Balloons and Airships

Balloons and Airships

In balloons and airships the gas is contained within envelopes of
cotton fabric lined with gold-beaters

1 skins or rubber impregnated.
Diffusion occurs through these comparatively impervious materials,

and, together with leakage, contaminates the enclosed gas, so that
densities greater than those given in the preceding article must be
assumed. Practical values for lift per thousand cubic feet are 68 Ib. for
hydrogen and 62 Ib. for helium, at low altitude. Thus the envelope of a
balloon weighing 1 ton would, in the taut state at sea-level, have a diameter
of 39-8 ft. for hydrogen and 41-1 ft. for helium ; actually it would be
made larger, filling only at altitude and being limp at sea-level.\

the variation of atmospheric pressure from the level of the top of

the open filling sleeve S to that of the crest of the balloon, OH the

corresponding variation of pressure through the bulk of helium

filling the envelope.

 The difference between these external arid

internal pressures acts radially outward on the fabric as shown to

the right. The upward resultant force and part of the force of

expansion are supported by the net N, from which is suspended the

basket or gondola B, carrying ballast and the useful load.

Balloons drift with the wind and cannot be steered horizontally.

Airships, on the other hand, can maintain relative horizontal velocities

by means of engines and airscrews, and are shaped to streamline

form for economy of power. Three classes may be distinguished.

The small non-rigid airship, or dirigible balloon (Fig. l(a)} has a

faired envelope whose shape is conserved by excess gas pressure

maintained by internal ballonets which can be inflated by an air

scoop exposed behind the airscrew. Some stiffening is necessary,

especially at the nose, which tends to blow in at speed. A gondola,

carrying the power unit, fuel, and other loads, is suspended on cables

from hand-shaped strengthening patches on the envelope. (Only

a few of the wires are shown in the sketch.)

In the semi-rigid type (b) some form of keel is interposed between

the envelope and gondola, or gondolas, enabling excess gas pressure

to be minimised. Several internal staying systems spread the load

carried by the girder over the envelope, the section of which is not as

a rule circular.