembered that the machine may occasionally, when landing or
getting off, pitch over on to the wing-tip skid, and if severe, the
shock transmitted to the spar may cause a fracture to develop which,
starting at the hole due to the strut fitting, and owing to the fabric
covering, would be difficult to detect. One or two similar mishaps,
with a consequent increase in the extent of the fracture, give distinct
possibilities of collapse in the air. Although one cannot give specific
instances, it is a feasible contingency, and one that should be
eliminated from the region of possibility.

Additional important features are the provision for rapid assembly and
detachment, ease of manufacture, and the absence of brazing, welding
and soldering as mediums for forming connections, at least for those
parts subject to any stress.

The qualities of strong construction and good design are paramount
considerations in the manufacture of these fittings, as the purpose
of an interstrut joint is not merely to form a connection between the
upper and lower planes, but also to distribute the intricate stresses
encountered in flight.


Brazing and Welding.

It is somewhat amazing that brazing as an essential operation in the
making of a joint should still be employed, as it is difficult to
imagine anything less suited to the conditions under which aircraft
operate. The advantages of a uniform high-grade steel possessing a high
ultimate tensile strength are dissipated by the intense heat necessary
for the action of brazing, resulting in the strength of the finished
joint becoming an extremely problematic quantity, indeed this is
rendered the more so by the individuality of the workmen.

Welding properly performed is less objectionable, indeed, its use may
be said to be constantly increasing, although it is well to recognize
its limitations. It should not be used for parts subject to any great
tensile stress, such as the fittings forming the subject of this
chapter. The efficiency of any welded joint is hard to determine, as
apparent soundness on the surface is no indication of the internal
nature of the weld. Regarded from the aphoristic “maximum strength for
minimum weight” view point, and taking into account the advantages in
this direction which can be obtained by the use of a high-grade steel,
brazing and welding are not to be commended.

The operation of soft soldering, requiring only a moderate heat, does
not weaken the material to any great extent, and for some items a
properly pegged and soldered joint is superior to the two methods of
jointing previously described.


Connections in Use.

[Illustration: FIG. 47.--Interplane strut attachment.]

The illustrations given indicate the varying degrees of practice,
taking as the standard for comparison the early Wright socket, Fig.
47. Although somewhat crude it was quite suitable for the purpose,
especially as the wing warping system in the Wright machines
necessitated a fair amount of flexibility in the joints. It serves also
to illustrate that some advancement has been made in constructional
work. The advantages of rapid erection and dismantling have been
realized and provided for in most machines since the early days of
the industry, and it is not surprising, therefore, that the salient
characteristic of the joint (Fig. 48) used by S. F. Cody on his famous
biplane was portability. The interstrut terminates in a kind of fork,
which in turn is pinned to the head of a special bolt slotted to
receive it. The fact that the wiring lugs were improvised from chain
links is interesting.

[Illustration: FIG. 48.--Interplane strut attachment.]

The method of packing the wings for transport consisted in detaching
the two outer cellules from the central structure, when the removal
of one set of wires enabled the planes to be folded one against the
other. It is possibly of interest to record the fact that in the
military trials of 1912 this machine was taken down and re-erected in
51 minutes, quite a good performance taking into account its large
dimensions. Although this attribute is scarcely necessary at the
present time, it will be undoubtedly required by the sporting owner of
the future with limited storage facilities. The fitting shown by

[Illustration: FIG. 49.--Interplane strut attachment.]

Fig. 49 is only suitable for machines with light wing loading. The plate
forming the anchorage for the wires is pressed out, the lugs bent to the
different angles, and then attached to the spar by an eyebolt, to which
is fixed the plane strut, the ends of the latter being capped with steel
tube of streamline section. A similar arrangement is that shown by

[Illustration: FIG. 50.--Interplane strut attachment.]

Fig. 50, the lug plate being pressed out and bent, but in this example
the strut terminates in a socket of oval steel tube welded to the
plate. It is connected to the spar by a bolt passing through the centre
of the socket, the strut end fitting over this.

[Illustration: FIG. 51.--Interplane strut attachment.]

The practice of anchoring wires to eyebolts, as in Fig. 51, forms
the nucleus of many strut connections, but as a method cannot be
recommended. Continual strain on the wire has a resultant in the
bending over of the head of the eyebolt as in Fig. 52. As a point
of fact the use of the eyebolt is distinctly elementary, and gives
the impression of a makeshift. The fitting illustrated by Fig. 53
constitutes an advance on the previous arrangements dealt with, and is
also indicative of modern practice.

[Illustration: FIG. 52.--Interplane strut attachment.]

[Illustration: FIG. 53.--Interplane strut attachment.]

The main body of this clip is a stamping from heavy sheet-steel, bent
up to the section of the spar, the bolts, it will be noticed, passing
horizontally through it. The anchorage for the wires is formed by lugs,
which have a direct pull on the bolts, and is so arranged that a slight
clearance exists between lug and spar.

[Illustration: FIG. 54.--Plane strut attachment.]

The plane-strut is shod with steel tubing, and connected to the fitting
by a bolt, as shown. Of the strut connections described so far, hardly
one can be said to conform to the leading principle of the ideal
fitting, _i.e._ the secure attachment to the spar without piercing the
latter for bolts. Fig. 54 gives a fitting which is as good a solution
of the problem as is constructionally possible. The basis of this
connection is the lug-plate, to which is welded the strut-socket,
the whole being fastened to the spar by four bolts, which are let in
the flange of the spar just half their diameter, and tighten on a
washer-plate on the opposite side. Lateral movement along the spars
is thus adequately prevented, although the outer strut-socket might
conveniently be bolted right through the spar, without materially
reducing the strength thereof. This is made possible by the fact that
the wing spars, disregarding the small wash-out at the extreme tip, are
generally parallel in depth from root to tip, the amount of material at
the point of intersection of the plane-strut being in excess of that
necessary for the stresses concerned. Another attachment achieving
similar results is shown in the diagram (Fig. 55), forming an example
of the fitting employed on the pre-war Avro biplane. It will be noticed
that in this case two bolts only are used for the connection, the pull
of the flying or lift-wires being counteracted by the duplicated wires
taken from the washer-plate to a fitting located on the single central
skid of the undercarriage.

[Illustration: FIG. 55.--Interplane strut attachment.]


Head Resistance of Strut Sockets.

A point calling for comment is the apparent oversight or neglect of
the amount of head resistance offered by the average strut fitting,
although great care is taken to ensure the strut and wing sections
being of correct form. It seems probable that some difference must
occur, especially at the high speeds now prevalent, between the air
flow across the plane and that which meets the strut terminal. Anyway,
some discontinuity of flow exists, and whether or no the aggregate
resistance of all the fittings is of any great moment provides matter
for discussion. It is quite possible to fair off any irregularities
in air-flow due to the strut connections by the attachment of
sheet-aluminium fairings, which could be beaten, pressed, or spun with
little difficulty. Although examples of this practice are very little
in evidence, the writer inclines to the belief that the additional
weight would be negligible compared with the ensuing reduction in head
resistance.

The foregoing examples cannot be said to constitute the latest
practice, nor is it possible under present conditions to give such
details, but sufficient has been said to indicate the progress and
trend of design.




CHAPTER VII.

WING-TRUSSING SYSTEMS.


Although the trussing of aeroplanes is carried out along certain
well-defined lines, there are occasional divergences from the orthodox.
The differences now existing are not nearly so great as those of
former days, this being explained by the fact that the progress of
any science or industry tends towards uniformity of method, while
practical experience eliminates the undesirable systems. This does
not necessarily mean that the present methods in vogue are incapable
of improvement, but merely denotes their suitability for present
requirements.


The Pratt Truss.

[Illustration: FIG. 56.--The Pratt truss.]

The basis of all modern trussing systems, with modifications, is
the Pratt truss (Fig. 56), familiar in bridge-building circles, the
basic principle of which is that the compression members are disposed
vertically, and while of minimum length are most favourably placed for
obtaining the maximum efficiency. There are other types of trusses
used in structural engineering, as, for instance, the Howe truss, in
which the compression members are arranged diagonally, and the Warren
lattice-type girder; but for various reasons these are not applicable
to the needs of aeronautical engineering. But a brief consideration
of the chief features of the Pratt or box-girder system of trussing
will suffice to illustrate its great advantages for aircraft work,
particularly for machines exceeding a certain span; and it is this
limiting span to which a monoplane can safely and efficiently be built
which is largely responsible for its present spell of unpopularity.


Monoplane Trussing.

From the standpoint of simplicity, the monoplane equals the biplane.
As each wing of the former may be considered as a cantilever, it
is the difficulty of adequately staying the wings above a certain
span which forms the deterrent feature, for it is obvious that, as
the span increases, in order to obtain a reasonable angle for the
wires, the king post, or cabane, must be increased in height. This
would necessitate an ungainly undercarriage, less able to withstand
rough landings, with a consequent increase in both weight and head
resistance. However, it seems that the monoplane will have a future
for sporting purposes, where the span will not exceed 30 ft., and will
probably be nearer 20 ft.

[Illustration: FIG. 57.--Monoplane wing bracing.]

Various attempts have been made to obviate this inherent defect of the
monoplane system of trussing, the first and most popular being the
king-post system (Fig. 57), in which short masts are incorporated in
the wing structure and wire-braced to the spars. From the points formed
by the crossing of the mast and spar the main bracing-wires are taken.
That this system is of real use is demonstrated by the fact that,
amongst others, the Antoinette, Flanders, and Martinsyde monoplanes
incorporated this system. It is worthy of note that this system also
characterized the huge Martinsyde trans-Atlantic ’bus, the wing-spread
being in the neighbourhood of 70 ft. Another original attempt at
improvement, the wing-bracing of the Deperdussin hydro-monoplane, is of
interest (Fig. 58). As regards the bracing, the machine was virtually a
biplane, the wings being stayed by a steel tube running parallel with
the wings, and connected to it at intervals by steel tubular struts,
with cross-bracing between, as in a biplane. The abolition of the top
wires rendered the machine of greater value for war purposes than other
tractor machines of that period. The logical conclusion of this system
is exemplified by the Nieuport scouting biplane, the lower plane of
which corresponds to the streamlined steel boom of the Dep.

[Illustration: FIG. 58.--Deperdussin monoplane bracing.]


Wireless Wing Structure.

Superficially, it would appear that the abolition of external trussing
and wiring would make for greater aerodynamical efficiency; and,
constructionally, it would be quite possible to build wings devoid of
external staying, and at the same time of sufficient strength. But when
it is considered that this would entail an excessive depth of spar at
the root of the wing, with a resultant increase of head resistance,
it is doubtful whether any appreciable advantage would accrue. In the
event of the wing becoming deformed or out of alignment, re-truing up
would be almost impossible, and would certainly require the uncovering
of the wing and partial reconstruction. Contrast this with the orthodox
wire bracing. It is simple of attachment, of relatively low cost, and
offers the utmost facility for truing up. A monoplane of note, built
without external trussing, was the special Antoinette, produced for the
French military trials of 1911. This had a span of approximately 46
ft., and the depth of spar at the root was about 2 ft. 3 ins., and at
the tip 9 ins., the consequent weight alone being abnormal.


Anchorage of Lift Wires.

The one-time practice of anchoring lift wires to various parts of the
undercarriage is bad in principle, as there is a distinct possibility
that a rough landing may damage the wire or its attachment, and
ultimately cause failure in flight. This practice undoubtedly arose
from a desire to obtain a good angle for the lift wires, a subsequent
improvement being the addition of a separate pylon or cabane.


Biplane Trussing.

[Illustration: FIG. 59.--Biplane Truss.]

The most common form of biplane truss is shown by the diagram (Fig.
59), sometimes, as in the case of various pusher types, or those for
long-distance work where a large wing area is necessary, extended to
three bays each side, which probably explains the partiality of German
designers for multiplicity of interplane struts, as, prior to the
outbreak of the war, the majority of German machines were designed and
built entirely for long distance and duration flying. By this means
light wing loading, which entails large wing area, was possible without
prohibitive weight, for by the addition of a pair of struts to the
two-bay type, a lighter wing spar for the same strength is possible.
In this type of truss the bays adjacent to the fuselage are varied
in width, in order more easily to apportion the stresses, which are
greater at the centre of the wing structure. A modification of Fig.
59 is indicated by Fig. 60, which illustrates diagrammatically the
arrangement of the Maurice Farman biplane, the improvement consisting
of the method of strengthening the interplane struts. The outer strut
is braced with a small king-post, and from this a wire is taken through
each side of the strut. On this machine the struts are of the light,
hollow-spar type, and this arrangement must therefore materially reduce
their tendency to buckling.

[Illustration: FIG. 60.--Farman wing structure.]

Another version of this system is that in which the top plane is of
greater span than the bottom, the extension thus formed being stayed
with lift and counter-lift wiring, or by means of a strut acting in
tension and compression.


Single Strut Systems.

The almost universal arrangement for the small single-seater scout
is the single bay, and from this method the progress of design has
inclined towards the elimination of as many struts and wires as
possible, which has its culminant in the type of truss embodying one
strut and one pair of wires, lift and counter-lift, each side of
the body. Quite a number of machines have incorporated the single
strut assembly, the earliest perhaps being the Brequet, and one also
remembers a small Avro scout, the strut in this case being built up
with spars and stringers, covered with fabric. The single-lift truss
is particularly suited to multiplane construction, where the chord of
the wings is narrow, and the bending moment, due to the movement of the
centre of pressure, is correspondingly reduced. A disadvantage exists
with this form of truss similar to that experienced with the wireless
monoplane truss, _i.e._ the difficulty of maintaining the correct
incidence from root to tip. However, some extraordinary machines of
recent construction embodying this feature, stand up to active service
demands, so that this defect can be of no great moment. A minor detail
consists in the circumstance of, for example, a lift wire coming adrift
or perhaps being shot away. With the single-lift truss total collapse
would ensue, but it is conceivable that the ordinary double-lift truss
offers more chances of escape.

[Illustration: FIG. 61.--Wireless wing structure.]

Another system which obviates the need for wires is illustrated by
Fig. 61, which was the particular system used on the Albatross “Arrow
biplane” of 1912. A drawback is the difficulty of readjustment, which
is the probable explanation of its failure to come into extensive use.
The direct antithesis of this arrangement, the elimination of struts,
is indicated by Fig. 62; but as this embodies all the defects of the
monoplane system of trussing, even of the attachment of wires to the
undercarriage, it must be considered of no practical utility.

[Illustration: FIG. 62.--Biplane truss without interplane struts.]


1½ Strut Machines.

The arrangement shown by Fig. 63 is responsible for the designation of
machines so built as “1½ strutters.” A later development of this system
consists of but four centre plane struts, the two struts forming an
inverted V between the fuselage longerons and centre being dispensed
with.

[Illustration: FIG. 63.--1½ strut wing structure.]

The system (Fig. 64) is illustrative of the form of staying in use on
a modern high-speed scout, and in respect of which a patent is held.
As this machine is designed with a very small gap, the lift wires
are consequently at a somewhat flat angle. The strut, about halfway
along each wing, is hinged at the point of intersection of the wires,
which, incidentally, do not run through from corner to corner, but are
attached in the centre to a fitting which also forms the anchorage
for the struts. By this method there is an apparent reduction in the
tendency of the wing spars to buckle under load between the points of
support.

[Illustration: FIG. 64.--A patented wing bracing.]


Drift Bracing.

So far the methods dealt with denote the methods of staying in a
vertical dimension, and it remains to consider the provision for
trussing in the fore-and-aft direction. There are two methods in use,
one being to brace the wings internally, which is the more general
practice, as by this arrangement the resistance of exposed wiring is
obviated, while the alternative method consists in taking wires from
various points along the wing to the nose and rear part of the fuselage.


Properties of the Various Types.

The necessity for increased size, with its inevitable sequence,
increased weight, must be realized without a very great addition to
the landing speed, the figure for the latter standing at approximately
45–50 m.p.h. This factor greatly influences the maximum wing loading
possible, without detrimentally affecting this, so that in the
design of the large machine a considerable increase in wing area is
unavoidable. This fact practically rules out the monoplane system for
the large aeroplane, as, although this arrangement possesses a superior
ratio of lift to drag to that of the biplane or multiplane, the great
span necessary to obtain the wing area is impracticable. It is quite
obvious that to brace adequately a monoplane structure of 100 ft. span
or so, a very complex system would be required, in addition to which
the spars would essentially be of larger and heavier section. The
biplane arrangement can be used successfully for spans up to 100 ft.,
and, assuming that the future commercial machine will necessitate still
greater wing area, it is a feasible supposition that the triplane, or
even quadruplane systems will be used. Certain modern triplanes have a
reputed excellent performance, the carrying capacity and engine power
being colossal. Against this we have the fact that the advantage of
the triplane system is purely structural, as aerodynamically it is not
nearly so efficient as the biplane, and it is at this stage that the
question of the limiting size of aeroplanes is encountered. Various
tests, both in model form and full size, have shown that the lift of
the middle plane of the triplane system is greatly inferior to that of
the top or bottom planes, this being due to the interference of the
free air flow by the upper and lower planes. This circumstance is an
indication that the biplane arrangement, viewed from the standpoints of
modern design, is the most economical form for future commercial use.




CHAPTER VIII.

FUSELAGE CONSTRUCTION.


The body, or fuselage as it is generally described, constitutes the
nucleus of the completed machine, and at the same time offers the
most interesting examples of constructional detail. It may be as well
to point out that the term “fuselage” is ordinarily applicable to a
body of a machine of the tractor type; the short body of the average
“pusher” or propeller aeroplane is termed the “nacelle.”

The material chiefly used in the construction of this component is
wood, and there are but very few instances where metal is used.


Fuselage Types.

The different types or methods of construction may be classified in the
following order:--

1. Box-girder of four longerons or rails, with cross-struts and wire
bracing (Fig. 65).

[Illustration: FIG. 65.--Arrangement of fuselage members.]

2. Tail portion of longerons, struts and wiring; in the front portion
the wire bracing is dispensed with, being replaced by diagonal wood
bracing, to which is screwed either three-ply or sheet aluminium alloy
(Fig. 66).

3. In this case wire bracing is entirely dispensed with, the four,
and occasionally six, longitudinals being connected together by cross
struts or formers cut to the required shape, the whole body being
covered with three-ply.

4. Laminated or monocoque type, formed by layers of wood and fabric,
crossed alternately and glued together.

[Illustration: FIG. 66.--Arrangement of fuselage members.]


Box-Girder Type.

Dealing with each type in greater detail, and in order of
classification, the details and methods of manufacture of type 1 may
be considered. The longerons are usually of ash or hickory, although
latterly silver spruce has come into use for this purpose, this being
due to the desire to reduce weight to the absolute minimum.

In the opinion of the writer, a spruce longeron should be of larger
section than one of hard wood, for one or two reasons. Spruce is a
soft wood, and the outside fibres are far more apt to get damaged by
a fitting which has been bolted home with too much pressure, also the
corners may get rubbed or knocked off, which all means a reduction in
strength. The use of a spruce longeron precludes any sharp bends in the
contour of the fuselage, as this wood does not lend itself to bending,
although it may be sprung to an easy curve. By disposing joints in
the longerons, it is possible to arrange the lengths so that the bend
is contained in one portion. This portion can then be of laminated
construction, _i.e._ it can be built up of a number of layers glued
together, and clamped to a block of the required shape until the glue
has set. In some cases the longerons from the engine mounting to the
rear cockpit, where additional strength is necessary, are of ash, while
aft of that, to the stern post, spruce is the material.

[Illustration: FIG. 67.--Longeron sections.]

It is usual, in this country at least, to spindle the rails to one of
the sections illustrated by Fig. 67, this spindling or channelling
running through from nose to stern post, or the front portion,
extending as far as the rear cockpit, is left solid, the tail part only
being spindled. This channelling is always stopped at the intersection
of the cross-struts with the rails, to provide the abutment for the
struts, and the extra material to compensate for any holes necessary
for the attachment of the fitting. In the shaping of the rails
longitudinally, two methods are available: they may be tapered or
gradually diminished from the front to the stern post, or the overall
section may be parallel to a point somewhere in the neighbourhood of
the pilot’s seat, and from that point diminished to the stern post. The
first method is obsolete, as all the fittings vary in size, which makes
for undue complication as well as increasing the number of jigs and
dies necessary to produce the stampings. The second method only partly
obviates this, and the only system which permits of the same size
fitting being used right through is that in which the rails are of the
same overall section throughout, but this is very rarely used.

Another arrangement consists of keeping the rail of equal thickness
for approximately 10 ft. from the engine bearers, and then diminishing
in a series of steps to the stern post. By this method only three or
four sizes of fittings are necessary. Some fittings are not affected by
the taper of the rails, and are made the same size throughout, but in
nearly every case the attachment to the rails is accomplished either by
bolts or screws. The piercing of the longeron, particularly when this
is of spruce, is hardly commendable practice, and certainly in view
of the many forms of clip fittings in use appears to be unnecessary.
A point which apparently escapes the notice of some designers, is the
necessity of some allowance being made for unfair stresses induced by
landing shocks and rough handling. There is a tendency to make the
tail portion separate from the front, the joint occurring just aft
of the rear cockpit, so that in the event of damage due to strains
transmitted by the tail skid, this portion can be detached and a new
portion substituted, which seems infinitely better than dismantling the
whole machine and returning the whole body to the works or depôt. In
the design of the body under consideration due regard should be given
to the necessity of occasional replacement of a damaged rail. Some
fittings afford the utmost facility for this, while others render this
procedure a lengthy and difficult operation.


Jointing of Longerons.

A popular method of jointing longerons consists usually of a plain butt
joint, clipped with some form of steel tube socket, or by fish-plates
flanged to clip the edges of the longerons and bolted through. A
spliced joint is sometimes used when timber is not procurable in
any great length, this consisting of an ordinary splice from 12 to
18 ins. long, glued and riveted, and afterwards, when the joint is
thoroughly set, bound with tape soaked in glue and subsequently doped
and varnished. As this is a somewhat lengthy operation the socket
method predominates. In modern aeroplanes the size of a longeron rarely
exceeds 1½ ins. square, and it will therefore be realized that this
construction is all that is possible, as, owing to the slightness of
material, no advantage would accrue from the employment of a joint of
the halved or scarfed variety.


Diagonal Wood Bracing.

A great deal of the foregoing applies to the second type, so far as the
longerons and tail portion are concerned. The diagonal wood bracing
is usually of spruce, and is, of course, heavier than wiring. The
aluminium or duralumin sheeting has latterly given place to three-ply
for the outside covering, which may be ascribed to the saving in weight
effected by its use, as a square foot of 20 B.W.G. aluminium, which is
the general thickness for this purpose, weighs 8 ozs., while a square
foot of 3/32 in. birch three-ply weighs approximately 5 ozs. This
gives a saving of 3 ozs. for every square foot of surface covered, and
moreover three-ply, properly glued and screwed or copper, nailed to
the framework, constitutes by far the better stiffening medium. The
disadvantages of this method of construction are: (1) the difficulty of
re-truing the front portion should distortion occur; (2) erection is
somewhat involved; and (3) it is heavier than the first type, although
it affords a more solid mounting for the engine, with a consequent
reduction of vibration.


Three-ply Fuselage.

The third system is typical of the method adopted for the series of
German Albatross machines. There are few, if any, examples of its use
in this country, although prior to the war a few constructors favoured
its use, and one successful monoplane of note was so built. The writer
is acquainted with one pioneer designer who very strongly believes
in this form of construction, and certain later developments in the
use of three-ply confirm this view. The advantages of this form of
construction are: (1) quickness of production; (2) great strength in
a vertical and horizontal direction; (3) the result of the longeron
being shot through would not endanger the structure to the same extent
as with a wire-braced system. Against this must be balanced the fact
that: (1) it entails a considerable increase in weight; (2) is weak
under a torsional strain, such as that produced by the combined actions
of elevator and rudder; and (3) cannot be trued up in the event of
distortion. Examples of this system in pre-war machines are afforded by
the Martinsyde and Blackburn monoplanes, although the framework in both
cases was so formed as to constitute a lattice girder. The tail portion
of the Martinsyde was lightened by cutting away diamond-shaped pieces
from each bay.

The formers of the Albatross are extremely simple. In the fore part
they are cut from three-ply, while at the rear they are just simple
frames composed of laths, reinforced where the longerons occur by
three-ply stiffeners. There are six longerons, the two middle ones
being fixed slightly more than halfway up each side, which are really
longitudinal stringers to prevent the three-ply buckling between the
points of attachment.


The Monocoque Type.

The monocoque system originated in France, several constructors having
produced machines incorporating this feature. The most successful
machine produced on these lines was the Deperdussin, and many will
recall the excellent streamline form of the machine exhibited at
the 1913 Aero Show. These bodies were built over formers of various
sections, which were removed when the glue joining the different layers
had set. The resultant shell, which was about four millimetres thick,
was then covered with fabric and varnished. Several factors militate
against its extensive adoption as a method. It is rather costly, and
does not seem to be suited to rapid production. In addition, the
attachment of such members as the chassis, wings, and interplane
struts, is more complicated. It should be noted, however, that various
modern machines are similarly built. The Borel firm produced a machine
with monocoque body, this being composed of three-ply covering on
ribs running diagonally the length of the body, and although this is
not such a lengthy operation as the Dep. system, it has not survived,
unless one considers flying-boat construction as its modern version.
A slight variation of the monocoque system is used for the bodies of
some modern aeroplanes. The framework consists of very small stringers
arranged at various points on light formers cut to the fuselage
section. To this structure is applied two thicknesses of three-ply in
the form of strips about 3¼ ins. wide, each thickness being disposed
diagonally in opposite directions, as shown by Fig. 68. This is covered
with fabric, the total thickness being no more than 1½ mm., and as this
is made up of six layers of wood and one of fabric, the fineness of the
ply-wood will be realized.

[Illustration: FIG. 68.--Arrangement of three-ply bands in monocoque
fuselage.]

It should be noted that the ply-wood strips do not completely encircle
the formers, but are jointed at the top and bottom, a light longeron
being arranged at these points.

A detail which would appear to be of great utility at the present
time is the arrangement wherein the nose of the body containing the
engine and accessories is a separate unit, and in the event of engine
breakdown can be detached and another substituted.


Fuselage Contours.

In the design of the contour of the fuselage the type of the motor
used is the determining influence. With the vertical “in line” engine,
it is possible to design a slim narrow body, while a rotary or radial
engine necessitates an increase in width, which also means increased
air resistance. With the Vee type engine, the popular practice is to
allow the tops of the cylinders to project through the cowling, which
permits of a narrower body than if the width of the body equalled the
overall width of the engine. Where a rotary engine is employed and
the mounting is of the overhung type, the width of the fuselage may
be reduced by allowing the engine to project over the sides, and the
cowling carried on an arrangement of formers and stringers, which
gradually merges into the main structure, as in Fig. 69. It is apparent
that the line of the body and that of the fairing should converge as
gradually as possible, as, should this be at all abrupt, there is a
distinct possibility that the air flow will take the course indicated
in Fig. 70, resulting in a dead air region and inefficiency.

[Illustration: FIG. 69.--Fuselage outline.]

[Illustration: FIG. 70.--Fuselage outline.]

It may be taken generally that the wider the body the greater the
weight, for the struts have not only to be made longer but also of
greater overall section. The practice in this country is to keep the
longerons parallel to the centre line on plan, as far as the rear
cockpit, tapering from that point to the stern post in a straight or
slightly curved line, as Fig. 71.

[Illustration: FIG. 71.--Fuselage outline.]

This simplifies the fittings, the sockets for the centre plane struts
are in line, and the different lengths of fuselage struts necessary
reduced to a minimum.

[Illustration: FIG. 72.--Fuselage outline.]

The plan outline of several German machines is shown diagrammatically
by Fig. 72. It will be seen that from the nose the body gradually
widens out until the maximum width, generally in the vicinity of the
front seat, is reached, from where it tapers to the tail. This shape
appears to satisfy aero-dynamic requirements more closely than either
of the foregoing examples; but in practice the difference is not
appreciable, and in any case the reduction of head resistance does not
compensate for the additional work.

[Illustration: FIG. 73.--Fuselage outline.]

[Illustration: FIG. 74.--Fuselage outline.]

In side elevation the general practice, with exceptions, is to arrange
the top longerons parallel to the line of thrust, _i.e._ the axis
of the motor, as in Fig. 73. This simplifies erection and affords a
convenient datum line for truing up.

[Illustration: FIG. 75.--Fuselage outline.]

On the German Rumpler and early Albatross biplanes, the upper longerons
are curved, as in Fig. 74, but in the most recent versions of the
Albatross they are level with the line of thrust. Fig. 75 illustrates
an arrangement where the top rails, from a point some distance along,
slope down to the nose. By this method the body weight is kept as low
as possible and the engine and accessories rendered more accessible.
Although it is usual to terminate the body in a vertical knife-edge,
formed generally by the rudder post, another arrangement, typical
of the Morane monoplane, finishes in a horizontal edge. The German
Fokker, obviously inspired by the French Morane, and the Albatross
DI, are similarly terminated. This system of tapering to a horizontal
knife-edge is not considered the best arrangement from a strength point
of view, the flat angle of the bracing wires permitting a certain
amount of movement, eventually resulting in slackness and loss of
alignment.




CHAPTER IX.

FUSELAGE FITTINGS.


The design and type of fitting employed for connecting the longerons,
cross and vertical struts of the fuselage, varies greatly, being
usually one of the distinctive constructional details of a machine.
This position renders uniformity of practice a comparatively
unattainable quantity, which, in view of present requirements, and
the absolute need of rapidity of output (which must commence as soon
as possible after a successful design is produced), can only be
considered as regrettable. This diversity of design is mainly the
result of the desire for originality of each individual designer,
and however commendable from this standpoint, is a position which is
almost certain to disappear with the progress of the industry. Take as
a hypothesis the case of, say, ten makes of scouting biplanes in use,
each with approximately the same arrangement of longerons and struts,
and with a similar overall size of fuselage. Each of these machines
will incorporate a different fuselage clip, which means that somewhere
highly skilled labour is being unnecessarily expended in the making
of jigs and press tools, whereas a suitably standardized clip for all
scout machines of certain dimensions would involve the making of one
set of press and bending tools only for the machines of the one type
built. Another aspect, quite as important, is the simplification of
the supply of spares. Acceleration of aircraft output, if achieved
only through the medium of small part production, is one of the most
important contributary factors towards ultimate success in the field.


Types in Use.

The sketches, explained in detail hereafter, are illustrative of some
of the many systems in use, and taken collectively fall under two
categories: (1) those in which attachment to the longeron involves
drilling, and (2) those in which the fitting clips or encircles the
longeron, friction only keeping it in position. The first method
permits of a clip of comparatively simple design, but it has the
serious disadvantage of weakening the material, and assuming the
longeron section is sufficient to account for this, then clearly a
fitting which is attached without the use of bolts would allow a
reduction in the size of a longeron (which means a saving in weight)
without depreciating the factor of safety. In the second method the
attachment is usually accomplished by the pressure of bolts, with
practically no weakening effect; but in this case the disadvantage lies
in the fact that at each point of attachment a differently dimensioned
clip will be necessary, this being due, as explained in the previous
chapter, to the longeron tapering towards the stern post. (A method
of reducing the number of different clips by suitably shaping the
longeron was also dealt with.) It is evident that most fittings must
inevitably form a compromise between the demands of production and
design, although it must be admitted that in some cases the fittings
collectively very successfully evade the requirements of both.

[Illustration: FIG. 76.--Fuselage fitting.]

The clip indicated by Fig. 76 is the particular form of construction
associated with the various versions of the Bleriot monoplane, and
favoured by the early pioneers generally. It was retained in the
Bleriot construction until some time after the outbreak of the war--as
a matter of fact, until the type was deleted for war purposes.

It is composed, as will be seen from the sketch, of simply two U-bolts,
the attachment to the longeron involving the drilling of four holes,
which constitutes the chief objection to this particular form of
clip, and has been the subject of criticism from the time of its
first appearance as an aircraft detail. The struts are slotted over
the bolt, and although this does not conform to the best principles,
it is simple, and may have been sufficient for a lightly loaded
machine. A point about this clip, which undoubtedly was the cause
of its popularity amongst the pioneers, with whom economy was an
evil necessity, is that the wires can be strained or tensioned by an
adjustment of the nuts on the longerons, thus rendering turnbuckles

[Illustration: FIG. 77.--Fuselage fitting.]

unnecessary. Fig. 77 shows the form of clip used on the Hanriot
monoplanes, and is a good example of the class of fitting bent up
from sheet metal. This is usually made to be slightly smaller than
the longeron, the pressure resulting from the tightening of the bolts
on the ends forming the cross-bracing lug, keeping it in place. The
defect of this arrangement is that any slight shrinkage of the longeron
will permit movement, and for this reason provision should be made for
the subsequent adjustment of the bolts. The struts are taken by the
lugs punched up from the body of the clip. This leaves very little
material to resist the tension of the bracing wires or tierods, but a
modification of this clip surmounts this difficulty.

[Illustration: FIG. 78.--Fuselage fitting.]

A similar clip is used by the German Aviatik firm, but it is certainly
inferior owing to the very poor connection of the struts. Instead of
the four lugs gripping the sides of the struts, they are punched up to
form a square or tenon, over which the cross-struts are mortised. In
any case it would not satisfy the standards maintained by our leading
constructors, and certainly not the technical advisers to the Air
Board. The clip indicated in Fig. 78 does not encircle the longeron,
but abuts against the two inner sides of the longeron only. The body
of this clip is a stamping, bent to a right angle, to which the square
sockets for the struts are welded. Attachment to the longeron is
effected by an eyebolt, which passes diagonally through it, this also
providing the anchorage for the cross-bracing wire. A form of this
clip has been used on a certain make of machine for a considerable
period, so that it has advantages that are not readily apparent. One
outstanding defect is existent in that the pull of the wires would tend
to lift the socket on the side opposite to the eye bolt, and this in
turn would cause distortion of the struts. A connection favoured by an
American firm is shown by Fig. 79, and possesses the merit of extreme
simplicity. The longerons are not drilled, the attachment being through
the agency of an L bolt, which also provides the anchorage for the
cross-bracing wire. To prevent movement the clip is additionally fixed
to the longeron by wood screws.

[Illustration: FIG. 79.--Fuselage fitting.]

[Illustration: FIG. 80.--Fuselage fitting.]

The method shown by Fig. 80 is that used on the Deperdussin monoplanes,
being patented by that firm as far back as 1912, and consists of two
cast aluminium sockets, bolted to the longeron. The struts, in this
case oval in section, are fastened in place by steel bushes, which
are driven through in the form of steel tube, and expanded and burred
round the socket, at the same time forming the anchorage for the
wires. This system has been used in the construction of a fast scout
of comparatively recent origin, but it embodies the same defects as
the Bleriot clip, _i.e._ four holes are needed in the longeron for
every joint; but it has the advantage over the latter in that a better
terminal is provided for the struts.

[Illustration: FIG. 81.--Fuselage fitting.]

Fig. 81 indicates the arrangement on the German L.V.G.
(Luft-Verkehrs-Gesellschaft) fighting biplane. This is an aluminium
alloy casting, fastened to the longeron by screws, and as it is not
affected by the taper of the longeron, all the fittings, or at any
rate, those in the tail portion, can be of the same dimensions. A
point which is often overlooked when using a fitting of this type is
that any strain on the wires is transmitted to the longeron by the
fastening screws only, or, in other words, the tendency of the wires
when tensioned to pull the fitting from the longeron is resisted by the
screws only. This does not impress one as being well suited to perform
the functions demanded of the average joint, and about the only detail
upon which its existence is justified is its ease of production. In the
writer’s opinion the clip, Fig. 82, is by far the finest connection
yet devised, and one that should be standardized. Its attachment is
accomplished without objectionable drilling; it provides an excellent
housing for the cross-struts; can be tightened up should shrinkage
occur in the longerons; and can be produced at an absurdly low figure.
This clip has been used on machines which have accomplished some
meteoric performances during the war, and, moreover, was designed and
in use a considerable period before the war.

[Illustration: FIG. 82.--Fuselage fitting.]

[Illustration: FIG. 83.--Fuselage fitting.]

The clip, Fig. 83, is simple and quite easily manufactured, being
stamped out of sheet metal, and bent up to shape. The lugs forming the
anchorage for the wires would have a tendency to straighten out at
the bends; but the amount of this, whether serious or otherwise, in
the absence of actual experience, is largely conjectural. However, a
fitting of this kind was used in the construction of the nacelle of a
seaplane exhibited at Olympia in 1914.


Steel Tube Fuselage Construction.

[Illustration: WELDED      WIRES 18 BWC

FIG. 84.--Welded joint in steel tube fuselage.]

In certain isolated instances, the fuselage is built up of steel
tubing, and on one machine of recent design the joints throughout are
effected by welding: a detail of the attachment of the vertical and
cross struts to the longerons is shown by Fig. 84. It will be noticed
that a small quadrant-shaped piece of tube or rod is welded to the
struts, and from this are taken the bracing wires. As the welded joints
impart a certain rigidity to the structure, the fact that the wires
are exerting a side pull on the struts may be of little consequence,
although this method could hardly be used in conjunction with the
fuselage construction of average English machines. A rather unusual
feature may be noticed in the attachment of the bracing wires, which
are not finished off with the orthodox wire ferrule, but are arranged
as a loop, the turnbuckle forming the anchorage for the two ends.
The trend of design in this country seems to incline towards the clip
stamped out from sheet steel and bent up. This class of fitting can be
produced accurately and quickly, and, in the writer’s opinion, is by
far the best manufacturing proposition. Aluminium castings are quite
obsolete, and the built-up fitting, involving welding or brazing, does
not seem greatly in vogue.




CHAPTER X.

UNDERCARRIAGE TYPES.


The present chapter deals with the general arrangement of the different
types of undercarriages, as distinct from the details of construction.
The principles of design embodied in the undercarriage are necessarily
a compromise, this position being due to the fact that its construction
has to be considered from two distinctly opposed view-points, and
undue attention to the requirements of either does not produce the
best results. Thus, on the one hand, we have the desirability of great
strength to withstand landings on very rough ground, ploughed fields,
and the like; and on the other hand, we have the considerations of
aerodynamical efficiency in flight, which, taken to one extreme, would
be best satisfied if the undercarriage did not exist, and at most calls
for a system in which the head resistance is brought to an irreducible
minimum. By the ordinary process of evolution the agglomeration of
ideas existing in the early days of flying with regard to the most
suitable form of landing gear, have given place to something which,
for machines of modern attainments, approaches finality. This has
resulted from improvements along the line of (1) simplification of
general design, (2) the reduction of head resistance and weight without
a consequent diminution in its powers as an alighting gear. A better
impression of the distinguishing points of the various types will be
gathered if we consider the desiderata of an ideal undercarriage.


Principles of Design.

One of the most important points is that rolling shocks should be
completely absorbed, and the least possible strain transmitted to
the fuselage or main structure, this calling for a good system of
wheel suspension. It must be capable of standing the considerable
strains sustained in alighting, not the least of which are those
attendant upon landing in a side wind; should offer the least possible
head resistance, while the weight must be reduced to a minimum.
Cross-country flying, which more often than not means “getting off” in
a restricted space, requires that the machine shall attain flying speed
in the shortest time, and conversely in alighting the machine should
come to rest in the quickest time. Innumerable smashes have been caused
after a perfectly good landing by failure to pull up before a hedge,
fence, or ditch. These are the main principles involved, and at least
they indicate how and why the undercarriage is necessarily a compromise.

[Illustration: FIG. 85.]

It is clear that in landing the speed of the machine relative to
the ground should be as low as possible, without developing into
the operation generally known as “pancaking,” or stalling, and the
usual method of accomplishing this is to bring the machine into the
wind, which, if of a moderate velocity, materially reduces the speed
relative to the earth. In ordinary circumstances, landing would be
accomplished by gradually increasing the angle of incidence until the
maximum, or angle of no lift, is reached, which is practically stalling
point. To satisfy this consideration, the heights of the main rolling
wheels and tail skid should be arranged to allow the wings to lie at
an angle a little in excess of this. With modern wing sections the
angle of maximum lift is between 14° and 16°, so that the angle of
18°, as shown in Fig. 85, is usually sufficient. This has additional
value in restricting the length of run after contact with the ground,
the wings acting as air-brakes. It will be realized that reduction
in height of the undercarriage, desirable as it is from the aspect
of head resistance, cannot be carried beyond a certain point without
the sacrifice to some extent of the foregoing qualities. So far we
have taken the principles of design as affecting the disposition
of the undercarriage members in a longitudinal direction, but, of
course, there are several details to be considered in its arrangement
laterally. A fundamental point is that the track of the wheels, _i.e._
the distance, centre to centre, should be of ample width, but several
constructional difficulties tend to restrict this to certain limits.
Where the undercarriage is of the type in which the main rolling
wheels are mounted on a single axle, it is clear that the wheel base
is limited to the greatest length the steel or duralumin tube can be
used without buckling under landing shocks. If this is to be exceeded a
bigger diameter tube of thicker gauge will be necessary, and this means
additional weight. Again, the fuselage width for the tractor machines
now in vogue does not greatly exceed 3 ft., being more usually under
that figure, so that a very wide base would mean raking the struts
at a flat angle, which would therefore require to be made of larger
section than would be the case if the wheel base was narrower; or, if
the same section strut is used, the strength is reduced. A wide wheel
base therefore means an undesirable increase in weight and resistance.
To make up for the deficiencies of the almost unavoidable narrow wheel
base, it is usual to make use of the wing tips by fitting skids of
malacca cane or laminated ash, which are brought into action when the
machine is excessively canted over sideways. At one time the wing tips
were almost invariably used to assist the undercarriage, the wing tips
of the Nieuport monoplane being specially constructed for the purpose,
and no skids were fitted. Earlier still the R.E.P. monoplane had only
one central rolling wheel, a smaller wheel being attached to each
wing tip. The wing tip wheels of the Cody biplane performed similar
functions, although these were used in conjunction with two main
wheels.


Undercarriage Types.

The type of landing gear in use to-day does not vary in principle to
any great extent, the differences usually occurring in the choice
of material, the system being that usually known as the Vee type,
from the fact that viewed in side elevation, the struts form a
V. While this type has much to commend it from the points of low
head resistance and great strength for weight, there are other
systems, some of which have been tried-out, while others still
exist, incorporating features designed for some specific purpose. Of
these the Farman type is an example of a landing gear designed for
the requirements of school work, consisting of two long ash skids,
which, extended from the rear end of the nacelle, being gradually
bent upwards to carry the front elevator. This was the arrangement
on the “Longhorn” machine, but on the “Shorthorn,” produced at

[Illustration: FIG. 86.--Side view of Farman landing gear.]

a later date, the skids, as shown by Fig. 86, terminated in short
bends. Each skid carried a pair of rolling wheels, attached to a
short axle, this being bound to the skids by rubber bands. The
wheel base being almost 9 ft., this type gave excellent results.
In the case of big machines, where it is desired to keep the load
on the tail skid as light as possible, three wheels are sometimes
used, two main rolling wheels and a light pilot wheel in the front.
This enables the main rolling wheels to be placed under the centre
of gravity, the pilot wheel preventing the consequent tendency to
pitch forward when rolling. A further development of this system
dispenses with the tail skid, two main wheels being placed under
the centre of gravity, and two smaller wheels a little forward of

[Illustration: FIG. 87.--Side view of four-wheeled landing gear.]

the propeller, as in Fig. 87. The skids were sometimes continued back
behind the rear struts, and saw-kerfed to increase the resiliency.
The base of support was formed by the rear wheels and the ends of the
skids, the machine being pulled on to the front wheels by the thrust
of the propeller. The short wheel base is bad for rolling on bumpy
ground, and frequent skid replacements are necessary with this system.
A similar type with no tail skid has the wheels disposed forward of the
C.G., while a single central skid, connected to the fuselage by

[Illustration: FIG. 88.--Side view of Nieuport undercarriage.]

a series of V struts, replaced the double skids, as in Fig. 88. This
type was used on the original Nieuport monoplane, and with minor
modifications on the Avro 80 h.p. Gnome tractor biplane. Its chief
advantage is low head resistance, but unfortunately with this system
a narrow wheel base, with the attendant defects, is inevitable. A
very distinctive system was that favoured by Bleriot, and used with
minor alterations on all the Bleriot monoplanes. This is shown,
diagrammatically, in side elevation, by Fig. 89, and was unusual in
that the wheels were arranged to swivel, this being an attempt to
counteract the side strains set up when landing in a side wind.
Although in the hands of some of our most famous exhibition pilots this
has functioned excellently, it is complicated and somewhat heavy.

[Illustration: FIG. 89.--Bleriot undercarriage.]


Recent Developments.

During the last three years the vital necessity of speed and climb, and
more speed and climb, has resulted in the gradual elimination of skids,
struts, and wires, until to-day the chassis for machines of average
dimensions is almost invariably the V type (Fig. 90). The wheels are
placed about a foot in front of the C.G., as, owing to the absence of
any forward skid, no other provision exists to counteract the tendency
to pitch over. In the actual construction of the Vee undercarriage,
some diversity of practice exists with regard to the material chosen.
In some cases the struts forming the Vees are constructed of a
streamline section steel tubing, in others round tubing, the streamline
section being obtained by a wooden fairing bound on, while a number of
constructors use wood for the struts.

[Illustration: FIG. 90.--Vee type undercarriage.]




CHAPTER XI.

UNDERCARRIAGE DETAILS.


The details of construction associated with the undercarriage are
those concerned with the forming of the struts and main members, and
the suspension of the axle. As noted in the previous chapter the Vee
undercarriage is greatly in favour at present, but the fact that with
this type no forward support exists to prevent pitching over when
obstructions are met in rolling, will almost certainly result in
some arrangement of wheels and skids for the touring machines of the
post-war period. Machines are now designed for air performance pure and
simple, so that an undercarriage of the simple Vee type is all that is
permissible; but in the post-war machine general utility will be the
desideratum sought for by designers. At one time the majority of the
undercarriage arrangements incorporated one or more skids. The material
most suited for this purpose is hickory, although some designers prefer
ash, steamed to the desired curve, and generally channelled out between
the points of intersection of the struts, fittings, etc., in a similar
manner to longerons and wing spars.

Where the bend is sharp, and therefore difficult to obtain by steaming,
it is usual to form the skid from a number of strips, or laminations,
glued together. Quite a good method of stream-lining the curved toe of
the skid is shown by Fig. 91, consisting of a spruce block attached to
the skid by screws, and it has additional value in ensuring permanency
of curve. Where the design is such that the rear end of the skid
performs the functions of a tail skid it is saw-kerfed, as in Fig.
92, the laminations so formed being stepped back, and the bottom
layer shod with a plate, or claw fitting, acting as a brake, and also
preventing wear produced by contact with the ground. At one time this
constituted popular practice, but it is a matter of some difficulty
to prevent the saw-cuts from developing into fractures. As a matter
of fact, on one type of machine replacements were so frequent that
eventually the skid end was left solid.

[Illustration: FIG. 91.--Streamlining curved toe of skid.]

[Illustration: FIG. 92.--Laminated skid end.]


Methods of Suspension.

In the preliminaries of design referred to in the last chapter, it was
observed that the action of rolling and alighting called for a good
system of suspension and shock absorption, and this is accomplished
on modern machines by binding the axle to the main members of the
structure with either rubber cord (this being a number of strands
of rubber about 1/16 in. square, compressed and bound together with
a woven twine casing) or plain rubber rings. The latter are more or
less obsolescent, at least in this country, the reason being found in
the better lasting qualities of the cord, which will also withstand a
much higher ultimate stress, the fabric covering contributing largely
to this. In a number of cases, and generally for heavy machines,
steel helical springs are fitted. Various attempts right from the
beginning of successful flight have been made to utilize steel springs
for suspension, but hitherto very few machines have successfully
incorporated them, and but a brief examination will show that their
use on machines of the average modern type is attended with some
unsatisfactory features. Firstly, they are much heavier than rubber,
but this in itself is no great disadvantage, as ease of attachment
probably compensates for this; but what is of moment is the fact that
steel springs are not nearly so efficient _shock-absorbers_ as the
rubber variety, while even the efficiency of the latter is capable of
considerable improvement. If we take the case of a machine rolling
over bumpy ground, all that is required of the suspension is that
the wheel movement over the inequalities shall not be transmitted to
the whole machine. So far both steel springs and rubber cord satisfy
these conditions, but in the operation of alighting the machine not
infrequently strikes the ground with some force, sometimes the result
of gusts or pancaking. With steel springs, and to a lesser degree
those of rubber, the energy of landing is not absorbed, but is stored
up, being given out again in the form of a rebound. With rubber,
elongation and its consequent depreciation of ultimate tensile strength
prevents any energy of moment being returned to the aeroplane, which is
why, for light machines of modern design, say, up to 2500 lbs. total
weight, rubber is the better material. Steel springs being deficient
in the power to damp out shocks, it becomes necessary to use these
in conjunction with some other medium possessing this quality, and
one of the most suitable arrangements extant is that known as the
oleo-pneumatic gear, consisting of a combination of helical coil spring
and oil plunger. It is usual to arrange the main compression members in
two halves, the upper half forming a piston, and the lower, attached to
the wheels, constituting the cylinder, is filled with oil. The weight
of the machine is taken normally during rolling by the helical spring,
wound round the upper half of the telescopic tube. Excessive shocks
cause the oil to be forced through a spring valve, adjusted to open at
a certain pressure, into the upper half, a back-pressure valve enabling
the oil to gradually return to the cylinder. The Breguet biplane, a
pre-war machine of original design, embodied in the undercarriage
arrangement a system analogous to the foregoing.


Shock Absorbing Effect of Tyres.

The assistance rendered by tyres of large diameter must not be
overlooked. The merits of the large tread are quite well known in
the sphere of the motor-car, and they are no less beneficial to the
aeroplane. It is of interest to record that a pre-war racing machine
had no other suspension and shock-absorbing medium than that provided
by the very large tyres fitted to the wheels, the axle being fixed
rigidly to the undercarriage struts. A similar arrangement existed on
a machine of much more recent date. One does not advocate this system,
as it can be of very little use for rough ground, the instance being
cited to emphasize the assistance so rendered to the ordinary type of
suspension.


Connections.

Various methods exist for connecting the rubber to the main members, a
typical arrangement with the Vee undercarriage of steel being shown by

[Illustration: FIG. 93.--Arrangement of suspension on steel tube
undercarriage.]

Fig. 93, and a variation of this, when wood is the material,
is indicated by Fig. 94. The web plate in Fig. 93 forms
a means for guiding the axle in its upward travel, and is
another version of the one-time popular

[Illustration: FIG. 94.--Arrangement of suspension on wood
undercarriage.]

radius rod. It is not considered necessary, in many instances, to
fit either web plate or radius rod, the movement of the axle

[Illustration: FIG. 95.--Farman type axle suspension.]

being of no great extent. Another system is shown by Fig. 95,
this being the method of suspension adopted for the Farman

[Illustration: FIG. 96.--Rubber cord suspension.]

machines. In this case rubber bands are attached to the main skids, the
short axle passing between the two. A similar arrangement in general
outline is shown by Fig. 96, although in this case the rubber takes the
form of cord.

[Illustration: FIG. 97.--Bridge type suspension.]

A method greatly in vogue in America is that indicated by Fig. 97,
known as the bridge type, and a characteristic Wright detail, the rings
being approximately two inches wide by two inches long. The fact that
very few examples of this system exist in this country may be ascribed
to the inferiority of rubber bands compared with the rubber cable.


Axle Fairings.

[Illustration: FIG. 98.--Axle fairing.]

It is now the practice to streamline the compression tubes between the
vees of the undercarriage with a fairing of aluminium or three-ply.
This is so arranged that in flight the axle lies in a slot formed in
the fairing, which appreciably reduces head resistance. A typical
arrangement is indicated by Fig. 98. The axle is usually formed
of steel or duralumin tube, and in the majority of undercarriage
arrangements is divided and hinged in the centre, a wire or wires from
this point to the fuselage accounting for any strain. Duralumin tube is
especially suited for this item, as a much stiffer axle is possible for
a given weight, although, unfortunately, this is slightly discounted by
the fact that duralumin does not form a good bearing surface for the
wheel hubs, and it therefore becomes necessary to fit either sleeves or
stub-axles of steel.


Undercarriage Brakes.

Additional means for restricting the length of travel after contact
with the ground is sometimes found in the employment of brakes of
various types. A very simple and widely used arrangement is to
terminate the tail skid in a claw fitting, as Fig. 99, so that in
alighting the tail is shoved hard down, bringing the skid into contact
with the ground. The disadvantage is that undesirable strains may be
carried to the fuselage members.

[Illustration: FIG. 99.--Tail skid with claw fitting.]

Another version recently patented is to construct small planes to
conform to the wing curve, and hinged so that by a system of wires and
pulleys, actuated from the pilot’s seat, they could be adjusted to
offer a normal surface to the direction of flight. The efficiency of
this arrangement at low speeds is not very great, moreover a landing
with the wind renders them quite useless. The best form of brake is
undoubtedly one acting direct on the main wheels, either of the rim or
band type, a good example of the latter being the system used on the 70
h.p. Bristol biplane. Closely allied to the question of brakes is that
of steering, and the requirements of this latter item are fairly well
satisfied by pivoting the tail skid and working it in conjunction with
the rudder from the foot-bar or wheel.


Housing of Undercarriage during Flight.

Numerous suggestions, ideas, and patents exist, having as their object
the housing of the undercarriage in the fuselage during flight, with
a resultant reduction in resistance; and excellent as the principle
is, its practical application is difficult of achievement--at least,
for machines of the present. In flight the undercarriage is a useless
encumbrance, adding weight and head resistance, so that an arrangement
whereby this component could be folded into the main structure would
apparently effect a saving in resistance. This would mean that the
fuselage would be of larger cross-sectional area, the natural sequence
being extra weight and resistance. It does not appear that the saving
effected in resistance, when the undercarriage is folded during flight,
would account for the additional weight of the operating mechanism and
the increased head resistance of the fuselage, so that altogether the
advantages of any so-called disappearing landing gear are very much
more apparent than real. There is also the very great possibility of
the undercarriage folding up or disappearing when it would be least
required to do so. In the construction of the problematic air-liners of
the future it may be possible to economically effect the housing of the
undercarriage.




CHAPTER XII.

CONTROL SYSTEMS.


The mechanism by which the aeroplane is controlled in flight forms
the connecting link between the pilot and machine, and constitutes
a vitally important and somewhat vulnerable item of the complete
structure.


Main Principles.

The control of all modern aeroplanes is effected in a lateral direction
by small planes hinged to the rear spar of the outer ends of the
wings, and known as “ailerons”; in a longitudinal or “fore-and-aft”
direction by the elevator planes; and for steering by the rudder.
Although these functions are alluded to separately, they are more
often than not combined in their actions. The correct proportion of
the controlling surfaces is an important factor in determining the
ease or otherwise with which a machine can be handled in flight,
and faults in this direction are responsible for the terms “heavy”
or “stiff” on the controls being applied to a machine. The use of
subsidiary flaps or ailerons for lateral control is a comparatively
modern innovation. At one time it was usual to warp the entire plane,
or in some cases the outer section only, and although the principle
is the same--that of forming a negative or positive surface to the
line of flight--structural considerations are wholly in favour of
ailerons. With warping, the whole plane is subjected to continuous
torsional movement, and to obtain this some of the trussing wires have
necessarily to be arranged as control wires, the result being that the
plane curvature loses its uniformity, and the whole girder system
of the planes is less efficient under load than if the wires were
permanently fixed; and the latter item is only possible with aileron
control. Although it is usual to attach ailerons to both top and bottom
planes of a biplane, there are occasions when sufficient control can be
obtained with ailerons to the upper plane only, usually when the span
of this plane is greater than that of the bottom.


Control by Inherent Stability.

With machines of the inherent stability class the lateral control
is effected by additional means, the planes being designed to
automatically right the effects of gusts. This element of inherent
stability is obtained by suitably grading the camber and incidence of
the wings, until at the wing tips the chord of the plane section forms
a negative angle to the line of flight. Although this arrangement is
undoubtedly of value, especially for the touring machine of moderate
power, its chief fault lies in the relatively slow righting movements,
which, although of no great consequence at a reasonable altitude,
becomes a source of danger when alighting, and certainly entail the use
of ailerons, or warp, to counteract it. The type was well exemplified
in this country by the Handley-Page monoplane and biplane, while in
Germany it achieved great popularity, surviving in some makes until
the latter part of 1916. In the matter of control-surface design it is
interesting to note the contrast between the preferences of English
and German designers. In almost all German machines the ailerons,
elevators, and rudder are balanced, _i.e._ surface is disposed each
side of the hinge-axis, this applying to the small Albatross scouts
and to the large machines of the Gotha class; while in this country
few examples of this practice occur. The reason for the balancing of
controls lies in the desire to reduce the manual strain on the pilot to
a minimum; and it appears that with large machines balanced surfaces
will be imperative. Several automatic controls have been produced,
the most notable perhaps being the Sperry gyroscopic, this being a
combination of servo-motor and gyroscope. This apparatus has been well
tried.

So far as the arrangement of the control surfaces is concerned, little
variation occurs, which condition has obtained from the early days of
aviation, but the mechanism governing or directing these movements
varied at one time considerably, and although in this country one type
of control is used, there are still instances of the use of widely
different systems. In former days the practice of individual makers
fitting different controls resulted in some arrangements being in exact
contradistinction to others, which not infrequently meant, to a pilot
taking on a new type, the unlearning of a great deal which practice had
rendered instinctive.


The Instinctive Principle.

All modern controls are based on the instinctive principle, _i.e._
the movements of the control lever coincide in direction with the
promptings of natural instinct. Thus, to change the course of a
machine flying level into an upward one, the column is pulled towards
the pilot, and for descent, the reverse, while to correct a bank,
the column is moved in a direction opposed to that of the bank. For
steering, a foot-bar is employed, so arranged that for a turn to the
left the left foot is pushed forward, and the reverse for a right turn.
On one well known machine of former days, the foot-bar actuated the
lateral control, which is sufficient indication of the great diversity
of opinion then existing.


Vertical Column Control.

[Illustration: FIG. 100.--Arrangement of vertical column control.]

A typical control of the immensely popular “joy-stick” type is shown
by Fig. 100. This consists of a vertical column pivoted through the
medium of a fork-joint to a rocking shaft. The elevator wires are
taken round pulleys mounted under the seat, and the aileron wires
from a form of bell-crank, flanged and welded to the steel tube. A
disadvantage with this system, in addition to the complication of the
wires, is that lateral movement also affects the elevator, although
the extent of this is of no great moment. It is obvious, although
somewhat paradoxical, that if the elevator is to be depressed by a
forward movement of the column, the control wires will required to
be crossed, _i.e._ the wire running from the base of the tube to the
pulleys will be attached to the arm on the top side of the elevator,
and _vice versâ_. On single-seater machines it is sometimes necessary
for the pilot to have both hands free of the controls, so that it
becomes necessary to install some form of locking device for the
elevator control, there being many simple ways of accomplishing this.
The locking of the control lever fixes the flight path of the machine,
but, of course, lateral equilibrium can be maintained by movements of
the lever sideways, and steering by the rudder bar. The German machines
of the Fokker and Albatross types are both fitted with the single lever
control with a locking arrangement. Another method which achieves the
same purpose consists of bracing the lever in a normal flying position,
with rubber cable or coil springs anchored to various parts of the
fuselage, and although this permits of movement, the control column
always tends to return to the normal position.


Wheel Controls.

[Illustration: FIG. 101.--Arrangement of wheel control.]

While the “joy-stick” type of control is greatly in favour, there are
various forms of wheel control in use. American machines are almost
entirely fitted with wheel controls, and all things considered, it
appears that modern practice is evenly divided between the two types.
The sequence of movements of the wheel type may be varied in a number
of ways, the general arrangement shown by Fig. 101 being typical of an
average system. In this case the hand-wheel is mounted on a central
column, which in turn is rigidly fixed by some form of Tee joint to
a transverse rocking shaft. A sprocket attached to the wheel centre
engages with a short length of chain, which connects to the aileron
control, while the elevator wires are connected to short tillers,
arranged to work on the outer side of the fuselage. With this system
the hand-wheel is rotated for the aileron movements, a fore-and-aft
rocking motion for the elevation, and the rudder is actuated by an
outward movement, with either foot on the rudder bar. A development
designated “three in one” embodies all these movements in the wheel
column, which in this case is pivoted at its base: a to-and-fro motion
in the column for the elevators, sideways for the ailerons, while the
rudder control is effected by the rotation of the wheel. This system is
fitted to a number of American machines, but it is a moot point whether
the rotation of the wheel for warping or steering is quite such an
instinctive action, as the sideways movement of the lever combined with
the movements of the foot on the rudder-bar; in any case, there is just
a suspicion of complication in its working which is undesirable, that
is, for machines intended for popular use.


The “Dep” Control.

[Illustration: FIG. 102.--“Dep” type control.]

The type of control used on the Deperdussin monoplanes of 1910 and
onwards has survived until the present day, and forms a distinctive
arrangement. Its chief attribute is that, compared with other systems,
much greater room and freedom is afforded the pilot, which is evident
by a consideration of the diagrammatic sketch, Fig. 102. The inverted
U-shaped lever is composed of either ash, bent to shape, or steel, or
duralumin tube, the general system of its working being the same as the
wheel control shown by Fig. 101. Incidentally, passing reference may
be made to the fact that the usual close proximity of the compass to
the controls precludes the use of steel in any great quantity for the
construction of the lever, as the various movements adversely affect
the compass readings.


The Wright System.

Another variant of the wheel control is instanced by the Wright system,
this consisting of a general lay out similar to that shown by Fig.
101, but no rudder-bar is fitted. The rudder control is provided by a
small lever, mounted concentric with the wheel, the latter carrying
a rigidly attached sprocket. The hand-lever is also connected to a
sprocket, this running free on the wheel shaft, so that by gripping
both hand-lever and wheel it is possible to operate the ailerons and
rudder simultaneously, this action being a characteristic feature of
all the Wright productions. Although there are many types of control
in use, those described in the foregoing chapter are illustrative of
general practice.




CHAPTER XIII.

WIRES AND CONNECTIONS.


In all aeroplanes the question of wires and the terminal connections
associated therewith is a matter of some importance, and while this may
vary in degree, there is little doubt that the efficiency of modern
wiring systems is largely responsible for the structural efficiency of
the aeroplane as a whole.

Aeroplane construction consists almost exclusively of a framework of
wood braced by wires, a condition of things which has obtained since
the inception of flight; as may be judged by the various engravings
of Henson’s projected monoplane of 1842. This machine incorporated
an arrangement of king-posts and wires approximating very closely to
modern practice, and the natural sequence of improvements have tended
towards the gradual elimination of exposed wiring.


Various Wires used.

The various wires used in construction may be classified into four
distinct types: the solid wire stay, the straining cord or cable used
for stay wires, the extra flexible cable used for controls, and the
swaged tie rods in plain or streamline form. The earliest form of
bracing was of the solid piano wire variety, this having been used
on most aeroplanes from the days of the Wrights onward. From the
view-point of the early pioneers, this wire was eminently satisfactory,
being cheap (a vital consideration) and simple to attach and replace.
Although the tensile strength of this wire cannot probably be excelled,
its hardness renders somewhat difficult the forming of the end loop
without fracture of the wire. For this reason piano wire gradually
gave place to a softer grade of wire which, while being strong, was
tough and ductile, enabling bends to be made with a lesser danger of
fracture. The original connection used for the piano wire stay is shown
by Fig. 103, this consisting of a loop or eye, the free end being
turned round a ferrule of soft copper tube, this being sometimes varied
by the use of a flat strip of tinned iron, wrapped round and soldered.
While this was fairly satisfactory for short stays, it was hardly
suitable for the main lift wires of the interplane bracing, owing to
the comparative ease with which, under load, the free end pulled or
cut through the ferrule, so that after a while the oval spring-wire
ferrule, Fig. 104, came into use. This is made of the same gauge wire
as the stay, and is from seven to nine convolutions in length. The eye
should be formed as an easy bend, and not kinked, the ferrule being
pushed tight against the shoulders, and the free end turned back.


Result of Tests.

Tests undertaken at the instance of the American Advisory Committee
for Aeronautics showed that 80 per cent. of the wires tested failed by
the free end pulling through the ferrule, the remaining 20 per cent.
failing by fracture, the stays possessing an average efficiency of
68 per cent. of the maximum strength of the wire. Although various
modifications, such as tying the free end to the ferrule with fine
wire, as in Fig. 105, resulted in an increase in total efficiency,
average European practice consists of that shown by Fig. 104. At the
present time the solid wire stay of the form dealt with is used mainly
for the bracing of the fuselage frame, and the internal wiring of the
tail planes.


Stranded Cable.

The gradual increase in engine power and total weight of aeroplanes led
to the adoption of stranded cable for all important loaded wires, this
being made in two distinct ways.

The cable employed for interplane bracing is composed of a number of
fine wires, varying from nineteen to thirty-seven according to the
different diameters, the end section being indicated by Fig. 106.

[Illustration: FIGS. 103–112.--Methods of forming wire connections.]

Where extra flexibility is required, such as for control wires running
round pulleys, the cable is composed of a number of strands, generally
seven, which in turn consists of a number of fine wires, usually
nineteen, the end section being shown by Fig. 107. English practice
designates this form of cable as extra flexible, and the single rope
of nineteen wires as straining cord. American classification is
practically the reverse, in that the single rope is known as stranded
cable, and the multi-strand as cord. Although the factor of strength
is an important one it does not entirely govern the selection of a
wire, as other considerations, such as flexibility and fatigue strain,
influence greatly the efficiency of a stay under active service
conditions. Under test the solid wire possesses the greatest ultimate
breaking weight, the next best being the single rope. It must be
understood that in flight a wire is subjected to constant and intensive
vibration, which must have a deleterious effect on the material, and
for this reason a flaw or slight fracture in a solid wire may escape
notice until complete failure in the air; whereas the cable, by the
unstranding of the damaged wires, would give warning of wear. Chiefly
owing to the difficulty of forming a satisfactory splice in the
single-strand cable, modern practice inclines toward the use of the
multi-strand cable for all purposes, as the construction of this wire
lends itself to the forming of a successful splice.


Cable Connections.

The earliest form of terminal connection for stranded cable consisted
of a loop, the free end being bound to the main part of the wire and
soldered. With the addition of a binding or serving of wire round the
loop to prevent injury, due to contact with the wiring lug, or strainer
eye, this wire, in a recent test, gave an efficiency of 100 per cent.
for all diameters up to ¼ in.

This result, considering the elementary nature of the joint, is
surprising. Unfortunately the effect of corrosion due to acid and
solder is a somewhat doubtful quantity; moreover, the appearance of
the joint is far from neat. An attachment which at one time achieved
some popularity is shown by Fig. 108, and is especially suitable for
the single-strand wire. This consists of a cone-shaped forked end with
a taper hole, into which the cable is inserted, the free end being
unstranded, spread out and soldered. The attachment has been used on
what was at one time one of our best products. The efficiency obtained
with this fitting is in the neighbourhood of 100 per cent.

In the method indicated by Fig. 109 a piece of flat copper tube is
passed over the wire, the free end of the latter being bent round a
brass thimble, and then passed through the copper tube, in a similar
manner to the connection for the solid wire in Fig. 102. The tube is
then given several turns, and the complete joint well soldered. This
system is reliable, and has given good results.

A distinctive terminal is indicated by Fig. 110, consisting of a brass
ferrule just sufficiently wide to accommodate the two thicknesses of
wire. The bolts are of the counter-sunk head variety, so that the
operation of screwing a bolt home also forces the wires into the
protuberances in the sides of the ferrule. Although the foregoing
methods have all been extensively used, they have now given place to
the thimble splice, Fig. 111, which, as a general proposition, is
undoubtedly the better terminal connection. The brass thimble protects
the strands from the wearing effect produced by contact with the
turnbuckle or wiring lug. It is the usual practice to wrap the splice
with a binding or serving of fine copper wire, or waxed twine. The
efficiency of this joint with a properly made splice may be safely
taken as 85 per cent. of the total strength of the wire. With this
joint the point of failure, as evidenced by numerous tests, always
occurs at, or near, the last tuck in the splice, at which point the
extra thickness of the splice is just merging into the normal thickness
of the wire. The disadvantage with all terminal connections which
necessitate the use of solder is the impossibility of determining just
how much the heating operation affects the strength of the wire, and
also the effects of corrosion, set up by the various species of flux
used in the process of soldering.


Relative Strengths.

For a given diameter the solid-wire stay possesses the greatest
strength, the next best being the single-stranded cable, as the
following comparison of stay strength, taken from the Report of the
National Advisory Committee for Aeronautics, 1915, of America, will
show:--

  --------------------+-----------+-------------+---------
       Material.      | Diameter. | Strength of | Strength
                      |           |  material.  | of stay.
  --------------------+-----------+-------------+---------
                      |   inch.   |     lbs.    |    lbs.
  Wire, solid         |   3/16    |     5500    |    5100
  Strand, single      |   3/16    |     4600    |    4100
  7 × 19 multi-strand |   3/16    |     4200    |    3500
  --------------------+-----------+-------------+---------


Streamline Wires.

Although in the quest for increased speed the number of exposed wires
were reduced to a minimum, the aggregate resistance still remained
considerable, this leading to the development of the swaged streamline
wire, the introduction of which is generally ascribed to the Royal
Aircraft Factory; and these wires are now generally used for all
exposed wiring. The points in favour of them are that, properly fitted,
a considerable reduction in resistance is obtained, there is a lessened
liability to slacken after some use, this rendering rigging a more
certain operation, and the nature of its connection obviates the use of
turnbuckles.

[Illustration: FIGS. 113–116.--Methods of forming wire connections.]

They have been variously criticized as being expensive to produce, that
the resistance may be increased if improperly aligned in the machine,
and also that any fracture or flaw is less liable to be detected before
complete failure during flight. In manufacture the solid rod is rolled
to the section shown by Fig. 112, a certain length each end being left
for the right- and left-hand thread. Two of the connections mostly used
are shown by Figs. 113 and 114, the latter being preferable, as the
universal joint permits of movement in two directions, which reduces
the tendency of the wire to crystallize as a result of excessive
vibration. To prevent wear at the points of intersection it is usual to
fit acorns of fibre or aluminium, a popular form being shown by Fig.
115. Some designers still prefer to use the wire cable for interplane
bracing, a fairing of wood being bound to the cables by tape at
intervals, this also preventing excessive vibration.

Some years ago various attempts were made, mostly on French monoplanes,
to utilize flat steel ribbon for exposed wiring, but, owing to the
difficulty of successfully forming a terminal, its use never became
extensive, although it may possibly be regarded as the precursor of the
modern streamline wire.

It is notable that, so far, the wiring of all German aeroplanes is
effected by cable, so that apparently the merits of the streamline wire
are not recognized. It is also surprising that no attempt has been made
to streamline the cable. A device for tying the wires and preventing
friction at the point of intersection, found on nearly all enemy
aeroplanes, is indicated by Fig. 116, and there are also instances of
quick release devices, these being popular in this country about 1912,
and now obsolete.

Although determined attempts have been made of late to entirely
eliminate exposed wiring, examples of this occurring in the recent
German Fokker triplane, it appears that the various alterations
engendered by this procedure in the structure of the machine more than
counteract the saving in head resistance.

Moreover, with modern methods of construction the ultimate strength of
a wireless wing structure leaves considerable room for improvement, and
the price paid for the saving is too great.

The arrival of the all-steel aeroplane would entirely alter the
condition of things, as with this construction much better chances
exist for the production of a reasonably strong wing structure without
exposed wing bracing.




CHAPTER XIV.

ENGINE MOUNTINGS.


The mounting of the engine and the general arrangement constitute
one of the most important and interesting sections of aeroplane
construction, and perhaps a brief outline of the various engines in
use will suitably preface a consideration of the mountings of the
different types. Although there are signs that certain revolutionary
engines may eventually come into use, the types in use on modern
aircraft are the stationary air and water-cooled, the radial air and
water-cooled, and the air-cooled rotary. The greater variety occurs
with the stationary type of engine, which may be sub-divided into those
in which the cylinders are arranged vertically in line, and those where
the cylinders viewed from the front form a =V=. Engines typical of
the former class are the Beardmore, Green, Mercedes, and Benz, all of
which are water-cooled; and of the latter class, Rolls-Royce, Sunbeam,
Hispano-Suiza water-cooled, and the Renault and R.A.F. air-cooled. The
types of radial engines which have been extensively used are confined
to two, these being the air-cooled Anzani and the water-cooled Salmson.
There is another radial engine of comparatively recent production; but
mention of this while present conditions obtain is not permissible.
Rotary engines of note are the Gnome, Le Rhone, and Clerget, all of
which are necessarily air-cooled.


Essential Requirements of an Engine Mounting.

The essential features of any mounting are absolute rigidity,
accessibility to permit ease of erection and dismounting; and it
should also be of a moderately low weight. Moreover, the general
arrangement must offer a minimum of head resistance, although in this
direction the type of engine used is a determining factor. Rigidity is
a paramount consideration, for the slightest tendency to slackness or
“play,” under the effect of engine vibration, speedily develops, until
either serious stresses are induced in the fore part of the fuselage or
the engine loses its correct alignment, with a consequent detrimental
effect on the flying qualities of the aeroplane. This, of course,
should be provided against in the general design; but it is also a
contingency which should be kept in mind during the actual construction
of the various components of the complete mounting.

A detail which does not always receive sufficient attention is the
provision of adequate bracing against the thrust of the engine. Where
the construction is such that the engine-bearers form an integral part
of the fuselage structure, there is generally little fault to find, but
with some sheet steel mountings, particularly those employed for the
rotary type of engine, the only bracing in a fore-and-aft direction
is that provided by the flanged edges of the plate, which are usually
much too narrow to be of real use. Further, the construction of both
the engine mounting and the fore part of the fuselage should be of the
necessary strength to ensure that the bearers supporting the engine
are always correctly in alignment and dead level. With some methods
of construction the weight of the engine and various landing shocks,
result after a time in the lowering of the bearers at the front, which
means that the angle of thrust is not in its correct position relative
to the centre of gravity and the incidence of the wings, this being
extremely detrimental to the flying properties of the machine.


Materials.

Wood, on account of its property of absorbing vibration, is
particularly well suited for the construction of the engine mounting,
and one finds examples of its use in a variety of ways. Perhaps
the most common form is that in which a bearer of ash or spruce,
channelled out between the fastening-down bolt holes for lightness,
is attached to steel brackets which in turn are bolted to the various
fuselage members. A development of this method consists of mounting
the bearers on either multi-ply formers of wood, or built-up wood
brackets stiffened with a three-ply covering on each side, and both
of these arrangements are being extensively used. Sheet steel is used
for the mountings of various machines, but it does not possess the
characteristic of absorbing vibration. In some instances one finds that
the engine has been specifically designed to be supported on bearers
of the tubular variety, in either steel or duralumin; but here again
rigidity is difficult of attainment. Although the use of welding,
that is to any extent, is not advisable in the construction of the
engine mounting, one finds this process very extensively used for the
mountings of some modern machines. In one particular instance, the
tubular bearers are supported from the steel tube fuselage by various
tubes, the whole structure being welded, and although every joint
successfully survived a smash which resulted in a considerable bending
and distortion of the fuselage, its use does not engender a sense of
security or reliability.


Rotary Engine Mountings.

[Illustration: FIG. 117.--Rotary engine mounting, in which engine is
supported between two plates.]

The mountings associated with the rotary type of engine fall under two
categories: those where the motor is supported between two or more
plates, and those in which the motor itself is overhung. The method
of mounting adopted for the first case is generally the type shown
by Fig. 117. The plates are pressed or bent up from sheet steel, and
all edges flanged to prevent buckling. The front plate embodies a ball
race, through which the propeller shaft runs, while to the rear bearer
is bolted the back plate of the engine. This arrangement with minor
variations has been extensively used for the different makes of small
scouting biplanes engined with the 80 h.p. and 100 h.p. Gnome motors.

Where the weight of the rotary engine used is excessive, as in the case
of the 160 h.p. Gnome with 20 cylinders, which is now out of date, a
mounting incorporating three bearers is used. The arrangement would be
similar to that indicated by Fig. 117, with the addition of an extra
bearer for the support of the crank-shaft extension.


Overhung Mounting.

[Illustration: FIG. 118.--Overhung rotary engine mounting.]

The overhung type of engine mounting which is used for both propeller
and tractor aeroplanes, is shown by Fig. 118. In this case the back
plate of the motor is bolted to the capping plate, while an extension
of the hollow crank shaft is supported by a smaller rear plate.
This system has been very widely used, chiefly by reason of its
extreme lightness, and the great facility afforded for the operation
of dismounting the engine; indeed, it would be difficult to find
an arrangement in which the demands of accessibility are so well
satisfied. Another form of overhung mounting, which has been used for
a radial Anzani motor, is shown by Fig. 119. In this case the four
longerons of the fuselage are capped by a single flanged steel plate,
to which the engine is attached by long bolts through the crank case.
Additional support is provided by light steel tube stays, which are
taken from various points on the front of the crank case to the centre
section of the upper plane, or other parts of the machine.

[Illustration: FIG. 119.--Anzani type engine mounting.]

[Illustration:

  _FRONT ELEVATION_      _SIDE ELEVATION_

FIG. 120.--An overhung mounting built up of steel tubes.]

A distinctly original type of overhung mounting is shown by Fig. 120
in front and side elevation, this being used on a machine incorporating
an all-steel fuselage. The ring to which the back plate of the engine
is bolted, is supported from the four corners of the fuselage by steel
tubing, while the bearing for the crank-shaft extension is formed by
a pyramid of tubes welded to a pressing of sheet steel, to which in
turn is bolted a ball-race housing. At each corner the three converging
tubes are welded together, and bolted to small angle plates, which are
also welded to the framework of the fuselage. It will be seen that the
strength of this mounting is entirely dependent upon the welding; but
such reliance, in view of the generally uncertain nature of this latter
process, is not to be recommended.


A Stationary Engine Mounting.

[Illustration: FIG. 121.--Stationary engine mounting.]

A mounting used for a 70 h.p. air-cooled Renault, which is designed to
be supported by short lengths of steel tube projecting from the crank
case, is shown by Fig. 121, this particular arrangement being used on a
propeller biplane. The four ash longerons of the nacelle are built up
in the form of a box girder, the struts immediately under the engine
bearers being reinforced with steel plates. The steel tubes from the
crank case embedded in a steel bearing, composed of two semi-circular
clips, which are let into the upper longerons, and are prevented from
moving sideways by steel collars sweated to the tubes and abutting
against the fixing clips. In this case, by the removal of the four
fixing clips and the necessary pipe connections, the engine can be
lifted bodily out.


Multi-Engine Mountings.

Several versions of the type of machine employing two or more engines,
and which, by the way, is regarded as being the type most suitable
for the commercial purposes of the future, are existent. The usual
arrangement with the twin-engined machine is to support the engines
between the planes on either side of the body, the bearers being
mounted on a structure of struts, which also serve as interplane
supports.

With regard to the flying-boat type of machine, a favourite practice
is to mount the motor on the four struts supporting the centre section
of the upper plane, which is braced by struts and wires from different
points on the hull or body.

Although the modern aircraft engine is of greatly increased power,
compared with the engine of the period 1912–1914, one does not find any
great difference in the structural features of the mounting employed,
and in view of the very diverse arrangements for mounting the same type
of engine which now exist, there is need for greater uniformity. With
regard to the materials employed, there is a very pronounced trend
towards the greater use of wood, which circumstance is certainly at
variance with the oft-portended approaching era of steel. As indicated
in previous chapters, wood possesses remarkable powers of resistance to
sudden shock, which, combined with its quality of absorbing vibration,
renders it peculiarly suitable for the structure of the aeroplane,
and despite its numerous defects, will undoubtedly continue in use
until either the available supplies of suitable timber are exhausted,
or until the production of a remarkably light alloy possessing high
strength values.




CHAPTER XV.

ERECTION AND ALIGNMENT.


The accurate erection and alignment or truing up of the aeroplane, is
a cogent factor in ensuring that the best performance is obtained,
and it is almost platitudinous to emphasize the fact that a machine
incorrectly aligned gives inferior results in flight, entails greater
attention on the part of the pilot, and may possibly seriously
interfere with the general stability of the aeroplane. The degree of
precision attained in the manufacture of the various components is
reflected in the ease or otherwise with which the complete assembly
is aligned; indeed, accuracy of erection is impossible without the
close observance of limits and general trueness in the production of
the different parts. For this reason the erection of the principal
components is surveyed as a necessary preliminary to a consideration of
their assembly in the complete structure.


Accurate Part Production.

In the production of the various struts, longerons and fittings of
the fuselage, the wing spars, compression and interplane struts of
the planes, the utmost accuracy must be observed. Although tolerances
are permissible with regard to the overall dimensions of the struts,
spars, longerons, etc., the lengths particularly of the fuselage
struts should be absolutely correct to drawing. The bad effects of a
strut, say 1 millimetre short, are not restricted to the particular
component of which it forms a part, but are noticeable in one way or
another in the complete structure. Similarly the ends of struts which
are required to be square should be dead square, and those which are
cut to a bevel should correspond with the correct angle. The result of
the slightest discrepancy in this respect becomes speedily apparent
when the defective struts are assembled, as the tension of the bracing
wires will result in the strut becoming bowed or bent, this being
due to the bedding down of the strut end in the socket or clip. It
is also advisable to trim the ends in a machine after being sawn to
something approaching the correct length, and the practice of sawing
to dead length should not be permitted. The surface of a sawn strut
end is formed of a number of more or less ragged fibres, which in
position in the machine and under pressure of the bracing wires tend
to gradually flatten down, this resulting in slack wires and loss of
alignment. Absolute accuracy and uniformity of part production can only
be obtained by the use of jigs, preferably of metal, and some form of
jig should certainly be used for cutting the various struts to length.
Referring again to the necessity of the strut ends being of the correct
angle, it is surprising to note the effect of the smallest inaccuracy.
The writer has frequently noticed fuselage struts considerably out
of straight, the grain of the timber being sometimes advanced as the
reason. However, the removal of the defective strut always resulted
in its return to a straight condition. It should be realized that the
effect of an initially bent strut is a reduction of strength, and as
this may prove a source of danger, it is in itself sufficient reason
for the rigid observance of length limits.


Drilling of Bolt Holes.

Of equal importance is the drilling of the various bolt holes for the
attachment of the fittings. It is not always advisable to drill the
holes in the spars and longerons before the fittings are applied, but
in numerous instances this is possible, and where interchangeability is
an important consideration it is imperative. The practice of setting
out the positions of the various holes from a drawing and then drilling
with a hand brace, is a procedure only justified when a small number
of machines of a certain type are to be produced, and ought by now
to be obsolete. Under such a system no two spars would be exactly
the same, as owing to the influence of grain in the wood, the drill
or bit always tends to “run” from the correct angle. Viewed from the
aspect of quantity production such a practice is very deficient. It
is only by the use of metal drilling jigs of suitable design that
anything approaching absolute accuracy is possible. Such jigs should
not only locate the hole, but should also form a guide for the drill.
In the attachment of the fittings to a properly jig-drilled spar, it
should not be necessary to again drill through, although this often
occurs. Where this is done, there is a distinct possibility of the
brace not being held true, which means that the hole becomes larger
than necessary and not infrequently oval in shape. An additional bad
point is the impossibility of detecting such a fault after the fitting
is bolted on, and it may not be realized until a noticeably slack
wire in the complete machine indicates the movement of the fitting.
In the foregoing, absolute accuracy in the various fittings has been
assumed, but unfortunately in practice almost the reverse is true.
Variation generally occurs in built- or bent-up fittings, and is
usually the result of jigs of either incorrect or bad design. Where
the variation includes a hole out of position, the use of this fitting
on a previously drilled wood part is only possible by the bad practice
of drilling through with the results explained above. It will thus be
realized that the uniformity and accuracy of component production is
only attainable by the utmost precision in the manufacture of both wood
and metal parts.


Locking of Bolts.

Throughout the complete machine it is necessary to lock the nuts of
the bolts, to prevent their gradual loosening under the vibration of
the engine, and different methods of accomplishing this are in use.
Undoubtedly the best form of lock is by the use of a castellated nut
and split pin. By this method one can readily ascertain whether or no
a bolt is locked, while by the withdrawal of the split pin the bolt
may be taken out. A disadvantage is that its use entails considerable
drilling, so that a modification consists of fitting castellated nuts
to all bolts liable to removal for minor adjustments; while elsewhere
the threaded portion of the bolt is left a little longer than the nut,
and then riveted over. Although this reduces labour, it is a somewhat
destructive method; and it is also difficult to determine the adequacy
of the riveting. Another method consists of filing the bolt end flush
with the nut, and then centre punching three or four dots in the joint
between nut and bolt.

This method is neat, the removal of a bolt is easily effected, and the
fact that it has been used in the construction of some fast scouting
biplanes is proof of its effectiveness.

Other systems include the use of two nuts, of a single nut soldered
to the bolt end, and the various patent lock-washers, which in this
country are not greatly in vogue. The practice of re-running down the
threads of bolts to ensure ease in the application of the nut is not
to be recommended--that is, indiscriminately done. Unless the die is
properly adjusted there is a possibility of too much thread being
taken off; the result, an extremely slack nut, being detrimental to
general reliability. The durability of an aeroplane in service is
dependent upon the good workmanship effected in the smallest and most
insignificant detail. Moreover, it should be remembered that the
absence of a split pin may eventually result in disaster.


Truing of Main Planes.

The planes or sections of a machine of the straight-wing type, as
distinct from a machine possessing arrow-shaped or retreating wings,
should, when erected on the fuselage, form a straight line from tip to
tip. This feature is dependent upon (1) the trueness of the planes,
and (2) the alignment of the attachments on the fuselage, the latter
being considered under the fuselage heading. To ensure that the plane
is quite square, it should be checked previous to covering by diagonal
measurements on the wing spars, these being taken from accurate set
positions such as are provided by the wing-root attachments and the
interplane strut fittings. Should a difference in the diagonals exist,
this can easily be rectified by a slight adjustment of the turnbuckles
incorporated in the internal plane wiring. As the ribs of the plane
are built up beforehand, and checked for correct contour by pattern,
little variation should occur in the camber. A point where differences
may occur is between the front spar and the leading edge, as the nose
formers are generally inserted during the assembling of the plane. For
the detection of faults in this direction the template illustrated by
Fig. 34 in Chapter IV. is of great utility.


Fabric Covering of Planes.

The evenness and correct tautness of the fabric covering contributes
largely to the trueness of the plane. Should the covering be stretched
unevenly or too tightly, the application of the dope will cause
distortion of the framework, which can only be obviated by re-covering.
The bad effects of this is more noticeable with regard to the ailerons,
elevators and rudder, which, being of very slender construction, are
more liable to deformation. Twisted or warped control surfaces should
never be used, as such surfaces not only offer increased resistance,
but also interfere with the balance of the machine in flight.


Fuselage Erection.

As the fuselage constitutes the nucleus of the aeroplane, accuracy of
alignment in this component is essential, and the degree of accuracy
obtained in the complete erection depends largely on the correctness
or otherwise of the fuselage. In different individual designs the
methods employed for the construction of the body will be found to vary
considerably. The process of erection adopted in many instances is to
assemble the sides first, upon a table or bench upon which the correct
disposition of the various parts have previously been set out. The
wires are adjusted until the sides conform to the setting out, which
are then packed up on a pair of trestles and the cross-struts attached.
It now remains to align the body so that it is perfectly symmetrical
in plan; and this is accomplished by marking the centre of each
cross-strut, preferably before insertion in the fuselage, and then
adjusting the plan-wires until a cord stretched from the stern-post to
the nose covers each centre line. The cross or sectional bracing-wires
are then tensioned until each diagonal coincides absolutely in length.
This procedure answers very well for a small fuselage of simple
construction, and of the wire-braced fabric-covered type; but where
the forward portion is covered with ply-wood, and the top rails of the
body are horizontal, viewed in side elevation, it is usual to true up
on a bench. This consists of a wooden structure built up of strong
sides, with legs at short intervals, the whole being well braced. The
top surface, on which the body lays, is composed of boards placed
wherever a plan-strut occurs. The bench should be rigidly fixed to a
concrete floor, the top planed until it is level both longitudinally
and transversely, and a centre line marked on each board, while these
lines, checked with a fine steel wire stretched from end to end, should
be in exact agreement with it. The fuselage, having been previously
assembled, with the wires inserted and the plan struts accurately
centred, is placed on it in an inverted position. All wires should be
then slacked off, and the top, which is now underneath, should be wired
until the centre on each strut coincides with the centres on the bench.
The side wires are then tensioned until the stern post is vertical,
or until various fixed points, such as wing-spar attachments, are in
agreement with points marked on the bench and squared or lined up,
and also until the longerons are touching every board. The sectional
wires are then tightened and adjusted so that each diagonal is of the
same length; and this will ensure the centre lines on the cross-struts
connecting the bottom rails being plumb or vertical over the centre
lines of the cross-struts connecting the top rails. Where the top rails
of the fuselage are not parallel to the line of flight, but slope down
towards the tail, it would be necessary, if the bench method is used,
to construct it so that the boards conform to the slope. With the
wire-braced fuselage minor adjustments to the wing-spar attachments,
which predetermine the angle of incidence of the main planes, can be
subsequently made. A type of fuselage which precludes this operation,
and which demands extreme accuracy in construction, is that in which
the bracing of the forward portion is effected by three-ply, all wiring
in a vertical dimension being eliminated, this system being described
in Chapter VIII. and illustrated by Fig. 66.

With this construction points such as the wing spar attachments are
fixed, and cannot be altered after the fuselage is built, so that
meticulous care must be taken in the setting of the short wing spars
across the body, or the fittings to which the wing roots are anchored.

Where a joint occurs in the fuselage it is usual to build the tail
separate from the front portion, and occasionally the two sections are
trued up independently. This does not give such good results as when
the two portions, although separately built, are joined together and
trued up complete.


Checking of Fuselage.

To check the fuselage for alignment it should be placed on a pair of
trestles, one underneath the forward undercarriage strut fixing and the
other under a vertical strut a short distance from the stern post. The
body should then be levelled up longitudinally by a straightedge placed
on two short straightedges of exactly similar widths, one being placed
at the front and the other towards the tail. It should then be packed
up on the trestles until the top longerons are dead level across. At
this point, if the body is in correct alignment, the engine-bearers
would be level both longitudinally and transversely, the incidence
of the main spar attachments should be correct and the stern post
perfectly vertical in all directions. Other tests should include the
placing of a straightedge at the nose, and another placed at the points
where struts occur, should, when sighted across the top edges, be “out
of wind,” that is in agreement. A point which should be carefully
levelled is that portion of the fuselage towards the stern post to
which is attached the fixed tail plane. Any inaccuracy here will result
in the tail being twisted in relation to the main planes. Each fitting
or attachment should also be equidistant from the stern post, and the
effect of variation here will be evidenced by the tail plane being
out of square with the centre line of the fuselage. Where the type of
machine is such that the engine is supported on bearers of wood, it
is usual to drill the holes for the accommodation of the holding-down
bolts to jig before the bearer is built in the structure. In this case
care should be taken to ensure that the corresponding bolt holes in
each bearer are square with the centre line. Any deviation will result
in the axis of the engine forming an angle with the centre line.


Alignment of Complete Machine.

In this connection it will be better to consider the alignment of a
type of machine in common use: a tractor-biplane in which the upper
plane is composed of two outer planes and a centre section, and the
lower plane in two sections, each abutting against the side of the
fuselage, this arrangement being shown in front elevation by Fig. 122.
The first operation is the levelling of the fuselage transversely by
placing the level across the engine bearers, and the attachment of
the centre section, which is mounted upon four struts which have been
previously cut to dead length and tested by jig. This, considered in
front elevation, should be centrally placed over the body, and this
is assured by adjustments in the wires A-A1. This can be checked by
dropping a plumb-line from the centre plane spar ends and measuring the
distance from the line to the side of the body, the distances on either
side should, of course, coincide. The next point is to brace the outer
sections to the correct dihedral. One method of accomplishing this,
as shown by Fig. 122, is by the use of a dihedral board, this being
prepared perfectly straight on one edge, the other being tapered to
the desired angle. The wires are then adjusted until the straightedge
is level. Another method is to use an ordinary straightedge placed
along the top surface of the plane, the angle being measured with a
protractor or clinometer, the latter instrument being most accurate. To
check the dihedral a line can be stretched between points immediately
above the top interplane struts on each side and then measuring to the
centre section, but it would be difficult to detect differences in the
angles of each wing. With regard to the undercarriage, the distances
between lines dropped from the fuselage sides and the wheel centres
should coincide.


Alignment of Machine in Side Elevation.

Considering the side elevation, Fig. 123, alignment here is concerned
with the incidence of the main planes, the distance forward of the top
plane from the lower plane or stagger, and the level of the engine
bearers in relation to the top longerons of the fuselage.

[Illustration: FIG. 122.--Showing use of dihedral board and
spirit-level.

FIG. 123.--Checking of main-plane incidence and stagger.

FIG. 124.--Template for checking incidence.]

The fuselage should be levelled longitudinally by placing the level
on the engine bearers, assuming the engine is not in place. When the
bearers are level, the top longeron should also be level, in any case
the incidence of the plane should only be adjusted in relation to the
engine bearers. To check the stagger, a plumb line should be dropped
from the leading edge of the centre plane, and adjustments made with
the incidence wires from the fuselage to the centre-plane struts, until
the required distance forward from the leading edge of the lower plane
is obtained. The incidence can be tested by a straightedge placed
under the plane and a clinometer, as in Fig. 123, and another device
sometimes used is shown by Fig. 124. This is made of dry wood, the
lengths of the legs to the tops of the spars being obtained from a
drawing of the wing section, and its incidence.


Plan Alignment of Machine.

[Illustration: FIG. 125.--Showing points to check for correct alignment
on plan.]

In the plan view, Fig. 125, the distances AB and AC must be equal,
the same applying to CD and BD. With modern machines external drift
wiring is obsolete, so that discrepancies in these measurements must
be rectified by alterations in the wiring of the fuselage, as it is
inaccuracy at some point in the latter component to which the trouble
may be ascribed. It is at this point that one realizes the need for
precision in the construction of the fuselage. In Fig. 126 is shown
a plan view in which the main plane is very obviously out of square
with the centre line of the body, the amount is not likely to occur in
actual practice, but it has been exaggerated in the drawing. The cause
of this trouble can be traced to the short wing spars in the fuselage,
to which the lower plane is attached, or in other cases to the
fittings, to which the lower plane is anchored, being out of centre,
possibly only an insignificant amount. The lengths of the fuselage wing
spars are also possible causes of trouble. Assuming that the rear spar
is the correct length, and the front spar is over the length, this
would result, when the outer sections were attached, in the latter
sloping backwards, which again emphasizes the need for accurate part
production.

[Illustration: FIG. 126.]

With regard to the tail plane, measurements taken from the extremities
of the back spar to some fixed point forward on the fuselage, to the
strut sockets on the planes, or to the rear wing spar anchorage, as in
Fig. 125, should be equal.

The primary consideration with regard to the rudder and fin is that,
viewed from the rear, they should be perpendicular, which can be
verified by a plumb-line dropped from the top of the rudder-post. In
plan view the fixed fin should correspond with the centre line of the
fuselage although there are exceptions to this rule, notably where the
fin is set over, to neutralize propeller torque, and in this case the
measurements given in the general drawings must be adhered to.


Tension of Wires.

The correct tensioning of wires is a matter upon which some variation
of opinion occurs. Although wires should not be left slack, conversely
they should not be over-tensioned, as this results in the spars, wires,
and struts, being initially stressed before any load due to flight is
applied. In this connection the importance of even or uniform tension
in the wires may be emphasized. The wires in one bay being of greater
tension than those in an adjacent bay, is the frequent cause of bent
or deformed struts. The more extended use of a tautness meter for the
interplane wiring would result in greater uniformity and the more equal
distribution of stresses.




INDEX.


  Aileron and warp control, 101

  Aluminium, 16

  Anchorage of wires to eyebolts, 55

  Ash, 10

  Ash, weight of, 10

  Axle fairings, 98


  Biplane trussing, 62

  Birch, 12

  Birch, weight of, 12

  Brazing and welding, 52

  Bridge-type undercarriage suspension, 98


  Control systems, 101

  Control by inherent stability, 102

  Control, vertical column, 103

  Control, wheel type, 104

  Control, the “Dep,” 106

  Control, the “Wright,” 107

  Control surfaces, balancing of, 102

  Cypress, 13


  Drift bracing, 65

  Drilling of bolt-holes, 124

  Duralumin, 17


  Engine mountings, 116

  Engine mountings, rotary, 118

  Engine mountings, overhung, 119

  Engine mountings, stationary, 121

  Engine mounting materials, 117

  Erection and alignment, 123


  Fabric, attachment of, 47

  Fabric, sagging of, 40

  Fabric, tension of, 48

  Fabric, friction blocks, 49

  False ribs, 41

  Fineness ratio, 26

  Fuselage construction, 67

  Fuselage covering, 71

  Fuselage contours, 75

  Fuselage erection, 127

  Fuselage fittings, 77

  Fuselage struts, 28

  Fuselage types, 67


  Glue, defects of, in wing spars, 33


  Hickory, 11


  Interplane struts, tapering of, 26

  Interplane strut connections, 51


  Leading and trailing edges, 43

  Lift wires, anchorage of, 62

  Locking of bolts, 125

  Longerons, 68

  Longeron sections, 69

  Longeron, jointing of, 70

  Longeron, shaping of, 69


  Mahogany, Honduras, 11

  Mahogany, Honduras, weight of, 11

  Mahogany, Cuban, weight of, 12

  Main planes, truing of, 126

  Main spars, shaping of, 32

  Materials, 6

  Maurice Farman strut arrangement, 62

  Metal construction, general, 4

  Metal wing construction, 45

  Methods of manufacture, general, 3

  Moisture content of timber, 8

  Monoplane bracing, 60

  Monocoque fuselage construction, 72

  Multi-ply wood, 13


  Oleo-pneumatic gear, 95

  Oregon pine, 12


  Parang, 13

  Piano wire bracing, 108

  Piano wire connections, 109

  Plane construction, 30

  Plane construction, details of, 40

  Planes, arrangement of, 33

  Poplar, 12

  Pratt truss, 59


  Raked wing tip, efficiency of, 43

  Ribs, spacing of, 40

  Ribs, types in use, 34

  Ribs under compression, 36


  Shakes, 14

  Shrinkage, 7

  Silver spruce, 8

  Silver spruce, weight of, 8

  Single-strut truss, 63

  Spar construction, hollow, 20–22

  Spar, laminated, 19

  Spar sections, 19

  Spars and struts, 18

  Spruce fir, 9

  Standardization of details, general, 2

  Steel, 15

  Steel tube, 16

  Steel-tube fuselage construction, 84

  Stranded cable, 109

  Stranded-cable connections, 111

  Streamline wires, 113

  Streamline wires, attachments, 113

  Strut sections, 23

  Strut sections, design of, 26

  Strut materials, 25

  Strut socket, “Wright,” 53

  Strut socket, “Cody,” 53

  Strut-socket fairings, 58

  Suspension springs, steel, 95

  Suspension springs, rubber, 97


  Tabulated tests, unreliability of, 8

  Three-ply fuselage construction, 71

  Thunder shakes, 11

  Timber, defects in, 14

  Trailing edge, wire, 43

  Twisted grain, 14


  Undercarriage brakes, 99

  Undercarriage details, 93

  Undercarriage, principles of design, 86

  Undercarriage skids, 93

  Undercarriage types, 89


  Walnut, 11

  White deal, 9

  Wing baffles, 44

  Wing fittings, attachment to spars, 51

  Wing-tip construction, 39

  Wing-tip details, 38

  Wing-trussing systems, 59

  Wireless wing structures, 61

  Wires and connections, 108

  Wires, results of tests on, 109

  Wires, relative strengths of, 112

  Wires, tension of, 134

  Wood, choice of a suitable, 6

  Wood, variable qualities of, 6


   PRINTED BY WILLIAM CLOWES AND SONS, LIMITED, LONDON AND BECCLES.




        Just Published. Over 100 pages. With numerous Diagrams.
              Fcap 8vo. Paper wrapper, Price 3s. 6d. net.

                              The Aircraft
                          Identification Book

                 A CONCISE GUIDE TO THE RECOGNITION OF
                 DIFFERENT TYPES AND MAKES OF ALL KINDS
                       OF AEROPLANES AND AIRSHIPS

                                   BY
                          R. BORLASE MATTHEWS

 Associate Member of the Institution of Civil Engineers, Member of the
        Institution of Electrical Engineers, Fellow of the Royal
             Aeronautical Society, Whitworth Exhibitioner,
               Author of the “Aviation Pocket-Book,” etc.

                                  AND
                             G. T. CLARKSON


EXTRACT FROM PREFACE

Just as one make of motor car can be distinguished from another by
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It is claimed, however, that this book will enable the reader to
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                                 LONDON
                        CROSBY LOCKWOOD AND SON,
          7 STATIONERS’ HALL COURT, LUDGATE HILL, E.C. 4, AND
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                   7 STATIONERS’ HALL COURT, E.C. 4.




Transcriber’s Notes


Punctuation, hyphenation, and spelling were made consistent when a
predominant preference was found in the original book; otherwise they
were not changed.

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marks were remedied when the change was obvious, and otherwise left
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originally appeared, even when that was mid-paragraph.

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references.





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