A ballonet is an airbag inside the outer 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 denser 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 pitch 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.
The ballonet valves were originally a butterfly type, actuated by springs. If pressure rose in the ballonet, a spring would pressure the spindle of the butterfly valve to turn, relieving the pressure. In more recent implementations, the valves are actuated electrically either by a linear actuator (driven open/closed) or a linear solenoid (spring return), the latter being the favoured fail-to-safe arrangement.
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