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Title: Dirigible Balloons

Creator: Charles B. Hayward

Release date: September 18, 2020 [eBook #63234]

Language: English



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                           DIRIGIBLE BALLOONS


                           INSTRUCTION PAPER


                              PREPARED BY

                           CHARLES B. HAYWARD


   MEMBER, SOCIETY OF AUTOMOBILE ENGINEERS; MEMBER, THE AERONAUTICAL
                                SOCIETY;

     FORMERLY SECRETARY, SOCIETY OF AUTOMOBILE ENGINEERS; FORMERLY

                  ENGINEERING EDITOR, "THE AUTOMOBILE"




                   AMERICAN SCHOOL OF CORRESPONDENCE

                            CHICAGO ILLINOIS

                                 U.S.A.




                       COPYRIGHT, 1912, 1918, BY

                   AMERICAN SCHOOL OF CORRESPONDENCE

                      COPYRIGHTED IN GREAT BRITAIN

                          ALL RIGHTS RESERVED




DIRIGIBLE BALLOONS



INTRODUCTION


Of the first attempts of men to emulate the flight of birds, we have no
knowledge, but one of the earliest, perhaps, is embodied in the myth of
Icarus and Daedalus. Xerxes, it is said, possessed a throne which was
drawn through the air by eagles. The Chinese have sometimes been given
credit for the invention of the balloon, as they have for many other
scientific discoveries. It is related that a balloon was sent up at
Pekin in celebration of the ascension of the throne by an emperor in the
beginning of the fourteenth century.

[Illustration: Fig. 1. De Lana Airboat.]

*Early Attempts.* Leonardo da Vinci devoted some time to the problem of
artificial flight. His sketches show the details of batlike wings which
were to spread out on the downward stroke and fold up with the upward
stroke.

Francisco de Lana planned to make a flying ship the appearance of which
was somewhat like that shown in Fig. 1, by exhausting the air from metal
spheres fastened to a boat. The boat was to be equipped with oars and
sails for propulsion and guiding. The method in which he purposed to
create the vacuum in the spheres consisted of filling them with water,
thus driving out the air, then letting the water run out. He thought
that if he closed the tap at the proper time, there would be neither air
nor water in the spheres. His flying ship was never constructed, for he
piously decided that God would never permit such a change in the affairs
of men.

*The First Flying Machine.* In 1781, Meerwein of Baden, Germany,
constructed a flying machine, and was the first, perhaps, to
intelligently take into account the resistance of the air. He took the
wild duck as a basis of calculation, and found that a man and machine
weighing together 200 pounds would require a wing surface of from 125 to
130 square feet. It is of interest to note that Lilienthal, who met his
death in trying to apply these principles, over one hundred years later
found these figures to be correct. Two views of Meerwein’s apparatus are
shown in Fig. 2. The construction involved two wood frames covered with
cloth. The machine weighed 56 pounds and had a surface area of 111
square feet. The operator was fastened in the middle of the under side
of the wings, and over a rod by which he worked the wings. His attempts
at flight were not successful, as his ideas of the power of a man were
in error.

[Illustration: Fig. 2. Meerwein Flying Machine]

*Classification.* All attempts at human flight have gone to show that
there are four possible ways in which man may hope to navigate the air.
He may imitate the flight of birds with a machine with moving or
flapping wings; he may use vertical screws or helices to pull himself
up; he may use an aeroplane and sail the air like an eagle; or, lastly,
he may raise himself by means of a gas bag and either drift with the
wind or move forward by means of propellers.

In these attempts, apparatus of several different types has been
developed. The types are classed in two general divisions based on their
weight relative to that of the atmosphere, viz, the _lighter-than-air
machines_ and the _heavier-than-air machines_. Lighter-than-air machines
are those which employ a bag filled with a gas whose specific gravity is
sufficiently less than that of the air to lift the bag and the necessary
attachments from the earth, and include simple balloons and dirigibles.
Heavier-than-air machines, which will neither rise nor remain in the air
without motive power, include all forms of aeroplanes.



SIMPLE BALLOONS


*Theory.* The balloon-like airship has been more highly developed than
any other type of aerial craft, probably because it offers the most
obvious means of overcoming the force of gravitation. It depends on the
law of Archimedes:

_"Every body which is immersed in a fluid is acted upon by an upward
force, exactly equal to the weight of the fluid displaced by the
immersed body."_

That is, a body will be at rest if immersed in a fluid of equal specific
gravity or equal weight, volume for volume; if the body has less
specific gravity than the fluid in which it is immersed it will rise; if
it has a greater specific gravity it will sink. Therefore, if the total
weight of a balloon is less than the weight of all the air it displaces
it will rise in the air. It is, then, necessary to fill the balloon with
some gas whose specific gravity is enough less than, that of the air to
make the weight of the gas itself, the bags, and the attachments, less
than the weight of the air displaced by the whole apparatus. The gases
usually employed are _hydrogen_, _coal gas_, and _hot air_.

At atmospheric pressure and freezing temperature, the weight of a cubic
foot of air is about .08 pound; the weight of a cubic foot of hydrogen
is about .005 pound, under the same conditions. According to the law of
Archimedes, a cubic foot of hydrogen would be acted upon by a force
equal to the difference, or approximately .075 pound, tending to move it
upwards. In the same way, a cubic foot of coal gas, which weighs .04
pound, would be acted upon by an upward force of .04 pound.

It is evident, then, that a considerable volume of gas is required to
lift a balloon with its envelope, net, car, and other attachments.

Further, it requires almost twice as much coal gas as hydrogen, under
the same conditions, for we have seen that the upward force on it is
only half as great. The lifting power of hot air is less than one-eighth
as great as that of hydrogen at the highest temperature that can
possibly be used in a balloon.

[Illustration: Fig. 3. Montgolfier Balloon]

The general type of lighter-than-air machines may be divided into
_aerostats_ (ordinary balloons, which are entirely dependent on wind
currents for lateral movement, and which are often the chief features at
country fairs) and dirigible balloons or _aeronats_ (air swimmers).
Dirigible balloons employ the gas bag for maintaining buoyancy, and have
rudders to guide them and propellers to drive them forward through the
air in much the same way that ships are driven through the water.

*The First Balloon.* For several years, Joseph and Steven Montgolfier
had been experimenting with a view to constructing a balloon: in the
first place by filling bags with _steam_; then by filling bags with
_smoke_, and finally by filling bags with _hydrogen_. These attempts
were all failures, for the steam rapidly condensed and the smoke and
hydrogen leaked through the pores in the bags. They finally hit upon the
idea of filling the bag with _hot air_, by means of a fire under its
open mouth. Several balloons were burned up, but the next was always
made larger, until, at their first public exhibition on June 5, 1783,
the bag had become over 35 feet in diameter. On this occasion, it rose
to a height of between 900 and 1,000 feet, but the hot air was gradually
escaping, and at the end of ten minutes the balloon fell to the ground.

The Montgolfiers then went to Paris, where, after suffering the loss of
a paper balloon by rain, they sent up a waterproofed linen one carrying
a sheep, a duck, and a rooster in a basket. A rupture in the linen
caused the three unwilling aeronauts to make a landing at the end of
about ten minutes. The Montgolfiers received great honor, and small
balloons of this type became a popular fad. One of these balloons is
shown in Fig. 3, making an ascension.

*Rozier.* The first man to go up in a balloon was Rozier, who ascended
in a captive balloon to a height of about 80 feet, in the latter part of
the year 1783. Later, in company with a companion, he made a voyage in a
free balloon, remaining in the air about half an hour. In these
balloons, the air within was kept hot by means of a fire carried in a
pan immediately below the mouth of the bag, as shown in Fig. 4.
Accidents were numerous on account of the fabric becoming ignited from
the fire in the pan.

[Illustration: Fig. 4. Rozier Hot-Air Balloon]

*Improvements by Charles.* The physicist, Charles, was working along
these lines at the same time. He coated his balloon with a rubber
solution to close up the pores, and was thereby enabled to substitute
hydrogen for the hot air. Shortly after the Montgolfiers’ first public
exhibition, Charles sent up his balloon for the benefit of the _Academie
des Sciences_ in Paris. The balloon, which weighed about 19 pounds,
ascended rapidly in the air and disappeared in the clouds, where it
burst and fell in a suburb of the city. The impression produced upon the
peasants at seeing it fall from the heavens was hardly different from
what could be expected. They believed it to be of devilish origin, and
immediately tore it into shreds. Charles subsequently built a large
balloon quite similar to those in use today. A net was used to support
the basket, and a valve, operated by means of ropes from the basket, was
arranged at the top to permit the gas to escape as desired.

*The Balloon Successful.* The English Channel was first crossed in 1785.
Blanchard, an Englishman, and Jeffries, an American, started from Dover
on January 7 in a balloon equipped with wings and oars. After a very
hazardous voyage, during which they had to cast overboard everything
movable to keep from drowning, they landed in triumph on the French
coast.

An attempt to duplicate this feat was made shortly afterward by Rozier.
He constructed a balloon filled with hydrogen, below which hung a
receiver in which air could be heated. He hoped to replace by the hot
air the losses due to leakage of hydrogen. Soon after the start the
balloon exploded, due to the escaping gas reaching the fire, and Rozier
and his companion were dashed on the cliffs and killed.



EARLY DIRIGIBLES


*Meusnier the Pioneer.* The fact that the invention of the dirigible
balloon and means of navigating it were almost simultaneous is very
little known today and much less appreciated. Like the aeroplane, its
development was very much retarded by the lack of suitable means of
propulsion, and the actual history of what has been accomplished in this
field dates back only to the initial circular flight of La France in
1885. Still the principles upon which success has been achieved were
laid down within a year of the appearance of Montgolfier’s first gas
bag. Lieutenant Meusnier, who subsequently became a general in the
French army, must really be credited with being the true inventor of
aerial navigation. At a time when nothing whatever was known of the
science, Meusnier had the distinction of elaborating at one stroke all
the laws governing the stability of an airship, and calculating
correctly the conditions of equilibrium for an elongated balloon, after
having strikingly demonstrated the necessity for this elongation. This
was in 1781 and Meusnier’s designs and calculations are still preserved
in the engineering section of the French War Office in the form of
drawings and tables.

But as often proved to be the case in other fields of research, his
efforts went unheeded. How marvelous the establishment of these numerous
principles by one man in a short time really is, can be appreciated only
by noting the painfully slow process that has been necessary to again
determine them, one by one, at considerable intervals and after numerous
failures. Through not following the lines which he laid down, aerial
navigation lost a century in futile groping about; in experiments
absolutely without method or sequence.

[Illustration: Fig. 5. Meusnier Dirigible Balloon]

Meusnier’s designs covered two dirigible balloons and that he fully
appreciated the necessity for size is shown by the dimensions of the
larger, which unfortunately was never built. This was to be 260 feet
long by 130 feet in diameter, in the form of an ellipse, the elongation
being exactly twice the diameter. In other words, a perfect ellipsoid,
which was a logical and, in fact, the most perfect development of the
spherical form. Although increased knowledge of wind resistance and the
importance of the part it plays has proved his relative dimensions to be
faulty, a study of the principal features of his machine shows that he
anticipated the present-day dirigible of the most successful type at
practically every point, barring, of course, the motive power, as there
was absolutely nothing available in that day except human effort. As the
latter weighs more than one-half ton per horse-power, it goes without
saying that Meusnier’s balloon would have been dirigible only in a dead
calm.

He adopted the elongated form, conceived the girth fastening, the
triangular or indeformable suspension, the air balloonet and its pumps,
and the screw propeller, all of which are to be found in the dirigibles
of present-day French construction, Fig. 5. It need scarcely be added
that the French have not only devoted a greater amount of time and
effort to the development of the dirigible than any other nation, but
have also met with the greatest success in its use. It was not until
1886, or more than a century after Meusnier had first elaborated those
principles, that their value became known. They were set forth by
Lieutenant Letourne, of the French engineers, in a paper presented to
the _Academie des Sciences_ by General Perrier.

In one form or another, the salient features of Meusnier’s dirigible
will be found embodied in the majority of attempts of later days. His
large airship was designed to consist of double envelope, the outer
container of which was to provide the strength necessary, and it was
accordingly reinforced by bands. The inner envelope was to provide the
container for the gas and was not called upon to support any weight.
This inner bag or balloon proper was designed to be only partially
inflated and the space between, the two was to be occupied by air which
could be forced into it at two points at either end, by pumps, so as to
maintain the pressure on the gas bag uniform regardless of the expansion
or contraction of its contents. Here in principle was the air balloonet
of today. Instead of employing a net to hang the car from the outer
envelope, the former was attached by means of a triangular suspension
system fastened to a heavy rope band, or girth, encircling the outer
envelope. At the three points where the lifting rope members met, a
shaft running the length of the car and carrying what Meusnier described
as "revolving oars" was installed. These constituted the prototype of
the screw propeller, invented for aerial navigation at a time long
antedating the use of steam for marine use. Thus he devised: (1) The air
balloonet to husband the gas supply and thus prevent the deformation of
the outer container or support, as well as to provide stability; (2) the
triangular suspension to attain longitudinal stability; and (3) the
screw propeller for propulsion, beside selecting the proper location for
the latter.



PROBLEMS OF THE DIRIGIBLE


*Ability to Float.* If ability to rise in the air depended merely upon a
knowledge of the principle that made it possible, it undoubtedly would
have been accomplished many centuries ago. As already mentioned,
Archimedes established the fact that a body upon floating in a fluid
displaces an amount of the latter equal in weight to the body itself,
and upon this theory was formulated the now well-known law, that every
body plunged into a fluid is subjected by this fluid to a pressure from
below, equivalent to the weight of the fluid displaced by the body.
Consequently, if the weight of the latter be less than that of the fluid
it displaces, the body will float. It is by reason of this that the iron
ship floats and the fish swims in water. If the weight of the body and
the displaced water be the same, the body will remain in equilibrium in
the water at a certain level, and if that of the body be greater, it
will sink. All three of these factors are found in the fish, which, with
the aid of its natatory gland, can rise to the surface, sink to the
bottom, or remain suspended at different levels. To accomplish these
changes of specific gravity, the fish fills this gland with air,
dilating it until full, or compressing and emptying it. In this we find
a perfect analogy to the air balloonet of the dirigible, which serves
the same purposes. The method by which lifting power is obtained in the
dirigible is exactly the same as in the case of the balloon.

But once in the air, a balloon is, to all intents and purposes, a part
of the atmosphere. There is absolutely no sensation of movement, either
vertically or horizontally. The earth appears to drop away from beneath
and to sweep by horizontally, and regardless of how violently the wind
may be blowing, the balloon is always in a dead calm because it is
really part of the wind itself and is traveling with it at exactly the
same speed. If it were not for the loss of lifting power through the
expansion and contraction of the gas, making it necessary to permit its
escape in order to avoid rising to inconvenient heights on a very warm
day, and the sacrifice of ballast to prevent coming to earth at night,
the ability of a balloon to stay up would be limited only by the
endurance of its crew and the quantity of provisions it was able to
transport. As the use of air balloonets in the dirigible takes care of
this, the question of lifting power presents no particular difficulty.
It is only a matter of providing sufficient gas to support the increased
weight of the car, motor and its accessories, and the crew of the larger
vessel, with a factor of safety to allow for emergencies, in order to
permit of staying in the air long enough to make a protracted voyage.

*Air Resistance vs. Speed.* Unless a voyage is to be governed in its
direction entirely by the wind, the dirigible must possess a means of
moving contrary to the latter. The moment this is attempted, resistance
is encountered, and it is this resistance of the air that is responsible
for the chief difficulties in the design of the dirigible. To drive it
against the wind, it must have power; to support the weight of the motor
necessary, the size of the gas bag must be increased. But with the
increase in size, the amount of resistance is greatly multiplied and the
power to force it through the air must be increased correspondingly. The
law is approximately as follows:

_Where the surface moves in a line perpendicular to its plane, the
resistance is proportional to the extent of the surface, to the square
of the speed with which the surface is moved through the air, and to a
coefficient, the mean value of which is 0.125._

This coefficient is a doubtful factor, the figure given having been
worked out years ago in connection with the propulsion of sailing
vessels. Its value varies according to later experimenters between .08
and .16, the mean of the more recent investigations of Renard, Eiffel,
and others who have devoted considerable study to the matter, being .08.
This is dwelt upon more in detail under "Aerodynamics" and it will be
noted that the values of the coefficient _K_, given here, do not agree
with those stated in that article. They serve, however, to illustrate
the principles in question.

In accordance with this law, doubling the speed means quadrupling the
resistance of the air. For instance, a surface of 16 square feet moving
directly against the air at a speed of 10 feet per second will encounter
a resistance of 16 X 100 (square of the speed) X 0.125 = 200 pounds
pressure. Doubling the speed, thus bringing it up to 20 feet per second,
would give the equation 16 X 400 X 0.125 = 800 pounds pressure, or with
the more recent value of the coefficient of .08, 512 pounds pressure.
The first consideration is accordingly to reduce the amount of surface
moving at right angles. The resistance of a surface having tapering
sides which cut through or divide the molecules of air instead of
allowing them to impinge directly upon it, is greatly diminished; hence,
Meusnier’s principle of elongation. If we take the same panel presenting
16 square feet of surface and build out on it a hemisphere, its
resistance at a speed of 10 feet per second will be exactly half, or a
pressure of 100 pounds.

By further modifying this so as to represent a sharp point, or
acute-angled cone, it will be 38 pounds. There could accordingly be no
question of attempting to propel a spherical balloon.

[Illustration: Fig. 6. Giffard Dirigible]

It is necessary to select a form that presents as small a surface as
possible to the air as the balloon advances, while preserving the
maximum lifting power. But experience has strikingly demonstrated the
analogy between marine and aerial practice—not only is the shape of the
bow of the vessel of great importance but, likewise, the stern. The
profile of the latter may permit of an easy reunion of the molecules of
air separated by the former, or it may allow them to come together again
suddenly, clashing with one another and producing disturbing eddies just
behind the moving body. To carry the comparison with a marine vessel a
bit further, the form must be such as to give an easy "shear," or sweep
from stem to stern.

[Illustration: Fig. 7. De Lome Dirigible]

That early investigators appreciated this is shown by the fact that
Giffard in 1852, Fig. 6, De Lome in 1872, Fig. 7, Tissandier in 1884,
and Santos-Dumont in his numerous attempts, adopted a spindle-shaped or
"fusiform" balloon. In other words, their shape, equally pointed at
either end, was symmetrical in relation to their central plan. However,
that the shape best adapted to the requirements of the bow did not serve
equally well for the stern, was demonstrated for the first time by
Renard, to whom credit must be given for a very large part of the
scientific development of the dirigible. Almost a century earlier,
Marey-Monge had laid down the principle that to be successfully
propelled through the air, the balloon must have "the head of a cod and
the tail of a mackerel." Nature exemplifies the truth of this in all
swiftly moving fishes and birds. Renard accordingly adopted what may
best be termed the "pisciform" type, viz, that of a dis-symmetrical fish
with the larger end serving as the bow; and the performances of the
Renard, Lebaudy, and Clement-Bayard airships have shown that this is the
most advantageous form.

The pointed stern prevents the formation of eddies and the creation of a
partial vacuum in the wake which would impose additional thrust on the
bow. Zeppelin has disregarded this factor by adhering to the purely
cylindrical form with short hemispherical bow and stern, but it is to be
noted that while other German investigators originally followed this
precedent, they have gradually abandoned it, owing to the noticeable
retarding effect.

*Critical Size of Bag.* Next in importance to the best form to be given
the vessel, is the most effective size—something which has a direct
bearing upon its lifting power. This depends upon the volume, while the
resistance is proportional to the amount of surface presented. Greater
lifting power can accordingly be obtained by keeping the diameter down
and increasing the length. But the resistance is also proportionate to
the square of the speed, while the volume, or lifting power, varies as
the cube of the dimensions of the container, so that in doubling the
latter, the resistance of the vessel at a certain speed is increased
only four times while its lifting capacity is increased eight times.
Consequently the larger dirigible is very much more efficient than the
smaller one since it can carry so much more weight in the form of a
motor and fuel in proportion to its resistance to the air. As an
illustration of this, assume a rectangular container with square ends 1
foot each way and 5 feet long. Its volume will be 5 cubic feet and if
the lifting power of the gas be assumed as 2 pounds per cubic foot, its
total lifting power will be 5 pounds. If a motor weighing exactly 5
pounds per horse-power be assumed, it will be evident that the motor
which such a balloon could carry would be limited to 1 horse-power,
neglecting the weight of the container.

Double these dimensions and the container will then measure 2 X 2 X 10
feet, giving a volume of 40 cubic feet, and a lifting power, on the
basis already assumed, of a motor capable of producing 8 horsepower, and
this without taking into consideration that as the size of the motor
increases, its weight per horse-power decreases. The balloon of twice
the size will thus have a motor of 8 horse-power to overcome the
resistance of the head-on surface of 4 square feet, or 2 horse-power per
square foot of transverse section, whereas the balloon of half the size
will have only 1 horse-power per square foot of transverse section. It
is, accordingly, not practicable to construct small dirigibles such as
the various airships built by Santos-Dumont for his experiments, while,
on the other hand, there are numerous limitations that will be obvious,
restricting an increase in size beyond a certain point, as has been
shown by the experience of the various Zeppelin airships.

To make it serviceable, what Berget terms the "independent speed" of a
dirigible, i.e., its power to move itself against the wind, must be
sufficient to enable it to travel under normally prevailing atmospheric
conditions. These naturally differ greatly in different countries and in
different parts of the same country. Where meteorological tables showed
the prevailing winds in a certain district to exceed 15 miles an hour
throughout a large part of the year, it would be useless to construct an
airship with a speed of 15 miles an hour or less for use in that
particular district, as the number of days in the year in which one
could travel to and from a certain starting point would be limited. This
introduces another factor which has a vital bearing upon the size of the
vessel. Refer to the figures just cited and assume further that by
doubling the dimensions and making the airship capable of transporting a
motor of 8 horse-power, it has a speed of 10 miles an hour. It is
desired to double this. But the resistance of the surface presented
increases as the square of the speed. Hence, it will not avail merely to
double the power of the motor. Experience has demonstrated that the
power necessary to increase the speed of the same body, increases in
proportion to the cube of the speed, so that instead of a 16-horse-power
motor in the case mentioned, one of 64 horse-power would be needed.
There are, accordingly, a number of elements that must be taken into
consideration when determining the size as well as the shape of the
balloon.

*Static Equilibrium.* Having settled upon the size and shape, there must
be an appropriate means of attaching the car to carry the power plant,
its accessories and control, and the crew. While apparently a simple
matter, this involves one of the most important elements of the
design—that of stability. A long envelope of comparatively small
diameter being necessary for the reasons given, it is essential that
this be maintained with its axis horizontal. In calm air, the balloon,
or container, is subjected to the action of two forces: One is its
weight, applied to the center of gravity of the system formed by the
balloon, its car, and all the supports; the other is the thrust of the
air, applied at a point known as the center of thrust and which will
differ with different designs, according as the car is suspended nearer
or farther away from the balloon. If the latter contained only the gas
used to inflate it, with no car or other weight to carry, the center of
gravity and the center of thrust would coincide, granting that the
weight of the envelope were negligible. As this naturally can not be the
case, these forces are not a continuation of each other. But as they
must necessarily be equal if the balloon is neither ascending nor
descending, it follows that they will cause the balloon to turn until
they are a continuation of each other, and in the case of a pisciform
balloon, this will cause it to tilt downward. Like a ship with too much
cargo forward, it would be what sailors term "down at the head."

As this would be neither convenient nor compatible with rapid
propulsion, it must be avoided by distributing the weight along the car
in such a manner that when the balloon is horizontal, the forces
represented by the pressure above and the weight below, must be in the
same perpendicular. This is necessary to insure static equilibrium, or a
horizontal position while in a state of rest. To bring this about, the
connections between the car and the balloon must always maintain the
same relative position, which is further complicated by the fact that
they must be flexible at the same time.

*Longitudinal Stability.* But the _longitudinal stability_ of the
airship as a whole must be preserved, and this also involves its
_stability of direction_. Its axis must be a tangent to the course it
describes, if the latter be curvilinear, Or parallel with the direction
of this course where the course itself is straight. This is apparently
something which should be taken care of by the rudder, any tendency on
the part of the airship to diverge from its course being corrected by
the pilot. But a boat that needed constant attention to the helm to keep
it on its course would be put down as a "cranky"—in other words, of
faulty design in the hull. A dirigible having the same defect would be
difficult to navigate, as the rudder alone would not suffice to correct
this tendency in emergencies. Stability of direction is, accordingly,
provided for in the design of the balloon itself, and this is the chief
reason for adopting the form of a large-headed and slender-bodied fish,
as already outlined. This brings the center of gravity forward and makes
of the long tail an effective lever which overcomes any tendency of the
ship to diverge from the course it should follow, by causing the
resistance of the air itself to bring it back into line. However, the
envelope of the balloon itself would not suffice for this, so just
astern of the latter, "stabilizing surfaces" are placed, consisting of
vertical planes fixed to the envelope. These form the keel of the
dirigible and are analogous to the keel of the ship. Stability of
direction is thus obtained naturally without having constant recourse to
the rudder, which is employed only to alter the direction of travel.

The comparison between marine and aerial navigation must be carried even
further. These vertical planes, or "keel," prevent rolling; it is
equally necessary to avoid pitching—far more so than in the case of a
vessel in water. So that while the question of stability of direction is
intimately connected with longitudinal stability, other means are
required to insure the latter. The airship must travel on an "even
keel," except when ascending or descending, and the latter must be
closely under the control of the pilot, as otherwise the balloon may
incline at a dangerous angle. This shows the importance of an unvarying
connection between the car and the envelope to avoid defective
longitudinal stability. Assume, for instance, that the car is merely
attached at each end of a single line. The car, the horizontal axis of
the balloon, and the two supports would then form a rectangle. When in a
state of equilibrium the weight and the thrust are acting in the same
line. Now suppose that the pilot desires to descend and inclines the
ship downward. The center of gravity is then shifted farther forward and
the two forces are no longer in line.

But as the connections permit the car to swing in a vertical plane, they
permit the latter to move forward and parallel with the balloon, thus
forming a parallelogram instead of a rectangle. This causes the center
of gravity to shift even farther, and as one of the most serious causes
of longitudinal stability is the movement of the gas itself, it would
also rush to the back end and cause the balloon to "stand on its head."
As the tendency of the gas is thus to augment any inclination
accidentally produced, the vital necessity of providing a suspension
that is incapable of displacement with relation to the balloon is
evident. Here is where the importance of Meusnier’s conception of the
principle of triangular suspension comes in. Instead of being merely
supported by direct vertical connections with the balloon, the ends of
the car are also attached to the opposite ends of the envelope, forming
opposite triangles. This gives an unvarying attachment, so that when the
balloon inclines, the car maintains its relative position, and the
weight and thrust tend to pull each other back in the same line, or, in
other words, to "trim ship."

*Dynamic Equilibrium.* In addition to being able to preserve its static
equilibrium and to possess proper longitudinal stability, the successful
airship must also maintain its dynamic equilibrium—the equilibrium of
the airship in motion. This may be made clear by referring to the
well-known expedients adopted to navigate the ordinary spherical
balloon. To rise, its weight is diminished by gradually pouring sand
from the bags which are always carried as ballast. To descend, it is
necessary to increase the total weight of the balloon and its car, and
the only method of accomplishing this is to permit the escape of some of
the gas, the specific lightness of which constitutes the lifting power
of the balloon. As the gas escapes, the thrust of the air on the balloon
is decreased and it sinks—the ascensional effort diminishing in
proportion to the amount of gas that is lost. The balloon, or the
container itself, being merely a spherical bag, on the upper
hemispherical half of which the net supporting the car presses at all
points, the question of deformation is not a serious one. Before it
assumed proportions where the bag might be in danger of collapsing, the
balloon would have had to come to earth through lack of lifting power to
longer sustain it. Owing to its far greater size, as well as to the form
of the surface which it presents to the air pressure, such a crude
method is naturally not applicable to the dirigible.

Dynamic equilibrium must take into account not only its weight and the
sustaining pressure of the air, but also the resistance of the air
exerted upon its envelope. This resistance depends upon the dimensions
and the shape of that envelope, and in calculations the latter is always
assumed to be invariable. Assume, for instance, that to descend the
pilot of a dirigible allowed some of the hydrogen gas to escape. As the
airship came down, it would have to pass through strata of air of
constantly increasing pressure as the earth is approached. The reason
for this will be apparent as the lower strata bear the weight of the
entire atmosphere above them. The confined gas will no longer be
sufficient to distend the envelope, the latter losing its shape and
becoming flabby. As the original form is no longer retained, the center
of resistance of the air will likewise have changed together with the
center of thrust, and the initial conditions will no longer obtain. But
as the equilibrium of the airship depends upon the maintenance of these
conditions, it will be lost if they vary.

*Function of Balloonets.* In the function of balloonets is realized the
importance of the principle established by Meusnier. It was almost a
century later before it was rediscovered by Dupuy de Lome in connection
with his attempts to make balloons dirigible. That the balloon must
always be maintained in a state of perfect inflation has been pointed
out. But gas is lost in descents and to a certain extent, through the
permeability of the envelope. Unless it is replaced, the balloon will be
only partially inflated. In view of the great volume necessary, it
requires no explanation to show that it would be impossible to replace
the gas itself by fresh hydrogen carried on the car. It would have to be
under high pressure and the weight of the steel cylinders as well as the
number necessary to transport a sufficient supply would be prohibitive.
Hence, Meusnier conceived the idea of employing air. But this could not
be pumped directly into the balloon to mix with the hydrogen gas, as the
resulting mixture would not only still be as inflammable as the former
alone, but it would also contain sufficient oxygen to create a very
powerful and infinitely more dangerous explosive. This led to the
adoption of the _air balloonet_.

In principle the balloonet consists of dividing the interior of the
envelope into two cells, the larger of which receives the light gas
while the smaller is intended to hold air and terminates in a tube
extending down to a pump in the car. In other words, a fabric partition
adjacent to the lower part of the envelope inside and subject to
deformation at will. In actual practice it consists of a number of
independent cells of this kind, longitudinally disposed along the lower
half of the interior of the envelope.

When the balloon is completely inflated with hydrogen, as at the
beginning of an ascent, these balloonets lie flat against the lower part
of the envelope, exactly like a lining. As the airship rises, the gas
expands owing to the reduction in atmospheric pressure at a higher
altitude, as well as to the influence of heat. With the increase in
pressure, uniform inflation is maintained by the escape of a certain
amount of gas through the automatic valves provided for the purpose.
Unless this took place, the internal pressure might assume proportions
placing the balloon in danger of blowing up. To avoid this, a pressure
gauge communicating with the gas compartment is one of the most
important instruments on the control board of the car, and should its
reading indicate a failure of the automatic valves, the pilot must
reduce the pressure by operating a hand valve. But as the car descends,
the increased external pressure causes a recontraction of the gas until
it no longer suffices to fill the envelope. To replace the loss the air
pumps are utilized to force air into the air balloonets until the sum of
the volumes of gas and air in the different compartments equals the
original volume. In this manner, the initial conditions, upon which the
equilibrium of the airship is based, are always maintained.

This is not the only method of correcting for change in volume, nor of
maintaining the longitudinal stability of the whole fabric, the
importance of which has already been detailed, but experience has shown
that it is the most practical. It is possible to give the balloon a
rigid frame over which the envelope is stretched and to attach the car
by means of a rigid metal suspension, as in the various Zeppelin
airships, or to take it semi-rigid, as in the Gross, another German type
in which Zeppelin’s precedent was followed only in the case of the
suspension. To prevent deformation by this means, the balloon is
provided with an absolutely rigid skeleton of aluminum tubes. This
framing is in the shape of a number of uniform cylindrical sections, or
gas compartments, each one of which accommodates an independent balloon,
while over the entire frame a very strong but light fabric constituting
the outer or protecting envelope is stretched taut. The idea of the
numerous independent balloons is to insure a high factor of safety as
the loss of the entire contents of two or three of them through accident
would not dangerously affect the lifting power of the whole. The
numerous wrecks which attended the landings of these huge non-flexible
masses during the early stages of their development led to the provision
of some form of shelter wherever they were expected to land. Even now,
they are practically unmanageable in the air during a fierce wind and
must be allowed to sail under control until the wind has spent itself.

The system of air balloonets has accordingly been adopted by every other
designer, in variously modified forms, as illustrated by the German
dirigible Parseval, in which but two air bags were employed, one at
either end. They were interconnected by an external tube to which the
air-pump discharge was attached, and were also operated by a
counterbalancing system inside the gas bag, by means of which the
inflation of one balloonet, as the after one, for example, caused the
collapse of the other.

_Influence of Fish Form of Bag._ But a condition of dynamic equilibrium
can not be obtained with the combined aid of the precautions already
noted to secure longitudinal stability and that of the air balloonet in
maintaining uniform inflation. Why this is so will be clear from a
simple example. If a simple fusiform or spindle-shaped balloon be
suspended in the air in a horizontal plane, the axis of which passes
through its center of gravity, it would be practically pivoted on the
latter and would be extremely sensitive to influences tending to tilt it
up or down. It would be in a state of "indifferent" longitudinal
equilibrium. As long as the axis of the balloon remains horizontal and
the air pressure is coincident with that axis, it will be in
equilibrium, but an equilibrium essentially unstable. Experiment proves
that the moment the balloon inclines from the horizontal in the
slightest degree, there is a strong tendency for it to revolve about its
center of gravity until it stands vertical to the air current, or is
standing straight up and down. This, of course, refers to the balloon
alone without any attachments. Such a tendency would be fatal, amounting
as it does to absolute instability.

If instead of symmetrical form, tapering toward both ends, a pisciform
balloon be tried, it will still evidence the same tendency, but in
greatly diminished degree. This is not merely the theory affecting its
stability but represents the findings of Col. Charles Renard, who
undoubtedly did more to formulate the exact laws governing the stability
of a dirigible than any other investigator in this field. His data is
the result of a long and methodically carried out series of experiments.
In the case of the pisciform balloon, the disturbing effect is due in
unequal degree, to the diameter of the balloon and its inclination and
speed, whereas the steadying effect depends upon the inclination and
diameter, but not on the Speed. The disturbing effect, therefore,
depends solely on the speed and augments very rapidly as the speed
increases. It will, accordingly, be apparent that there is a certain
speed for which the two effects are equal, and beyond which the
disturbing influence, depending on speed, will overcome the steadying
effect.

To this rate of travel, Renard applied the term "critical speed," and
when this is exceeded the equilibrium of the balloon becomes unstable.
To obtain this data, keels of varying shapes and dimensions were
submitted to the action of a current of air, the force of which could be
varied at will. In the case of the La France, the first fish-shaped
dirigible, the critical speed was found to be 10 meters, or
approximately 39 feet per second, a speed of 21.6 miles per hour, and a
24-horse-power motor suffices to drive the airship at this rate of
travel. But the internal combustion motor is now so light that a
dirigible of this type could easily lift a motor capable of generating
80 to 100 horse-power. With this amount of power, its theoretic speed
would be 50 per cent greater, or 33 miles an hour. But this could not be
accomplished in practice as long before it was reached the stability
would become precarious. As Colonel Renard observed in the instance just
cited, "If the balloon were provided with a 100-horse-power motor, the
first 24 horse-power would make it go and the other 76 horse-power would
break our necks."

_Steadying Planes._ It is accordingly necessary to adopt a further
expedient to insure stability. This takes the form of a system of rigid
planes, both vertical and horizontal, located in the axis of the balloon
and placed a considerable distance to the rear of the center of gravity.
With this addition, the resemblance of the after end of the balloon to
the feathering of an arrow is apparent, while its purpose is similar to
that of the latter. For this reason, these steadying planes have been
termed the _empennage_, which is the French equivalent of "arrow
feathering," while its derivative _empennation_ is employed to describe
the counteraction of this disturbing effect. In the La France, which
measured about 230 feet in length by 40 feet in diameter, the area of
the planes required to accomplish this was 160 square feet, and the
planes themselves were placed almost 100 feet to the rear of the center
of gravity. By referring to the illustrations of the various French
airships, the various developments in the methods of accomplishing this
will be apparent.

[Illustration: Fig. 8. La Ville de Paris Showing Balloonets]

In the Lebaudy balloon, it took the form of planes attached to the
framework between the car and the balloon. In La Patrie and La
Republique, the resemblance to the feathered arrow was completed by
attaching four planes in the form of a cross directly to the stern of
the balloon itself. But as weight, no matter how slight, is a disturbing
factor at the end of a long lever, such as is represented by the
balloon, Renard devised an improvement over these methods by conceiving
the use of hydrogen balloonets as steadying planes. The idea was first
embodied in La Ville de Paris, Fig. 8, in the form of cylindrical
balloonets, and as conical balloonets on the Clement-Bayard. These
balloonets communicate with the gas chamber proper of the balloon and
consequently exert a lifting pressure which compensates for their
weight, so that they no longer have the drawback of constituting an
unsymmetrical supplementary load.

*Location of Propeller.* The final factor of importance in the design of
the successful dirigible is the proper location of the propulsive effort
with relation to the balloon. Theoretically, this should be applied to
the axis of the balloon itself, as the latter represents the greater
part of the resistance offered to the air. At least one attempt to carry
this out in practice resulted disastrously, that of the Brazilian
airship Pax, while the form adopted by Rose, in which the propeller was
placed between the twin balloons in a plane parallel with their
horizontal axes, was not a success. In theory, the balloon offers such a
substantial percentage of the total resistance to the air that the area
of the car and the rigging were originally considered practically
negligible by comparison. Actually, however, this is not the case.
Calculation shows that in the case of any of the typical French airships
mentioned, the sum of the surface of the suspending rigging alone is
easily the equivalent of 2 square meters, or about 21 square feet,
without taking into consideration the numerous knots, splices, pulleys,
and ropes employed in the working of the vessels, air tubes
communicating with the air balloonets, and the like. Add to this
equivalent area that of the passengers, the air pump, other transverse
members and exposed surfaces, and the total will be found equivalent to
a quarter or even a third of the transverse section of the balloon
itself.

To insure the permanently horizontal position of the ship under the
combined action of the motor and the air resistance, a position of the
propeller at a point about one-third of the diameter of the balloon
below its horizontal axis will be necessary. Without employing a rigid
frame like that of the Zeppelin and the Pax, however, such a location of
the shaft is a difficult matter for constructional reasons.
Consequently, it has become customary to apply the driving effort to the
car itself, as no other solution of the problem is apparent. This
accounts for the tendency common in the dirigible to "float high
forward," and this tilting becomes more pronounced in proportion to the
distance the car is hung beneath the balloon. The term "deviation" is
employed to describe this tilting effect produced by the action of the
propeller. Conflicting requirements are met with in attempting to reduce
this by bringing the car closer to the balloon as this approximation is
limited by the danger of operating the gasoline motor too close to the
huge volume of inflammable gas. The importance of this factor may be
appreciated from the fact that if the car were placed too far from the
balloon, the propulsive effect would tend to hold the latter at an angle
without advancing much, owing to the vastly increased air resistance of
the much larger surface thus presented.

*Relations of Speed and Radius of Travel.* The various factors
influencing the speed of a dirigible have already been referred to, but
it will be apparent that the radius of action is of equally great
importance. It is likewise something that has a very direct bearing upon
the speed and, in consequence, upon the design as a whole. It will be
apparent that to be of any great value for military or other purposes,
the dirigible must possess not only sufficient speed to enable it to
travel to any point of the compass under ordinarily prevailing
conditions of wind and weather but also to enable it to remain in the
air for some time and cover considerable distance under its own power.

_Total Weight per Horsepower Hour._ As is the case in almost every point
in the design of the dirigible, conflicting conditions must be
reconciled in order to provide it with a power plant affording
sufficient speed with ample radius of action. It has already been
pointed out that power requirements increase as the _cube of the speed_,
making a tremendous addition necessary to the amount of power to obtain
a disproportionately small increase in velocity. In this connection
there is a phase of the motor question that has not received the
attention it merits up to the present time. The struggle to reduce
weight to the attainable minimum has made weight per horsepower
apparently the paramount consideration—a factor to which other things
could be sacrificed. And this is quite as true of aeroplane motors as
those designed for use in the dirigible. But it is quite as important to
make the machine go as it is to make it rise in the air, so that the
question of _total weight per horsepower hour_ has led to the
abandonment of extremely light engines requiring a great deal of fuel.

Speed is quite as costly in an airship as it is in an Atlantic liner. To
double it, the motor power must be multiplied by 8, and the machine must
carry 8 times as much fuel. But by cutting the power in half, the speed
is reduced only one-fifth. The problem of long voyages in the dirigible
is, accordingly, how to reconcile best the minimum speed which will
enable it to make way effectively against the prevailing winds, with the
reduction in power necessary to cut the fuel consumption down to a point
that will insure a long period of running.

When the speed of the dirigible is greater than that of the prevailing
wind, it may travel in any direction; when it is considerably less, it
can travel only with the wind; when it is equal to the speed of the
latter, it may travel at an angle with the wind—in other words, tack, as
a ship does, utilizing the pressure of the contrary wind to force the
ship against it. But as the air does not offer to the hull of the
airship, the same hold that water does to that of the seagoing ship, the
amount of leeway or drift in such a manoeuver is excessive. This applies
quite as much to the aeroplane as it does to the dirigible.



FRENCH DIRIGIBLES


*The First Lebaudy.* The interest evidenced by the German War Department
in Zeppelin’s airship was more than duplicated by that aroused in French
military circles by the success of the Lebaudy Brothers. Since 1900
these two brothers had been experimenting with dirigible balloons. Their
first dirigible—built by the engineer Juillot—made thirty flights, in
all but two of which it succeeded in returning to its starting point.
This machine was somewhat similar to the later types built by
Santos-Dumont and carried a 40-horsepower Daimler motor. A speed of 36
feet per second, or about 25 miles per hour, was obtained. During tests
in the summer of 1904, the balloon was dashed against a tree and almost
entirely destroyed.

*Lebaudy 1904.* The next year the "Lebaudy 1904" appeared. This was 190
feet long and had a capacity of 94,000 cubic feet of gas. The air bag
was divided into three parts and contained 17,600 cubic feet of air. It
was supplied with air from a fan driven by the engine, and an auxiliary
electric motor and storage battery were carried to drive the fan when
the gas engine was not working. The storage battery was also used to
furnish electric lights for the airship. A horizontal sail of silk was
stretched between the car and the gas bag, which had an area of
something over 1,000 square feet, and a sort of keel of silk was
stretched below it. A horizontal rudder, shaped like a pigeon’s tail,
was used at the rear, and immediately behind it were two V-shaped
vertical rudders. A small vertical sail was carried, which could be used
to assist in guiding the airship. The car was 16 feet long and was
rigidly hung 10 feet below the bag. It was provided with an inverted
pyramid of steel tubes meeting at an apex below the car to prevent
injury in alighting. Sixty-three ascents were made in 1904 with this
balloon, all of them comparatively successful, the longest being a
journey of 60 miles in two hours and forty-five minutes.

[Illustration: Fig. 9. La Patrie, French War Dirigible]

The next year a new and larger balloon equipped with a more powerful
motor was used. Many flights were made in tests for the French War
Department.

*La Patrie.* La Patrie was then built for the French government by the
Lebaudy Brothers and was of the same design as their earlier airships.
In speed it was nearly equal to Zeppelin’s, and its dirigibility was
nearly perfect. Fig. 9 shows a view of this airship in flight.

It was 200 feet long, and the 70-horsepower engine drove two propellers.
It could carry seven people and one-half ton of ballast. It carried four
people at a speed of 30 miles per hour. On its last trip it covered 175
miles in seven hours. A few days afterward, a heavy wind tore it away
from its moorings and it was blown out to sea and lost.

*La Republique and Le Jaune.* Two more airships of the same type, La
Republique and Le Jaune, followed this. These were tried by the French
government, in 1908, and both proved successful. La Republique is
illustrated in Fig. 10. The shape and equipment of the car are shown in
Fig. 11. The automobile type of radiator may be seen attached to the
side of the car. During a flight in the fall of 1909, a propeller blade
broke and was thrown clear through the balloon envelope, causing the
balloon to fall from a height of 500 feet. The four officers who formed
the crew of the dirigible were killed instantly.

*Clement-Bayard II.* The numerous factors that must be considered in the
design of a successful dirigible balloon as well as the many conflicting
conditions that must be reconciled have already been referred to in
detail. How these are carried out in practice may best be made clear by
a description of what may be considered as an advanced type of
dirigible, the Clement-Bayard II, Fig. 12, of French design, and the
most successful of the French military air fleet. Its predecessor, the
Clement-Bayard I, Fig. 13, made thirty voyages, some of them of
considerable distances, without suffering any damage, but a study of its
shortcomings led to their elimination in the following model.

[Illustration: Fig. 10. La Republique, French War Dirigible]

[Illustration: Fig. 11. Car of La Republique]

The pisciform shape of the first Clement-Bayard was retained but given
more taper, the dimensions being 248.6 feet overall by 42.9 greatest
diameter, this being but a short distance back of the bow. This gives it
a ratio of length to diameter of 5.76. The gas balloonet stabilizers
were eliminated altogether, Fig. 12. The total gas capacity is
approximately 80,000 cubic feet. Like all French dirigibles it is of the
true flexible type, the only rigid construction being that of the
framework of the car itself. To the latter are attached all rudders and
stabilizing devices, instead of making them a part of the envelope as
formerly. The latter is made of continental rubber cloth.

[Illustration: Fig. 12. Clement-Bayard II, French Dirigible]

Light steel and aluminum tubing are employed in the construction of the
frame supplemented by numerous piano-wire stays. This frame extends
almost the entire length of the envelope and carries at its rear end a
cellular, or box-kite, type of stabilizing rudder, instead of the former
gas balloonets employed on the Clement-Bayard I, Fig. 13. This cellular
rudder is in two parts, consisting of two units of four cells each, the
two groups being joined at the top, with a space between them. In
addition to acting as a stabilizer, this is also the direction rudder,
its leverage being increased by making the end planes somewhat larger
than the partitions of the cells. Between the cellular stabilizing
rudder and the envelope is placed the horizontal rudder for ascending or
descending. In the illustration this appears to be a flag, but it is in
reality a long rectangular plane, which may be tilted on its
longitudinal axis, the latter being at right angles to that of the
balloon. There are two air balloonets of about one-third the total
capacity of the balloon itself, and they are designed to be inflated by
large aluminum centrifugal blowers driven from the main engines
themselves.

[Illustration: Fig. 13. Clement-Bayard I]

There are two motors, each of 125 horsepower, both being of the same
conventional design, _i.e._, four cylinder four cycle vertical water
cooled. In fact, they are merely light automobile motors. The cylinders
have separate copper water jackets and the motors themselves are
muffled, which is a departure from the usual custom. Each drives a
separate propeller carried on top of the main frame through bevel
gearing.

The Clement-Bayard II made itself famous by its rapid and successful
flight from the suburbs of Paris across the Channel to London, in
October, 1910.

*Astra-Torres.* In reviewing the specifications of any of the big
dirigibles, the observer cannot fail to be struck by the excessive
amount of power necessary to drive them at speeds which are lower than
the minimum, or landing speeds, of many aeroplanes. When a speed of 45
miles per hour was first reached by a dirigible, it was acclaimed as a
great feat. But this comparatively moderate rate of travel was surpassed
only by increasing the number of motors and their horsepower until the
fuel consumption became exceedingly high. This necessitated the carrying
of a great weight of fuel and cut down correspondingly the useful load
that the dirigible was capable of lifting as well as restricted its
radius of flight at full speed. Until aerodynamic research had
demonstrated the contrary, the necessity for such a tremendous amount of
power was considered necessary to overcome the head resistance of the
balloon itself. Research brought out in a striking manner how great a
proportion of the total head resistance of an aeroplane was due to the
struts and bracing wires. In the construction of the different types of
airships illustrated, it will be noted that the gear provided for
suspending the car or cars below the balloon requires a great number of
cables. Later developments showed that by eliminating the great amount
of head resistance caused by these numerous surfaces, the speed of a
dirigible could be increased by over 50 per cent with the same amount of
power.

[Illustration: Fig. 14. Section of Astra-Torres, Illustrating Method of
Suspension. _CB_, Bracing of Heavy Fabric Bands; _SR_ and _A_,
Suspension Ropes and Cable Passing through Envelope; _S_, Expansion
Sleeve in Envelope; _CC’_, Ropes to Sides of Car; _E_, Envelope]

_Improved Suspension._ The shortcoming of the dirigible with reference
to suspension was realized more than ten years previous by a
Spaniard—Torres—but owing to lack of financial support, he was unable to
put his idea into execution. The principle he evolved is made clear by
Fig. 14, which gives a section of an Astra-Torres dirigible illustrating
the method of suspension. Instead of the ropes _SR_ used to suspend the
car being attached to bands passing around the envelope, these
reinforcing bands _CB_ and also the ropes fastened to them are placed
inside the envelope, thus eliminating head resistance from those
sources.

_Performance._ Failing to obtain any encouragement in Spain, Torres
finally succeeded in interesting the French Astra Company, which built a
vedette, or scouting airship, of a little over 50,000 cubic feet
capacity. It was pitted against the Colonel Renard, at that time the
leading unit in the French aerial navy and the fastest airship in
commission. The small Torres dirigible so completely outclassed its huge
competitor that another of close to 300,000 cubic feet capacity was
built and tried against the Parseval with similar results. An
Astra-Torres dirigible built for the British government showed a speed
in excess of 50 miles per hour. This particular dirigible has been at
the front in France almost since the outbreak of hostilities and has
rendered considerable valuable service. Its success led the French
Government to order a huge replica of it, having a capacity of over
800,000 cubic feet and with motors developing 1,000 horsepower, which
would give it an indicated speed of 60 miles per hour. So confident were
its builders of attaining or even exceeding this, that an order for a
second and even larger airship of the Astra-Torres design was placed
before the first one was finished. This is also fitted with motors
aggregating 1,000 horsepower and displaces 38 tons, making it larger
than any Zeppelin that had been constructed up to the time it was built.
As its construction and trials were undertaken during the war, no
details have been published, but it is said on good authority that its
speed exceeds 60 miles per hour, so that it is faster than any of the
German dirigibles.

_Construction._ Unlike the German dirigibles, the larger types of which
have been characterized by a rigid frame, the Astra-Torres is a flexible
airship and, owing to its method of suspension, its external appearance
is decidedly unconventional, since the envelope instead of being of the
usual cigar shape is more like a triangular bundle of three cigars with
the third one on top. At the point where the three envelopes join, as
shown in section, Fig. 14, heavy cloth bands _CB_ are stretched across
the arcs, forming a chord across each arc, the three chords comprising
an inverted triangle. The suspension ropes _SR_ are attached to the
opposite ends of the base of this inverted triangle and converge in
straight lines downward through the gas space, so that the air
resistance offered by the ropes is practically eliminated since only a
very small part of the suspension system appears outside the envelope.
This external part consists of vertical cables _A_ attached to the
collecting rings of the bracing system and extending downward through
special accordion sleeves _S_ which permit the free play necessary at
the points where they pass through the outer wall of the envelope. These
sleeves also have another function—that of permitting the escape of gas
under the pressure of expansion. A short distance below the envelope _E_
each of these cables splits into two parts _C_ and _C’_ attached to
opposite sides of the car.

The British airship mentioned is provided with but one car, but the
larger French ships have two placed tandem, each of which carries a
500-horsepower motor driving two two-bladed propellers of large
diameter. While the form of envelope made necessary by this construction
increases the frictional resistance, this is negligible in comparison
with the great saving in power effected by the method of suspension, not
to mention the greater simplicity of construction.



GERMAN DIRIGIBLES


*Early Zeppelin Airships.* At the same time that Santos-Dumont was
carrying on his hazardous experiments, the problem was being attacked
along slightly different lines by Count Zeppelin.

It will be remembered that Dumont experienced much trouble on account of
the envelope of his balloon being too flexible, causing it to crumple in
the middle and to become distorted in shape from the pressure of the
air. His efforts to overcome this by the employment of air bags did not
meet with great success, even in his later types.

[Illustration: Fig. 15. Zeppelin Dirigible Rising from Lake Constance]

_Construction._ Zeppelin employed a very rigid construction. His first
balloon, which was built in 1898, was the largest which had ever been
made. It is illustrated in Fig. 15, which shows his first design
slightly improved. It was about 40 feet in diameter and 420 feet long—an
air craft as large as many an ocean vessel. The envelope consisted of
two distinct bags, an outer and an inner one, with an air space between.
The air space between the inner and outer envelopes acted as a heat
insulator and prevented the gas within from being affected by rapid
changes of temperature. The inner bag contained the gas, and the outer
one served as a protective covering. In the construction of this outer
bag lies the novelty of Zeppelin’s design. A rigid framework of strongly
braced aluminum rings was provided and this was covered with linen and
silk which had been specially treated to prevent leakage of gas. The
inner envelope consisted of seventeen gas-tight compartments which could
be filled or emptied separately. In the event of the puncture of one of
them, the balloon would remain afloat. An aluminum keel was provided to
further increase the rigidity. A sliding weight could be moved backward
or forward along the keel and cause the nose of the airship to point
upward or downward as desired. This would make the craft move upward or
downward without throwing out ballast or losing gas. Lender each end of
the balloon a light aluminum car was rigidly fastened and in each was a
16-horsepower Daimler gasoline engine. The two engines could be worked
either independently of each other or together. Each engine drove a
vertical and horizontal propeller. The propellers each had four aluminum
blades. As will be seen from Fig. 15, the ears were too far apart for
ordinary means of communication and so speaking tubes, electric bells,
and an electric telegraph system were installed.

_First Trials._ Very little was known as to the effects of alighting on
the ground with such a rigid affair as this vessel, therefore the cars
were made like boats so that the airship could alight and float on the
water. The first trials were made over Lake Constance in July, 1900. The
mammoth craft was housed in a huge floating shed, and the vessel emerged
from it with the gas bag floating above and the two cars touching the
water. She rose easily from the water, and then began a series of
mishaps such as usually fall to the lot of experimenters. The upper
cross stay proved too weak for the long body of the balloon and bent
upward about 10 inches during the flight. This prevented the propeller
shafts from working properly. Then the winch which worked the sliding
weight was broken and, finally, the steering ropes to the rudders became
entangled. In spite of all this, a speed of 13 feet per second, or about
9 miles per hour, was obtained. These breakages made it necessary to
descend to the lake for repairs and in alighting the framework was
further damaged by running into a pile in the lake. The airship was
repaired and another flight was made later in the year, during which a
speed of 30 feet per second, or 20 miles per hour, was obtained.

_Second Airship._ Zeppelin had sunk his own private fortune and that of
his supporters in his first venture, and it was not till five years
later that he succeeded in raising enough money to construct a second
airship. No radical changes in construction were made in the new model,
but there were slight improvements made in all its details. The balloon
was about 8 feet shorter than the original and the propellers were
enlarged. Three vertical rudders were placed in front and three behind
the balloon, and below the end of the craft horizontal rudders were
installed to assist in steering upward or downward. The steering was
taken care of from the front car.

The most important change was made possible by the improvement in
gasoline engines during the preceding five years. Where, in the earlier
model, he had two 16-horsepower engines, he now used an 85-horsepower
engine in each car, with practically the same weight. In fact, the total
weight of the vessel was only 9 tons, while his first airship weighed 10
tons.

His new craft made many successful flights. One was made at the rate of
38 miles per hour and continued for seven hours, covering a total
distance of 266 miles.

*Later Zeppelins.* The later Zeppelins embody no remarkable changes in
design, the principal alteration being in size. One of these is
illustrated in Fig. 16. In this the gas bag was increased to 446 feet in
length and it held over 460,000 cubic feet of gas. This gave it a total
lifting power of 16 tons. With this, Zeppelin made a voyage of over 375
miles. He was in the air for twenty hours on this trip and carried
eleven passengers with him.

[Illustration: Fig. 16. Zeppelin Airship in Flight]

In August, 1908, the Zeppelin left its great iron house at
Friedrichshafen and sailed in a great circle over Lake Constance. The
day after it started, however, it was destroyed by a storm, and sudden
destruction from one cause or another has ended the existence of
practically every one of the Zeppelins built since, usually after a very
brief period of service.

*Shape and Framing.* In the early days of dirigible design the data upon
which the shape and proportions of the envelope were based were purely
empirical. Schwartz, Germany’s pioneer in this field, adopted the
projectile as representing the form offering the least air resistance
and accordingly designed his envelope with a sharply pointed bow and a
rounded-off stern, giving it a length four times its diameter. Zeppelin
did not agree with these conclusions and adopted a pencil form, rounded
at the nose and tapering to a sharp point at the stern, making the
length nine to ten times the diameter. Subsequent research work in the
aerodynamic laboratory has demonstrated that the most efficient form for
air penetration is one having a length six times its maximum diameter
with the latter situated at a point four-tenths of the total length from
the bow. It has likewise been proved that an ellipse is more efficient
than either the projectile or pencil form and that tapering to a sharp
point at the stern offers no particular advantage. As a result, the most
approved form resembles the shape of a perfecto cigar, the nose being
somewhat blunter than the after end. This form is likewise that of the
swiftest-swimming fishes and has been shown to have the least head
resistance as well as the minimum skin friction; it results in a section
to which the term _stream-line_ has been applied, and it is now employed
on all exposed non-supporting surfaces on aeroplanes, such as the struts
and even the bracing cables. Laboratory research has demonstrated that
it is worth while to reduce the head resistance of even such apparently
negligible surfaces as those presented by these wires and cables and,
therefore, they are stream-lined by attaching recessed triangular strips
of wood to their forward sides.

_Framing Details._ Despite this, the builders of the Zeppelins have
adhered to the original pencil shape with but slight modifications at
the bow and stern, probably because that shape is much easier to build
and assemble from standard girders. The form of girder employed is shown
in Fig. 17, while the complete assembly of the frame is illustrated in
Fig. 18. The girders form the longerons, or longitudinal beams, running
the entire length of the rigid frame and supported at equidistant points
by ring members built of similar girder sections. The fourth ring from
the nose and each alternate ring after that are further braced by being
trussed to the longitudinal beams around their entire circumferences, as
shown in Fig. 18. The larger V-shaped truss at the bottom forms the
gangway, which is now placed inside the envelope instead of being
suspended beneath it, as formerly. This is done to eliminate the head
resistance set up by the additional surface thus exposed. In the first
instance in which this gangway was incorporated in the envelope, no
provision was made for ventilation, and the ship was wrecked by a gas
explosion. Regardless of how tight the fabric is made, gas is always
oozing out through it to a greater or less extent. This fact is now met
by providing ventilating shafts leading from the gangway to the upper
surface of the envelope. Additional shafts through the envelope lead to
gun platforms, forward, amidships, and aft, and are reached by aluminum
ladders.

[Illustration: Fig. 17. Trellis Type of Aluminum Girder used in
Longitudinals of Zeppelin Frame]

[Illustration: Fig. 18. Aluminum Frame Construction of Zeppelin Hull]

_Framing of Schutte-Lanz Type._ It has become customary to refer to all
large German airships as Zeppelins, but many of those used during the
past three years have been of the Schutte-Lanz build, which is also a
rigid frame type of dirigible but has been designed with a view of
overcoming some of the disadvantages of the aluminum frame construction
encountered in the use of the Zeppelin. The length and diameter of the
latter airships are such that, no matter how rigidly the framing is
assembled, there is more or less sag. When the sag exceeds a certain
amount, the frame is apt to buckle at the point where it occurs,
involving expensive repairs or wrecking the airship altogether. To
overcome this difficulty, the Schutte-Lanz type employs a rigid frame of
flexible material, namely, laminated wood in strip form, held together
at joints and crossings by aluminum fittings and braced inside by
cables. As shown by Fig. 19, no rigid longitudinal beams are employed,
the only girders used being rings, to which a network built of the wood
strips is attached. Starting at the nose, each continuous strip follows
an open spiral path such as would be traced in the air by a screw of
very large pitch, in fact, approximating the rifling of a gun barrel. It
will also be noted from the illustration that the form of the
Schutte-Lanz airship is the cigar-shape, which laboratory research has
shown to be the most efficient.

[Illustration: Fig. 19. Schutte-Lanz Type of Frame Construction of
Laminated Wood with Aluminum Fittings]

The use of wood in conjunction with the spiral construction of the
supporting members of the framing affords the maximum degree of
flexibility, since the displacement of any of these members under
stresses of either tension or compression would have to be very great to
cause damage to the frame as a whole. The frame not being rigid,
strictly speaking, either as units or as a complete assembly, stress at
any particular point would simply cause all the members near that point
to give in the direction of the strain, and the rest of the frame would
accommodate itself to their change of position by either elongating or
shortening slightly. In addition to these advantages, the Schutte-Lanz
type of construction is said to be lighter than the Zeppelin for an
airship of the same load-carrying capacity.

*Power Plant.* Compared with their successors of war times, the early
Zeppelins were mere pigmies where power is concerned. Many of these
pioneers were driven by less than 100 horsepower all told, whereas in
the later types no single motor unit as small as this total has been
employed. The motors used most largely have been the 160-horsepower
Mercedes and the 200-horsepower Maybach, both of which are described in
detail under the title "Aviation Motors." From five to ten of these
units have been used on a single ship, giving an aggregate in some of
the latest types of close to 2,000 horsepower. Power has been applied
through five or six propellers to limit their diameter and to guard
against the breakdown of any one of the units putting the power plant
out of commission as a whole. To distribute the weight of the engines
equally and to insure each propeller a position in which it can work in
undisturbed air, the engines have been placed at widely separated points
on the airship and in different planes so that no two are coaxial. The
main engine room is usually located in a cabin just back of the
operating bridge and wireless room, while the remaining motors are
suspended in independent gondolas at different points along the sides.
Where more than 1,000 horsepower has been used, each of these gondolas’
has been fitted with two motors placed side by side and so coupled that
either one or both may be employed to drive the single propeller carried
by the propelling car. All the more recent propellers have been of the
two-bladed type.

*Control Surfaces.* The numerous expedients formerly resorted to by
various designers in providing for stabilizing, steering, and elevating
surfaces have been abandoned for forms that are practically a
duplication of aeroplane practice. Experience demonstrated that the
different types of multiplane rudders, elevators, and stabilizing
surfaces employed in earlier days not only offered no operating
advantages but were actually detrimental, in that they increased the
head resistance unnecessarily. Moreover, their complication meant
increased weight and weaker construction. They have accordingly been
displaced by monoplane surfaces which are of exactly the same type of
construction as those used on the aeroplane and the location and
proportions of which are very evidently based on aeroplane practice.
Both the horizontal and vertical stabilizers are of approximately
triangular form and have the steering and elevating surfaces hinged to
them at their after ends, so that, except for the pointed extremity of
the envelope which extends beyond them, the tail unit of the later
Zeppelins is practically the same as the empennage of an aeroplane. The
horizontal surfaces are apparently depended on entirely to effect the
ascent and descent, there being no evidence of swiveling propellers by
means of which the power of the engines could be employed to draw the
airship up or down. The great weight of ballast carried is, of course,
in the form of water, but this is discarded in order to ascend only when
the power of the engines exerted against the elevating planes is no
longer capable of keeping the airship at the altitude desired. In the
low temperatures encountered in night flights, however, the contraction
of the hydrogen gas is so great that the crew has found it necessary to
reduce the weight by discarding not only every pound of ballast but, as
far as possible, everything portable. Despite this, several airships
have fallen when their fuel supply was exhausted, one coming to the
ground in Scotland, two dropping into the North Sea, and three or four
falling in France.

*Operating Controls.* All the operating controls are centered at the
navigating bridge, which is inclosed to form the commander’s cabin. By
means of push buttons, switches, levers, and wheels every operating
function required is set into motion from this central point. Whether
auxiliary motors are carried for the purpose of pumping air into the
balloonets or this is one of the duties of the main engine just back of
the wireless room does not appear, but with the aid of a push button
board the amount of air in any of the balloonets may be increased or
decreased at will. There is a control button for each operation, or two
for each balloonet, which fact necessitates a rather forbidding looking
board, since the more recent Zeppelins have seventeen to nineteen gas
bags within each of which is incorporated an air balloonet.

The amount of fuel supplied to any one of the motor units can likewise
be controlled from a central board, and this is also true of the ballast
release apparatus, so that water can be emptied from any one of the
ballast tanks at will, thus facilitating ascent or descent by lightening
one end or the other. Elevating and steering surfaces are operated by
small hand-steering wheels with cables passing around their drums, a
member of the crew being stationed at each of these controlling wheels.
Owing to the number of motors used, the instrument board is the most
formidable appearing piece of apparatus on the bridge, since there is a
revolution counter for each power unit in addition to the numerous other
instruments required. Some of these instruments are the aneroid
barometer for indicating the altitude, transverse and longitudinal
clinometers to show the amount of heel and the angle at which the
airship is traveling with relation to the horizontal, the anemometer, or
air-speed indicator, manometers, or pressure gauges, for each one of the
gas bags, fuel and ballast supply gauges, drift indicators, electric
bomb releasers, mileage recorders, and the like. In addition to these,
there are a large chart and a compass, so the navigating bridge of a
Zeppelin combines in small space all the instruments to be found in the
engine room and on the bridge of an ocean liner besides several which
the latter does not require. That the proper coordination of all the
functions mentioned is an exceedingly difficult task for one man seems
evident from the numerous Zeppelins that have apparently wrecked
themselves.

*Crew Carried.* In the various Zeppelins that have been captured or shot
down by the British or French, the personnel has varied from fifteen to
thirty men but in the majority of instances has not exceeded twenty. The
positions and duties are about as follows: The commander,
lieutenant-commander, and chief engineer, and possibly a navigating
officer are stationed at the bridge. Two or three of the crew are also
stationed there to work the manually operated controls. In the cabin
just back of the bridge are two wireless operators and one or two engine
attendants for the motors in the engine room behind the wireless room. A
similar number of engine attendants are stationed in the after engine
room and there is at least one attendant for each of the other motor
units. One man is stationed at each machine gun, of which there are
three to five on the "roof" and two in each car, and at least as many
bombers are needed to load the "droppers." As a reserve there are
usually an additional gun pointer for each gun and an extra engine
attendant, since to run continuously most of the crew would have to
stand watch and watch as in marine practice. The sleeping accommodations
consist of canvas hammocks slung in the gangway.

*Explosives Carried.* In addition to a liberal supply of ammunition for
the machine guns, a large weight of bombs is carried, though the
quantity as well as the size of the bombs themselves has been
exaggerated in the same or even greater ratio than that which has proved
characteristic of the German military press-agency service. The bombs
are carried suspended in racks amidships, and the bomb droppers are also
located at that part of the ship so that the release of the bombs will
not upset the longitudinal equilibrium of the craft. The bomb-dropping
apparatus is controlled electrically from the navigating bridge but may
also be operated by hand from the same point. It has been reported by
the Germans that their latest types of Zeppelins are capable of dropping
bombs weighing 1 ton each. In view of the effect that the sudden release
of a weight of 1 ton would have on the airship itself, this is
manifestly very much of an exaggeration. Zeppelin bombs that have failed
to explode have never exceeded 200 to 300 pounds and many of those
employed are doubtless still lighter. So far as the total amount carried
is concerned, many of the later airships doubtless are capable of
transporting 2 to 3 tons and still carrying sufficient fuel, though
adverse conditions would prevent their return, as has frequently
happened.



BRITISH WAR DIRIGIBLES


*Adoption of Small Type.* German designers have continued to pin their
faith blindly to the huge rigid type, despite the fact that prior to the
war almost a dozen of these costly machines met with disaster as fast as
they could be turned out. Since the war started, their destruction has
kept pace pretty closely with their building without their accomplishing
anything of military value. The British naval aeronautic service, on the
other hand, appreciated the futility of such tremendous and unwieldy
construction and, after a single demonstration of its uselessness,
abandoned it altogether. This single attempt was the ill-fated Mayfly,
which was most appropriately named, since its performance resolved into
a certainty the doubt expressed by its title. In being taken out of its
shed, the framing of the airship was damaged, and it collapsed a few
minutes later so that it never did fly. One of the early types of small
British dirigibles is shown in Fig. 20.

Attention has since been concentrated in most part on the construction
of aeroplanes in constantly increasing numbers, although the dirigible
has not been given up altogether. However, its restricted usefulness as
well as the necessary limitations of its effective size has been
recognized. Early in the war Great Britain planned the construction of
fifty small dirigibles, of both the rigid and nonrigid types, all of
which have undoubtedly since been completed. They are small airships
designed chiefly for scouting and short-range bombing raids over camps
when in army service and for coast patrol and submarine hunting as an
aid to the naval forces. While no specifications are available, the
cubic capacity of these patrol airships probably does not exceed 50,000
to 75,000 cubic feet, their over-all length being approximately 100 to
125 feet.

[Illustration: Fig. 20. An Early Type of Small British Dirigible]

*Aeroplane Features.* To simplify the construction and at the same time
minimize the amount of head resistance, the car consists of an aeroplane
fuselage of the tractor type, fitted with a comparatively small
motor—under 100 horsepower—and having accommodations for a pilot and an
observer in two cockpits, placed tandem. The control surfaces are also
similar to those used in aeroplane construction. Despite their low
power, these dirigibles can make 40 miles an hour, owing to their
greatly reduced head resistance. Instead of employing either an
auxiliary blowing motor or a blower driven by the motor itself, the
supply duct to the air balloonet is made rigid and is sloped forward so
that its open end comes directly in the slip stream of the propeller;
thus the latter serves to inflate the balloonet as well as to drive the
dirigible. The desired amount of inflation is controlled by a valve.

[Illustration: Fig. 21. Side and End Views of British Astra-Torres
Dirigible Used for Anti-Submarine Patrol Service]

*Use in Locating Submarines.* Many of these small scouting and
naval-patrol dirigibles have given a good account of themselves and
comparatively few have met with accident or have been destroyed by the
enemy. On frequent occasions they have been very successful in locating
submarines below the surface, since the body of the under-water boat is
readily detected from an altitude of a thousand feet or more, even
though submerged to a great depth and despite a heavy ripple on the
surface that makes the water absolutely opaque when viewed from the deck
of a ship. Doubtless they will be employed to an increasing extent as
the hunt for the submarine becomes more and more intensive, though their
use is very much restricted during the winter months, owing to the
frequent and severe storms encountered.

*British Astra-Torres.* A number of comparatively small Astra-Torres
dirigibles have also been built in Great Britain for coast patrol and
anti-submarine work. The line drawing at the left of Fig. 21 illustrates
the general design and construction of these small airships, while the
various letters indicate the different parts of the gas container, air
balloonets, suspension and car, and the end view at the right of the
figure shows the small amount of head resistance offered by the
suspension of this type as compared with that of the usual form of
nonrigid dirigible. _A_ is the balloon itself, or main gas container,
the pressure relief valve for which is located at _M_. _BB_ are the air
balloonets connected with the blower _H_ in the car. In the illustration
these balloonets are shown fully inflated as they would be after the gas
bag had lost a considerable proportion of its original contents through
leakage or expansion. At the beginning of a flight, when the gas bag is
fully inflated with hydrogen, they lie perfectly flat along the lower
side of the envelope, being brought into service only as they are needed
to keep the envelope distended to its full volume.

The novel method of suspension to which this type of dirigible owes its
greater speed and fuel economy, because of the reduction of the head
resistance, is shown by the numerous supporting ropes _O-O-O_, which
terminate in a comparatively few cables attached to the car. In the
small British airships referred to here, there is but one small car
designed to carry a crew of two men and the engine is of comparatively
low power, driving a propeller at either end of the car, but in the
large French dirigibles of the same type, two large cars are placed
tandem some distance apart and are fitted with 500-horsepower motors.
The various parts indicated by the letters are: _CC_ propellers, _D_
motor, _F_ space for pilot and crew, _G_ fuel and oil tanks, _J_ guide
rope, _K_ gas valve, _LL_ air valves, _NN_ balloonet cable, _P_ rudder,
_Q_ stabilizer, _RR_ bracing cables, and _S_ the car itself.



MILITARY USES OF ZEPPELINS


*Limitations of Use.* Nothing excites the Teutonic imagination so
strongly as things military to which the characteristic German adjective
_kolossal_ can be enthusiastically applied. It was for this reason that,
despite its uniform record of tragic disaster for years before the war,
the Germans pinned their faith to the Zeppelin as a weapon that could
not fail to strike terror to the hearts of the British and French and
make them hasten "to sue for peace." However, apart from its reputed
employment on the single occasion that the German grand fleet left the
security of the Kiel Canal, it is not known to have been used in any
purely military operation. The aeroplane has been developed to a point
that, in spite of the ability of the Zeppelin to ascend rapidly when
hard pressed, would make it suicidal for one of the huge gas bags to
sally forth in daylight, unless attended by a large number of battle
planes to prevent enemy flying machines from attacking it. No such use
of the Zeppelin has been recorded thus far. Consequently, it has been
used only in nocturnal bomb-dropping expeditions, chiefly directed
against London and only undertaken when weather conditions made
detection difficult. In order to carry these out, it has been necessary
to establish stations in Belgium, since the fuel consumption of the
Zeppelin is so great that, even with its tremendous fuel supply of 3 to
5 tons, a flight to London and return to points well within the German
border is impracticable. The first raids of this character were carried
out successfully, but subsequent attempts were marked by the loss of one
or two airships on each occasion, so that the practice was abandoned as
being too expensive for the results attained and aeroplanes were
substituted.

*Number Built.* Taking it for granted that the numbering of the German
airships has been consecutive, the total number built during the first
three and one-half years of the war by the Germans would be between
eighty and one hundred. All large German airships have come to be
commonly termed Zeppelins, but a number of them were of the Schutte-Lanz
type, almost equally large and also characterized by rigid construction,
which, however, was of wood with aluminum fittings instead of being all
metal, as it was found that the huge metal frame accumulated a static
charge of high potential that was responsible for igniting the gas in
one or two instances.

*Weakness of Type.* The L-I (_Luftschiff_, or airship), the first of the
German airships designed for purely military purposes, was a Zeppelin
525 feet long by 50 feet in diameter, of 777,000 cubic feet capacity,
and 22 tons displacement. Its three sets of motors developed 500
horsepower and it had a speed of 52 miles per hour. It was launched at
Friedrichshafen in 1912, and after a number of successful cross-country
trips, it was tried in connection with naval maneuvers off Heligoland.
Before the trial had proceeded very far, a sudden squall broke the
backbone of the huge gas bag and hurled it into the sea, drowning
fifteen out of the crew of twenty-two. It is a striking commentary on
the frailness of these aerial monsters that every one of the big
airships built up to that time had met disaster in an equally sudden
manner but from a totally different cause in each instance. The L-II was
slightly shorter but had 5 feet longer beam and displaced 27 tons. She
was designed particularly for naval use, had four sets of motors
developing 900 horsepower, and was fitted with a navigating bridge like
that of a ship. It was confidently thought that all possible
shortcomings had been remedied and success finally achieved in the L-II,
but before there was any opportunity to demonstrate its efficiency, the
airship exploded in mid-air, killing its entire crew.

_Effectiveness Grossly Overrated._ Despite this unbroken chain of
disasters, the German official press bureau spread broadcast the prowess
of the Zeppelin, its magnificent ability, and its remarkable
achievements as an engine of war—in theory, since this was a year or two
prior to the outbreak of hostilities. Had it not been for the forced
descent of the Zeppelin IV at Luneville, where it was taken possession
of by the French, these tales might have been accepted at their face
value. But the log of the commander of this airship showed that its
maximum speed was but 45 miles per hour, the load 10,560 pounds, and the
ascensional effort 45,100 pounds. The fuel consumption averaged 297
pounds per hour while the fuel capacity was only sufficient for a flight
of seven hours. During its flight, it had reached an altitude of only
6,250 feet, to accomplish which over 3 tons of ballast had to be
dropped. It was also shown that the critical flying height of these huge
airships is between 3,500 and 4,000 feet, Zeppelin himself declaring
that his machines were useless above 5,000 feet. This probably accounts
for the fact that the early raids on English towns were carried out at a
height but slightly in excess of 2,000 feet. Later types, however, are
said to have reached high altitudes.

[Illustration: Fig. 22. Zeppelin L-49 Brought Down Intact by a French
Airman, Resting on Hillside near Bourbon-Les-Baines
_Copyright by Underwood and Underwood, New York_]

Shortly before the outbreak of the war the L-5 was completed. This had a
capacity of about 1,000,000 cubic feet, motors aggregating 1,000
horsepower or over, and a reputed speed of 65 miles per hour. Just what
was the fate of this particular ship did not become known, since
information of a military character has not been permitted to leak out
of Germany from that time on. But capture or destruction has accounted
for many of the intermediate numbers of the series; big German airships
have been brought down in England, in the North Sea, in France, and at
Saloniki, their loss culminating in the disaster to four out of the
fleet of five that attempted a raid over London but were caught by
adverse winds which exhausted their fuel supply so that they were blown
out of control, toward the south of France. French anti-aircraft
batteries or aeroplanes accounted for three of these, while the fourth,
the L-49, was captured intact.

[Illustration: Fig. 23. Nose of Giant L-49 and Group of Sightseers
_Copyright by Underwood and Underwood, New York_]

*L-49.* An essential part of the equipment of every form of German
military apparatus is a means of destroying it in case of capture. In
the case of the big airships, the officers are provided with revolvers
loaded with incendiary bullets, which are fired into the gas bag, so
that until the L-49 was forced to descend in the south of France by the
activities of a battle plane, plus a lack of fuel, no airship of a
recent type had ever been captured intact. In this case, the commander
fired his pistol at the balloon but missed and was prevented from firing
again by a French peasant who "covered" him with a shotgun. The wireless
operator succeeded in using a sledge hammer on some of the apparatus of
the very completely equipped wireless cabin before he was captured but
did not do sufficient damage to prevent reassembly of the parts with
little trouble. With the exception of the earlier type of Zeppelin that
was forced to descend at Luneville prior to the war, the L-49 was the
first that was ever known to have landed undamaged in hostile territory,
as practically all the others were destroyed in the air, most of them
having been wrecked either by aeroplane or anti-aircraft fire. Fig. 22
shows the L-49 as it rested on a hillside at Bourbon-les-Baines, France,
and Fig. 23 shows a close view of the nose of the monster.

_Standardized Parts._ Comparing the L-49 with many of its predecessors
led to the conclusion that it was one of the latest types, but an
inspection of its construction revealed the use of many parts produced
in quantities from standard patterns as well as a lack of the finish
that has always characterized airship construction. Appearance and
comfort had both been sacrificed with a view to saving the last ounce of
superfluous weight in order to carry more fuel and ammunition. Evidently
the production of these large airships has been reduced to a
manufacturing basis and they are constructed in series in much the same
manner as motor cars, though on a reduced scale.

_General Design._ In its general construction the L-49 was along the
same lines that have characterized the Zeppelin since its inception, the
outer envelope being stretched over a rigid frame of aluminum girders,
inclosing a large number of independent balloons inflated with the usual
hydrogen gas, no trace being discovered of the non-inflammable gas, the
discovery of which had been hailed by the German press. The commander’s
cabin was suspended well forward with the wireless room directly behind
it, while a V-shaped gangway, recessed in the envelope proper so as to
present no additional head resistance, ran back from the latter the
whole length of the ship. This and the gun platform on top, mounting two
machine guns and reached by a ladder suspended in a well amidships, have
been familiar features of all the recent Zeppelins. The main envelope
contained nineteen independent gas bags, each of which was made integral
with an air balloonet to take care of the expansion and contraction of
the hydrogen with varying altitudes and temperatures. Distributed along
the lower part of the frame inside the envelope were a series of
50-gallon water-ballast tanks.

_Power Plant._ No less than nine large motors were employed to drive the
huge gas bag, the maximum horsepower probably aggregating 1,600 to
2,000. The motors were distributed in five different locations, the
largest being suspended just abaft the wireless room. The remainder were
placed in self-contained units in the form of gondolas suspended from
the sides of the frame, as shown in Fig. 24, the outline being that of a
blunt-nosed fish. Each of these gondolas carried two motors placed side
by side and coupled up so that either one or both could be employed to
drive the single propeller. For cruising speeds one motor in each
gondola supplied sufficient power or in some gondolas both motors could
remain idle. No accommodation was provided for attendants in the
gondolas, any of which could easily be reached by light ladders from the
inclosed gangway.

To insure greater safety, the fuel supply was divided among sixteen
tanks, all of which were interconnected with each other and the engines
so that gasoline from any tank or tanks could be diverted to any
particular engine. The supply of lubricating oil for each engine was
carried in a tank in the gondola itself.

_Control._ Vertical and horizontal stabilizing surfaces of conventional
form were built on the sharply tapering rear end of the frame, the
elevator and rudder being similar to those used in aeroplane
construction, except that the rudder was in two sections, the larger of
which was placed on top of the envelope. The control of these surfaces,
the operation of all the engines, the control of the water ballast, the
air supply to the balloonets, and the fuel supply to the motors were all
concentrated at a panel board in the commander’s cabin, the forward end
of which bore a close resemblance to the bridge of a man-of-war. By
means of thirty-eight push buttons, half red and half white, air could
be released from or pumped into the balloonets, while in a similar
manner the contents of any one of the water-ballast tanks could be
emptied. Elaborate controls were provided for the power plant, it being
possible to vary the speed or stop any one or more of the motors from
the bridge. The rudder and elevators were operated by means of small
hand wheels, similar to a marine steering wheel. One of the most
prominent features of the operating cabin was a huge chart frame,
capable of carrying a large scale map covering a considerable area, as
well as an ample supply of maps. Few instruments were found in the
captured ship and it is thought highly probable that everything not
fastened in place had been dumped overboard at the last to increase its
lifting power.

[Illustration: Fig. 24. One of Six Gondolas, or Power Units of the
Zeppelin L-49
_Copyright by Underwood and Underwood, New York_]

Apart from the use of standardized fittings and parts and the employment
of a great deal more power in a slightly different manner than had
characterized the earlier types of Zeppelins, the L-49 revealed nothing
of unusual importance in airship design and certainly none of the
world-beating features that German propaganda had been heralding for
some time previous.

*Destruction of Zeppelins.* Mention has already been made of the fact
that practically the only use made by Germany of her huge airships has
been the bombardment of open cities, and that always at night. From the
first of September, 1914, up to the end of 1917, between thirty and
forty had met disaster, but only two were captured intact. The first of
these was discovered by a Russian cavalry patrol while at anchor and its
crew of thirty men were made prisoners. This was at an early period in
the war, while the second one to be captured was the L-49, already
referred to, which formed one of a squadron of five evidently sent out
on a bombing expedition against London. Owing to adverse winds, they
never reached their destination and four of them were known to have been
put out of action, all except the L-49 being destroyed in the air. Not a
few of these big airships have fallen victims to their own weakness and
succumbed to the elements, in one instance a high wind tearing the
airship loose from its moorings while the crew was not aboard. This was
at Kiel, and after traveling a number of miles unguided, the big bag
fell into the North Sea. In quite a number of other cases head winds
have prevented the return of the raiders to their base and they have
either been destroyed by their crews or wrecked at sea in attempting to
return. In still other instances the unwieldy monsters have been wrecked
by high winds when attempting to land, as was so frequently the case
prior to the war.

_Aeroplane and Anti-Aircraft Fire Effective._ Before the war broke out
the ability of either the aeroplane or the anti-aircraft gun to overcome
the Zeppelin was purely theoretical, but actual experience has
demonstrated that much of the theory was well founded. At least three
Zeppelins have been destroyed by British aviators in mid-air, all or
most of the crews being killed, while probably an equal number have been
accounted for by French aviators in open battle. The war had not been
under way a month before French anti-aircraft gunners showed their skill
by bringing down-a "Zep," while only a week later a Russian battery
accomplished the same feat, in this instance killing the entire crew. In
1916, British and French gunners succeeded in either "winging" or
setting on fire three or four, while two dropped into the North Sea and
one was blown up by its crew, having run out of fuel while raiding
Scotch towns.

_Bombing Raids against Zeppelin Sheds._ Not the least of the
disadvantages from which such huge and unwieldy craft suffer is the fact
that the correspondingly large structures required to house them make
exceedingly easy marks for the raiding aviator. Bombing, however, is
such an uncertain art that even such large buildings as these cannot be
struck from any altitude with a fair degree of accuracy. Consequently,
in the number of raids that have been carried out against Zeppelin
sheds, success has been due very largely to the temerity of the
aviators, who have descended within a few hundred feet of their mark
despite the fire directed at them from all quarters. At least three and
probably more of the big airships have been destroyed in this manner by
British aviators, who have made flights of several hundred miles to
reach their destination, while the destruction of as many more has been
ascribed by the Germans to the "accidental" explosion of a bomb in the
shed. In view of the great precautions taken against accident from the
explosion of the bombs carried by the airship itself, it is not
considered at all likely that there was anything accidental about the
wrecking of these craft.

One of the earliest attempts against Zeppelin headquarters at
Friedrichshafen on Lake Constance, which resulted in the destruction of
the L-31, is typical of the plan followed in attacks of this kind. Two
British aviators flew from their base in France, about 250 miles
distant, at a high altitude. They became separated before reaching their
destination owing to a mist. This, however, prevented their discovery
until they had dropped within a few hundred feet of the surface of the
lake, which it was necessary to do to obtain a view of the airship
sheds. The first pilot dropped his cargo of bombs from a height of only
100 feet or so over the shed and was rewarded by seeing it catch fire.
He had hardly straightened out on his return course before he heard the
attack of his companion. The latter was not so fortunate in escaping
unscathed, as a bullet pierced his fuel tank and compelled him to
descend. In the majority of instances, however, the raiders have
succeeded not only in carrying out their task but in escaping undamaged
as well.



CAPTIVE BALLOONS


*Military Value.* As an aid to military operations, the use of the
captive balloon dates back many years. It was extensively employed in
the Civil war and more recently in the Boer war, but with the advent of
both the dirigible and the aeroplane, it was generally considered
outside of Germany that its reason for existence had passed away. The
German military plans included a large number of balloons for artillery
observation purposes and they were used right from the start. It was
only when the fighting settled down to trench warfare, however, that
they came into prominence and the aid that they rendered the German
batteries put their opponents at a serious disadvantage. Like the
bayonet, which was also generally considered to have been relegated to
military operations of the past, the captive balloon is now playing a
very important role, particularly on the western front. In favorable
weather, anywhere from ten to forty of these aerial observation posts
will be visible from a single point on the line.

*Spherical Type Defective.* The captive balloon of the present day,
however, bears no resemblance to its predecessors. From a sphere, it has
been developed into a form that more nearly resembles the dirigible and
at the same time, it embodies some of the features of the aeroplane. The
old spherical balloon was always at the mercy of the wind, which not
only governed the altitude to which the balloon would rise but also made
things extremely uncomfortable as well as dangerous for the observers.
With 1,000 feet of cable out, such a balloon rises to an equivalent
height on a perfectly, calm day. But even a light wind cuts this height
down by 100 or 200 feet, while if a strong wind is blowing, the balloon
is held down to within a few hundred feet of the ground regardless of
the length of cable paid out. Every strong gust beats it over at a
perilous angle and the resulting shocks to the basket are so severe that
its occupants can have little thought for anything but their own safety.
Strong cross gusts set both the bag and basket to spinning and jumping
in a manner that would make the results of the severest storm at sea
seem mild by comparison, since the movements of the basket are executed
with such rapidity that they seem to be in almost every plane
simultaneously. As a result, the old type of captive balloon was
available for service only in the calmest weather.

*Modern Kite Balloon.* It should not be supposed that the improved type
of observation balloon now in use in such large numbers provides any
unusual amount of ease or comfort, since it is also prey to the wind and
does a great deal of swinging about as well as jerking when the wind is
more than 15 or 20 miles an hour. But it has been improved to a point
where the wind not only serves to elevate, instead of depressing it, but
also to steady it. The new type. Fig. 25, is technically known as a kite
balloon, because, in addition to the appendages attached to the bag
itself for steadying purposes, it is equipped with a tail to assist in
keeping it heading into the wind. This consists of a number of
bucket-shaped pieces of heavy canvas attached to the tail cable by
bridles so as to catch the wind and hold it, thus placing a heavy strain
on the cable and preventing the balloon from swinging violently. As is
the case with practically everything used at the front, the technical
name of the new type of balloon is prominent by its absence. It is a
_Drache_ (kite) to the Germans and a "blimp" to Tommy Atkins. Both its
shape and attitude when aloft bear a close resemblance to a huge
sausage, so that the term "sausage" is used by all the belligerents in
common to a large extent. A side view of an American type is shown in
Fig. 26.

[Illustration: Fig. 25. Head-On View of Modern Kite Balloon, Showing
Details of Tail Buckets
_Copyright by Central News Service, New York City_]

It will be noted from Figs. 25 and 26 that the suspension of the basket
and the appendages attached to the balloon at the rear hold it in a
position which is roughly the equivalent on a large scale of the curve
of an aeroplane wing. It has both camber and an angle of incidence, so
that the wind serves to elevate it instead of beating it down. This
lifting effect is further increased by tubes of large diameter, open at
the forward end only and curving around the end of the gas bag at the
rear. (It is also equipped with an air balloonet, the same as a
dirigible.) The wind enters the lower end of this tubular member, which
is in a line with the longitudinal axis of the balloon, but it must pass
around the curve at the end of the gas bag before it can fully inflate
it, so that it performs the double function of increasing the lift and
steadying the balloon, though the latter is its chief purpose. The
basket is suspended quite a distance below the gas bag and has
accommodation for two observers. Like scores of other inventions that
the Germans were the first to utilize on a large scale in the present
war, the kite balloon was not a German creation but was originally
developed in France.

[Illustration: Fig. 26. American Kite Balloon of Latest Type Ascending
_Copyright by Committee on Public Information, Washington, D C._]

*Methods of Inflation.* The average capacity of the kite balloons used
for observation purposes is 28,000 cubic feet. They are inflated with
hydrogen either from a portable generating plant forming part of the
equipment of the balloon company or from a supply carried under high
pressure in heavy steel "bottles" similar to those used for transporting
oxygen or carbonic acid gas intended for industrial use. Since the
balloon companies are stationed about 4 miles back of the firing line,
the use of the portable plant is practical, but it has been found more
economical and more convenient to generate the gas on a large scale at
special establishments in France and England and send it to the front in
containers. With a portable plant, several hours are necessary to
inflate the gas bag, whereas with a large supply of the gas at hand
under high pressure, the operation may be carried out in less than an
hour.

The balloon naturally works under the same difficulties as all
lighter-than-air craft, that is, there is a constant leakage of the
hydrogen through the fabric in addition to that lost by the expansion of
the gas on warm days when the summer sun beats down directly on the gas
bag. Where a field generating plant is employed, quick inflation of a
new balloon or replacement of loss is accomplished by the used of
several "nurses", Fig. 27. These are simply large gas bags which are
kept replenished by the gas plant working constantly, in other words,
they are storage tanks, and when it is necessary to inflate the balloon
quickly, their contents are simply transferred to it.

[Illustration: Fig. 27. Landing Big Kite Balloon at Training Station
"Somewhere in England." "Nurse" in Background
_Copyright by Underwood and Underwood, New York_]

*Balloon Company.* Though aeronautical in character, the kite balloon
service is actually a branch of the artillery, to which it is directly
attached. A balloon company accordingly consists of twelve to twenty
artillery officers of varying ranks and about 120 to 130 men. Of the
officers, six to eight are artillery lieutenants or captains and go
aloft as observers, this number being necessary because the strain of
watching constantly is very great and the observers must be relieved at
frequent intervals, the balloon otherwise being kept up continuously,
both day and night. There are also a number of sergeants, each of whom
is in charge of a different branch of the work, such as the inflation,
transport, telephone service, and winding machine. No less than fifteen
3-ton to 5-ton motor trucks are necessary for each balloon company
besides two or more motorcycle messengers, the care of the machines
usually being entrusted to the corporals of the company. The remainder
of the company are practically laborers, whose chief duties are to
attach the ballast bags to the ropes when it is intended to hold the
balloon on the ground for any length of time and to utilize their own
weight for the same purpose when the balloon is about to go aloft or is
only on the ground temporarily. In addition, every company has its
surgeon and assistants, quartermaster, cooks, company clerk, and other
attaches necessary to complete its organization, since a balloon company
serves as an independent unit.

*Equipment.* The paraphernalia required is quite as elaborate as that
necessary to keep several aeroplanes aloft, though naturally of a
different nature. It must all be readily portable, for a balloon company
has to change camp more or less frequently, or as often as the enemy
artillery happens to discover its range. To secure mobility is the
purpose of the great number of motor trucks employed. One of these is
equipped with a hoisting winch and a large drum capable of holding 3,000
or 4,000 feet of about 3/8-inch steel cable. The winch is driven by the
same engine that propels the truck, and in case of emergency the engine
may be applied to the two purposes alternately within a short space of
time. For instance, in case of attack either by shrapnel from an enemy
battery or by a hostile aviator, it may be used to quickly haul in or
let out cable to change the altitude of the balloon, or it may be
employed to drive the truck to another and more favorable location with
the balloon in tow.

Another truck houses a complete telephone exchange, since the observers
in the balloon may wish to communicate with any one of a number of
batteries which they are serving. Telephone communication is established
by means of an insulated wire which forms the core of the cable, while
the steel cable itself acts as the return wire to complete the circuit.
In some cases, a separate copper cable is employed, using the steel
cable as the return half of the circuit. In addition there is a truck
for transporting the balloons, for the company must always have
duplicate equipment at hand in case of the destruction of the balloon it
is using or, as more frequently happens, damage of a nature that
requires hours or days to repair. In addition to the balloon itself,
there are covers and the ground cloth, as in inflating a balloon no part
of its fabric must be allowed to touch the ground because of the danger
of stones or sticks tearing rents in it. The balloon proper and its
immediate accessories utilize at least one and sometimes two motor
trucks.

To hold the balloon on the ground when out of service, there are eighty
sacks of sand weighing 25 pounds each, or an aggregate of 1 ton of
ballast, in addition to which there are necessary a large number of
steel screw stakes, spare ropes and parts, ladders and the like, besides
the basket and its equipment. The stakes are employed to hold the
balloon down in a heavy wind by "pegging" it in the same manner as a
tent. Three or four trucks are required to carry the large supply of
hydrogen necessary, which entails the transportation of 130 to 150
containers. Each container holds several thousand cubic feet of gas
under high pressure, which is released through a reducing valve. Some of
the other transportation units required are the "cook wagon,"
quartermaster’s stores truck, truck for carrying tents, blankets, and
other impediments for the men, and the "doctor’s wagon" (ambulance).

*Advantages of Kite Balloon.* It became a necessity to resurrect the
captive balloon and bring it up to date, not simply because the Germans
were employing it in numbers, but because experience demonstrated that
it possessed numerous advantages over the aeroplane for artillery
observation. The observer in an aeroplane is carried back and forth over
and around the location he wishes to watch, at high speed and at a
constantly varying altitude. He must communicate by means of either
signals or wireless, and it is not always possible for him in either
case to know whether his signals have been received and understood,
since it is possible to transmit messages by wireless from an aeroplane
but a very difficult matter to receive. The observers in a kite balloon,
on the other hand, have the advantage of being able to scrutinize a
certain sector constantly with the aid of powerful glasses. With a few
weeks of experience in observing a given terrain they become so familiar
with it that any changes or the movements of troops or supplies are
quickly distinguished. The greatest advantage, however, is that the
information thus acquired may be instantly transmitted not merely to one
but to any one or all of a group of batteries extending over a mile or
two of front in either direction, the balloons being stationed 4 to 6
miles apart. The observers are fitted with portable head sets so that
they speak directly into their telephones without the necessity of
removing the glasses from their eyes, which enables them to watch the
fall of the shells and tell the battery attendant in the dugout
alongside the gun whether a shell fell "short", "over," "left," or
"right," and the amount of correction needed before the smoke from the
explosion has cleared away. With the aid of close corrections of this
nature the battery commander is in a position to get the range exactly
without the great expenditure of ammunition that firing entirely by map
or with the assistance of aeroplane observers entails. Instances are
recorded in which a 9.5-inch shell has been landed right in a concrete
"pill-box" not over 15 feet square from a distance of 3 miles after six
trial shots had been fired to obtain the range. Such a shot is reported
back to the battery by the balloon observer as a "direct hit," and it is
only necessary to fire the gun at the same range and direction to score
as often as necessary.

*Duties of Balloon Crew.* Each kite balloon carries aloft two observers,
Fig. 28, both of whom can concentrate their entire attention on the work
of "spotting," since they have nothing to do with the control of the
balloon itself, except to give orders. Their chief duties consist of
"counter-battery" observation, that is, spotting the location of enemy
batteries, and being constantly alert to detect any suspicious movements
back of the enemy’s lines, such as movements of troops, ammunition, or
supplies. The batteries controlled from observation balloons are the
"heavies," which are located 1 mile or more back of the front line
trenches and to the gunners of which the objects they are firing at are
never visible. Some of the heaviest guns mounted on specially
constructed railway trucks are often fired from points 5 miles or more
back of the lines. In fact, when balked in their attempt to take Calais,
the Germans bombarded the town with the aid of long-range naval guns
from a distance of over 15 miles and every shot dropped into either some
part of the city or its outskirts. Buildings, hills, or specially
constructed and concealed observation towers are frequently utilized in
conjunction with captive balloons to serve as auxiliary observation
posts, so that the base line connecting the two may be used to
triangulate distances and thus calculate them more accurately than is
possible by direct observation from a single point.

[Illustration: Fig. 28. French Kite Balloon Observers about to Ascend
_Copyright by Committee on Public Information, Washington, D.C._]

*Risks Incurred.* _Enemy Fire._ While the observers in a kite balloon
are not subjected to all the risks that the aviator must encounter when
he goes aloft or, at least, not to the same extent, their lot is far
from being free from danger. One of the duties of the reconnoitering
aviator is to destroy observation balloons by means of incendiary bombs
equipped with fishhooks which catch in the fabric or by the use of his
machine gun. Enemy batteries may also succeed in getting the range of
the balloon and fire at it with large caliber shrapnel, which spreads
its fragments over an area 100 yards or more in diameter when it bursts.
So many of the German balloons were downed by French and British
aviators in the early part of the war—and the Germans retaliated in
kind—that a battle plane is now always detailed to keep watch above the
balloon to ward off attacks by aeroplanes.

_Escape of Balloon._ In addition to the risk of being shot down, there
is the ever-present danger of the balloon being wrecked by a sudden
squall or of its breaking away from its windlass through the parting of
the cable and floating over the enemy lines. Balloons have been lost
through both causes in a number of instances. Each of the two observers
wears a heavy harness to which is attached a parachute suspended by a
light cord from the rigging of the balloon, so that in case of emergency
they may save themselves by jumping without having to make any
preparations for their sudden drop.

In case of the breakage of the cable, which usually results from a
strong wind coming up suddenly and putting a terrific strain on the
steel line by jerking it, the observers are guided in their actions by
the direction in which the balloon moves. When it is carrying them back
over their own territory, they navigate in the same manner as a free
balloon, coming to the ground as soon as a favorable landing place can
be reached. Instruction in free ballooning is accordingly an important
part of the curriculum that the kite balloon observers must go through.
Should the wind be in the opposite direction, however, as only too often
proves to be the case, all instruments, papers, and maps are immediately
thrown over the side and the observers promptly follow suit in their
parachutes, abandoning the balloon to its fate. As the balloon travels
with the speed of the wind, once it is released, and the parachute of
the descending observer is carried in the same direction, prompt action
is vital to prevent coming to the ground in the enemy’s territory. In a
30-mile wind, for example, only eight minutes would elapse from the
moment that the balloon broke away until it traversed the 4 miles
intervening between its station and the enemy’s lines. On some
occasions, kite balloons which were not fit for further service have
been loaded with explosives and released from a height that would cause
them to land well within the enemy’s territory with disastrous results
to the men detailed to capture them.

*Marine Service.* The kite balloon was first used by the British naval
forces in their operations against the Dardanelles and proved so
valuable that they have since been employed in fleet expeditions in the
North Sea as well as for anti-submarine work. In the latter form of
service, they have the same superiority over the aeroplane for
observation that they possess in land operations. The ship naturally
cannot run the risk of remaining stationary, but as the speed of the
balloon is the same as that of the ship towing it, the observers do not
pass over a given area with anything like the velocity of an aeroplane,
while their elevated position affords the same advantages for detecting
the presence of the submerged submarine or the approach of enemy
vessels.




                           EXAMINATION PAPER




                           DIRIGIBLE BALLOONS


*Read Carefully:* Place your name and full address at the head of the
paper. Any cheap, light paper like the sample previously sent you may be
used. Do not crowd your work, but arrange it neatly and legibly. _Do not
copy the answers from the Instruction Paper; use your own words, so that
we may be sure you understand the subject._

   1. What essential features of design did Meusnier’s first dirigible
      incorporate?
   2. Describe the difference between rigid, semi-rigid, and flexible
      types of dirigibles.
   3. State the laws governing the increase of resistance with speed,
      the increase of power necessary for a given increase of speed, and
      the ratio in which the volume and area of the gas bag increase
      with increased dimensions.
   4. What provides the lifting power of the dirigible and how is this
      lifting power utilized? Why should this lifting power be so much
      less at night than in the daytime? What is net lifting power?
   5. What are air balloonets? How and for what purpose are they used?
   6. What is the most efficient form of envelope for the dirigible, and
      why?
   7. Why cannot the ordinary spherical balloon be propelled as a
      dirigible?
   8. Is the form of the stern as important as the bow?
   9. What is longitudinal stability and how is it obtained?
  10. How is stability of direction obtained? What are stabilizing
      planes?
  11. Why must a form of suspension for the car that cannot be
      accidentally displaced with relation to the balloon be provided?

12. Theoretically, where should the propulsive effort be applied to a
dirigible? What factors affect the placing of the propeller and what has
been proved to be the most practical solution of the problem?

  13. Discuss the advantages of the kite balloon over the aeroplane for
      observation.
  14. What is the effect of the wind on a modern kite balloon?
  15. What is the difference between "pounds per horsepower" and "pounds
      per horsepower hour" as applied to the motor of a dirigible? Which
      is more important?

  10. Sketch and explain the Astra-Torres suspension.

  17. What differences exist between a Zeppelin and a Schutte-Lanz
      dirigible?
  18. Describe the "L-49", discussing power plant and control.
  19. Define static and dynamic equilibrium as applied to the dirigible.
  20. Is the Zeppelin effective? Discuss fully.

*After completing the work, add and sign the following statement:*

I hereby certify that the above work is entirely my own.

(Signed)




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