FURTHER LINES OF PROGRESS
Progress toward the modern airship has, as we have seen, been by
short and laborious flights. The disappointments and disasters have
been almost numberless. Endless patience, perseverance, and dauntless
courage have been demanded. Moreover, in the past the would-be master
of the air has needed very considerable resources. On account of a lack
of funds many promising designs have come to no definite end. In the
earlier days of flying the work of construction was done chiefly by men
of leisure and means. Not till a comparatively recent date has the work
been put on a commercial basis and done by large manufacturing firms.
One of the chief difficulties to be overcome was to discover an object
of sufficient strength to be driven through the air, and yet so light
that it could displace more than its own weight of air. No very great
difficulty was experienced in constructing the spherical balloon, for
the sphere is, of course, the natural shape which any flexible envelope
will take. No framework was needed to stiffen the flimsy covering of
such a balloon. The sphere is, in itself, a natural shape, and it has
no tendency to change. The distorting action upon it is that due to the
weight of the car; but by using a large net bag, enclosing the whole
balloon, this has been so spread that the distortion is very slight,
and the natural shape not interfered with to a very appreciable extent.
The great pressure of the air has, of course, constituted many
difficulties. At sea-level the air pressure is 14·7 lbs. per square
inch. A vessel containing a vacuum has therefore to be strong enough
to support 15 lbs. on every square inch of its surface. To make the
envelope of a balloon strong enough to contain a vacuum is impossible
for the purpose. Too great weight would be required.
It has been found that the best course is to fill the balloon with
hydrogen, the lightest of gases. In this way the difficulty as regards
pressure is overcome, for the hydrogen presses upwards as strongly as
the air presses inwards. Stated in round figures, 1,000 cubic feet of
hydrogen weighs about 5½ lbs., and the same quantity of air about 80
lbs. It has been found, then, that 75 lbs. represents the gross lifting
weight, and that from it must be deducted the weight of the envelope to
arrive at the desired lifting effect.
With the increased size of the balloon many difficulties have been
removed, for the lifting weight increases faster than the superficial
area of the envelope. The contents of a sphere increase as the cube
of a diameter, but the area grows only as the square of the diameter.
Therefore, if you double the diameter of a balloon you increase its
capacity and consequently its gross lift by eight times. Even if it
should be necessary to increase the thickness of the fabric of which
the balloon is made, there is still a good margin left in favour of the
larger balloon.
But the aim has been to obtain something more than the ordinary
spherical balloon, which simply drifts in the air-currents. Such a
balloon is helpless as far as direction is concerned. It simply ‘goes
with the wind.’ Its weight may be varied, but not its direction. The
aim of the inventors of steerable balloons has been to overcome
helpless drifting by means of propellers and rudders, and by various
means designed to avoid loss of gas in ascending and descending.
Inventors in time past found that it was no easy matter to drive a
large spherical object of a light and flimsy construction through the
air. With the huge area which a spherical balloon offers to the wind,
it was found impossible to make any headway at all, except in perfectly
calm weather, or with the wind behind. Consequently the steerable
balloon took on an elongated shape, the nose growing more and more
pointed, so that it could ‘cut’ the air.
But now a fresh call arose for new ways and means of construction.
The simple bag, which served in spherical form, was useless for the
new design. A rigid framework of suitable lightness and strength was
called for—an extremely difficult matter. Indeed, even in the case of
a ship built for the sea there are troubles in this direction. ‘The
water supports it all along, while the load which it carries is more
or less in lumps, distributed irregularly from end to end. A ship in
still water, without any attacks by storms from without, is in danger
of breaking its back. If it be divided up into short sections some
will be found to possess great buoyancy and little load, while others
will be carrying loads far in excess of their buoyancy. The ship
must therefore be strongly constructed, so that the lightly loaded
parts may be able effectually to assist the heavily loaded parts. As
great longitudinal stiffness is required in a ship as in a bridge. In
fact, the modern ship is actually modelled upon a railway bridge. The
method of construction which made the great liner of to-day possible
was invented by I. K. Brunel, who got the idea from the Menai Straits
Bridge of Robert Stephenson.’
Longitudinal stiffness is, then, an absolute essential to any structure
of the kind now in mind. The buoyancy must be fairly constant from end
to end, the cars being suspended at intervals. That is to say, it has
been found that the necessary stiffness must be attained whereby the
weight of the suspended cars will be distributed in due proportion to
every part of the balloon, not simply to the parts immediately above.
This has been attained by means of a cleverly constructed framework of
aluminium, and on a line with this improvement have come a number of
drum-shaped gas-bags, made of rubber fabric and placed in allotted
spaces in the framework. A kind of keel has also been introduced
beneath the frame, giving additional stiffness and keeping the airship
from rolling, just as in the case of seafaring craft.
Improvement has followed improvement. In some designs two light frames
have been spread out from the main structure of the airship, each
carrying a propeller. Frames have also been introduced at the back of
the airship, thus giving four propellers in all—two forward and two
aft. With these have come fins or planes, designed with the view to
keeping the nose of the airship foremost to the wind. Moreover, groups
of planes have been employed, lying in horizontal position but capable
of movement, and making it possible to steer upward at both ends or at
one only, as required.
Whilst these structures, which led to the Zeppelin, were in course of
preparation, other designs of importance were being made, which led by
degrees to airships of the nature of the Parseval. In these designs
there was no elaborate framework. The balloon portion was in one—a
huge shape, stout in the middle with a pointed tail and rounded nose,
and carrying triangular planes, placed horizontally. This strange
shape, not unlike a fish, was maintained simply by the formation of
the bag, distended by pressure of the gas. Difficulties as regards the
car were overcome by long ropes, the car being suspended some distance
below. The ropes were attached to the balloon at intervals, thus
distributing the weight of the car throughout almost the full length of
the balloon.
Later came improvements which permitted the car of the airship to
slide, so to speak, upon the suspending ropes, thus giving greater
freedom to the action of the propeller. To the design were also added
two smaller ballonets, inside the large one, carrying air-ballast. And
by means of clever manipulation these bags made it easier to keep the
airship at an even keel. This aim was also aided by a small horizontal
plane or elevator placed beneath the bow. Underneath the stern was hung
a vertical plane, to the end of which the rudder was hinged. The motor
was in the car, and drove two propellers, supported upon a framework,
between the car and the balloon. These craft gradually grew to about
300 feet in length, and about 50 feet in diameter at the thickest
parts.
Other designs, which led to the Astra-Torres, an airship of French
origin, had a balloon of ‘trefoil’ shape. The car was hung low, as in
other models of the kind, and was distributed by a number of wires,
some of which passed into the balloon itself and were attached inside.
Indeed, it was this mode of attaching the car that led to the trefoil
shape. Two planes were attached to the rear, and two elevators and the
rudder were placed beneath the rear end.
In another fairly successful design of a similar nature a long girder
ran underneath the balloon, supported by wires from the balloon,
the car being attached to the centre, thus distributing the weight
throughout the whole length of the balloon.
Many of these designs had their origin in France, but the British have
not been idle. Many improvements have had their birth in England, and
we know that these, as in the case of other designs here mentioned,
have led to definite results. Out of persevering efforts, checked again
and again by misfortune and often by disaster, have come the modern
airships with which we are familiar. In their wake are many victims.
Yet, as we have seen, and shall see afresh in these pages, they have
called forth many heroic deeds.