THE ZEPPELIN AND OTHER MODERN AIRSHIPS
The keenest interest and curiosity is very naturally felt in the
Zeppelin airship. Much has been written concerning its peculiar
construction—much that is founded on doubtful evidence, and much that
is mainly true. At this point we shall limit ourselves to a brief
description of the construction of the Zeppelin, and seek to show in
simple terms how the type of airship rises and falls. With the heroic
acts the Zeppelins have called forth we shall deal later.
Now, imagine a long cage tapering to a rounded point at either end. At
intervals are thin walls or partitions of aluminium sheet, dividing
the cage lengthwise into a large number of drum-shaped compartments,
while every part is stiffened and straightened by crossed bars forming
diagonal bracing, tying and holding all together into a structure of
remarkable strength. Such is the basis of a Zeppelin airship.
The whole of the framework is covered with waterproof fabric, the
length of some of the patterns being 492 feet in length and 47½ feet in
diameter.
Beneath is fixed a light framework, forming a kind of keel, and giving
additional stiffness. In some designs a cabin is formed in the keel.
The cars, which are not unlike the form of a boat, are hung under the
keel, one near either end. Near the front, on either side, two light
frames spread out, each of which carries one of the propellers, and
another pair of frames are fixed in like manner toward the end. At the
after end are a number of fins or planes, the purpose of these being to
keep the nose of the ship foremost to the wind, as shown in a previous
chapter.
Now as regards rising and falling. To many people the manœuvring of a
Zeppelin in the air is still a matter of mystery. It is certainly not
easy for the lay mind to grasp and hold the fact that a monster vessel
made of metal, and weighing nearly 20 tons, can float in a medium
through which a feather falls. The Zeppelin, in effect, is lighter
than a feather, volume for volume, and this lightness is obtained by
creating an enormous space within the carcase of the ship and filling
this space with hydrogen gas, which is about fifteen times lighter than
air.
If we imagine that a steel boiler 50 feet long has the same width and
height as a Zeppelin and weighs 20 tons, it is easy to understand that
if this were filled with hydrogen gas it would not float in the air.
But imagine the boiler to be drawn out until it was 500 feet long, and
one gets some idea of the lightness of the Zeppelin structure. Each
plate of metal in the boiler would be increased to ten times its normal
length, and thus would become exceedingly thin. Of course, in the
Zeppelin lighter materials are used, with the result that for a small
weight we get an enormous volume.
Then, by filling this space with hydrogen the ship displaces its own
volume of air, but this volume of air is so much heavier than the
ship’s weight that the vessel rises.
The most remarkable feature of the Zeppelin is the ingenious manner
in which the volume of hydrogen is controlled, and through this
control the altitude of the ship is regulated. In principle the method
resembles that of the air bladder of a fish. When the eighteen
gas-bags of a Zeppelin are filled with hydrogen the ship is at its
maximum of buoyancy or lightness. It then has a lifting power which
unless restrained by heavy weights would take the vessel high up into
the air until a thin atmosphere was reached, where the ship would float
motionless in a medium of less density. But if we replace the hydrogen
with air when the ship is held to the ground, we increase the weight of
the vessel so much that it will not rise.
Thus in the Zeppelin, by the alternative use of light hydrogen and
heavy air, we can so alter the weight that the vessel can be made
to rise or sink. By a highly-developed system of tanks, pumps, and
valves the relative volumes of hydrogen and air can be controlled with
wonderful accuracy.
In the older system of airships the hydrogen was allowed to escape when
it was desired to make the ship heavier, but the modern Zeppelin, when
it takes hydrogen from the gas-bags, is able to store the gas in metal
tanks under pressure, and it also has a reserve supply to make up for
unavoidable leakage.
Each gas-bag is mounted above an air-bag, and when the gas-bag is
inflated to the maximum the air-bag is almost empty. The ship is then
at its most buoyant stage. To reduce this buoyancy the air pumps are
put in motion, and they force air under pressure into the air-bags.
This pressure, acting on the gas-bags, forces out the hydrogen through
pipes and non-return valves to the storage tanks. If at any time it
is required to make the vessel ascend, the air-bags are deflated and
the gas supply pipe with its pump is employed to force more hydrogen
into the gas-bags. One thousand cubic feet of hydrogen have a lifting
power of nearly 75 lbs. at sea-level, and this lifting power acts very
quickly. Thus a Zeppelin changes its altitude rapidly when the weight
is altered, and at the same time there is automatic control whereby
the vessel can be kept at the same level if necessary. When a Zeppelin
drops a bomb it suddenly becomes lighter, and it rises in consequence.
This circumstance is very disconcerting to gunners, for if, say, a 200
lb. bomb were dropped, the ship would leap up nearly 200 feet in the
air, unless the captain desired to check the ascent. The discharge
of water ballast produces the same rising effect, and with almost
equal suddenness the ship can sink by using its powerful air pumps to
press out the hydrogen. Moreover, when the Zeppelin is in motion it
can use its elevating planes for changing altitude in the manner of
an aeroplane. Thus, in addition to its power of steering from left
to right in the same plane, and of climbing and descending along an
inclined path by the use of the elevators, the Zeppelin can rise and
fall vertically, and by its system of storage tanks these manœuvres can
go on for a long period.
[Illustration: SECTIONAL VIEW OF ZEPPELIN AIRSHIP, SHOWING THE
ARRANGEMENT OF THE HYDROGEN AND AIR BALLONETS WHICH CONTROL THE WEIGHT
OF THE AIRSHIP, THUS ENABLING IT TO RISE AND FALL AS REQUIRED.
(1) Section of one of the eighteen ballonets. (2) Hydrogen gas-bag
partly inflated. (3) Air. (4) Rear gondola. (5) Outer covering of
fabric. (6) Metal work. (7) Air space between gas-bag and frame. (8)
Hydrogen gas-bag fully inflated. (9) Flexible gas-pipe. (10) Inner
ballonet deflated. (11) Metal gas tank into which hydrogen is pumped
under pressure. (12) Forward gondola. (13) Flexible pipe from pump to
ballonet. (14) Keel cabin.
(Diagram from a photograph taken from a point at the forward part of a
Zeppelin Airship.)]
There is a good deal of difference of opinion as to the altitude which
the Zeppelin can attain. When fully loaded in war trim the latest ships
can rise to about 5,000 feet, but by the time they reach London, for
example, and have used nearly half their fuel, ammunition, &c., they
are several thousand feet higher. The practical limit to airship work
is said to be about 10,000 feet. Above that height the cold is so
intense, the air so rarefied, and the conditions for men, engine, and
ship so distressing, that there is no inducement to rise further.
It is noteworthy that the latest type of Zeppelin is fitted with a
switchboard for dropping bombs, as, for example, in the airship brought
down in the north of London in the early part of October, 1916.
The German Schütte-Lanz, a well-known type, is an attempt to secure
the advantages of a rigid type, without the fragilities of the
Zeppelin. The framework is made of fir wood, and contains separate gas
compartments. Exceptional strength is claimed for these compartments.
A centrifugal pump is employed for distributing the gas. The volume
of the airship is 918,000 cubic feet—an extremely large structure,
surpassing even some of the largest types of airship. It is believed in
authoritative quarters that one of the first airships brought down in
flames on British soil was a ship of this type.
The German Gross airship has been described as more or less a
reproduction of the Lebauchy type, which is, of course, of French
origin. It is built partially on the rigid and partly on the non-rigid
system.
The Parseval airship is portable, and therefore a particularly useful
type. On account of its subtleness it has been remarkably free from
accidents. It is small in size, and is fitted for many purposes for
which larger airships would be useless. The dimensions, however, of
the Parseval vary considerably, the smallest being 3,200 cubic metres.
(This particular ship was built in the year 1908.) The more recent and
larger designs have a far greater capacity.
There are, of course, many other types on similar lines, but we are
chiefly concerned in these pages with the purpose and fate of airships
of the rigid type, and in our next chapter we shall see how our airmen
have fitted themselves for the task of dealing with Zeppelins.