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.