British Airships, Past, Present and Future | Page 6

they have been ripped and deflated and
brought back to the station without incurring any but the most trifling
damage. Experience in the war has proved that for military purposes
the large rigid, capable of long hours of endurances and the small
non-rigid made thoroughly reliable, are the most valuable types for
future development. The larger non-rigids, with the possible exception
of the North Sea, do not appear to be likely to fulfil any very useful
function. Airship design introduces so many problems which are not
met with in the ordinary theory of structures, that a whole volume
could easily be devoted to the subject, and even then much valuable
information would have to be omitted from lack of space. It is,
therefore, impossible, in only a section of a chapter, to do more than
indicate in the briefest manner a few salient features concerning these
problems. The suspension of weights from the lightest possible gas
compartment must be based on the ordinary principles of calculating

the distribution loads as in ships and other structures. In the non-rigid,
the envelope being made of flexible fabric has, in itself, no rigidity
whatsoever, and its shape must be maintained by the internal pressure
kept slightly in excess of the pressure outside. Fabric is capable of
resisting tension, but is naturally not able to resist compression. If the
car was rigged beneath the centre of the envelope with vertical
suspensions it would tend to produce compression in the underside of
the envelope, owing to the load not being fully distributed. This would
cause, in practice, the centre portion of the envelope to sag downwards,
while the ends would have a tendency to rise. The principle which has
been found to be most satisfactory is to fix the points of suspension
distributed over the greatest length of envelope possible proportional to
the lift of gas at each section thus formed. From these points the wires
are led to the car. If the car is placed close to the envelope it will be
seen that the suspensions of necessity lie at a very flat angle and exert a
serious longitudinal compression. This must be resisted by a high
internal pressure, which demands a stouter fabric for the envelope and,
therefore, increased weight. It follows that the tendency of the envelope
to deform is decreased as the distance of the car from the gas
compartment is increased. One method of overcoming this difficulty is
found by using the Astra-Torres design. As will be seen from the
diagram of the North Sea airship, the loads are excellently distributed
by the several fans of internal rigging, while external head resistance is
reduced to a minimum, as the car can be slung close underneath the
envelope. Moreover, the direct longitudinal compression due to the
rigging is applied to a point considerably above the axis of the ship. In
a large non-rigid many of these difficulties can be overcome by
distributing the weight into separate cars along the envelope itself. We
have seen that as an airship rises the gas contained in the envelope
expands. If the envelope were hermetically sealed, the higher the ship
rose the greater would become the internal pressure, until the envelope
finally burst. To avoid this difficulty in a balloon, a valve is provided
through which the gas can escape. In a balloon, therefore, which
ascends from the ground full, gas is lost throughout its upward journey,
and when it comes down again it is partially empty or flabby. This
would be an impossible situation in the case of the airship, for she
would become unmanageable, owing to the buckling of the envelope

and the sagging of the planes. Ballonets are therefore fitted to prevent
this happening. Ballonets are internal balloons or air compartments
fitted inside the main envelope, and were originally filled with air by a
blower driven either by the main engines or an auxiliary motor. These
blowers were a continual source of trouble, and at the present day it has
been arranged to collect air from the slip-stream of the propeller
through a metal air scoop or blower-pipe and discharge it into an air
duct which distributes it to the ballonets. The following example will
explain their functions: An airship ascends from the ground full to
1,000 feet. The ballonets are empty, and remain so throughout the
ascent. By the time the airship reaches 1,000 feet it will have lost
1/30th of its volume of gas which will have escaped through the valves.
If the ship has a capacity of 300,000 cubic feet it will have lost 10,000
cubic feet of gas. The airship now commences to descend; as it
descends the gas within contracts and air is blown into the ballonets.
By the time the ground is reached 10,000 cubic feet of air