Ballonet

A Ballonet is an air-filled flexible container (or airbag) that is 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 pitch of the airship.