J air pump,
KK flap-valves in the air lines,
LL air inlets into the ballonets,
M maneuvering vent,
U overpressure valve in gas cell,
VV overpressure valves in ballonets.
Ballonets are air-filled flexible containers (or airbags) that are located inside the envelope of a non-rigid or semi-rigid airship. Such an airship can have one or more ballonets, commonly one fore and one aft. Because air is heavier than the lifting gas (usually helium but, in the early days of lighter-than-air transport, hydrogen), the ballonets are deflated or inflated with air to maintain the external shape of the airship during ascent or descent. They can also be used to control the trim, or levelness, of the airship.
The lower image illustrates the principle of a balloon within a balloon. The outer balloon represents the airship's outer envelope or gasbag; the red inner balloon represents the ballonet. In an airship the ballonet would be much smaller relative to the size of the gasbag, e.g. in the French airship Lebaudy Patrie the ballonet's volume was approximately one-fifth that of the envelope.
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