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.
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