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_]