|An airship or dirigible is a type of aerostat or lighter-than-air aircraft which can navigate through the air under its own power. Aerostats gain their lift from large gas bags filled with a lifting gas that is less dense than the surrounding air. |
In early dirigibles, the lifting gas used was hydrogen, due to its high lifting capacity and ready availability. Helium gas has almost the same lifting capacity and is also nonflammable, but is rare and relatively expensive. Significant amounts were first discovered in the United States and, for a while, helium was rarely used for airships outside the United States.
Most airships built since the 1960s have used helium, though some have used hot air.
The outer envelope of an airship may be formed from its single gas bag, or may be a separate supported skin. Besides the main envelope, an airship also has engines and crew and/or payload accommodation, typically in a gondola hung beneath the envelope.
The main types of airship are non-rigid, semi-rigid, and rigid.
- Non-rigid airships, often called "blimps", rely on internal pressure to maintain the shape of the airship.
- Semi-rigid airships maintain the envelope shape by internal pressure, but have some form of supporting structure, such as a fixed keel, attached to it. Rigid airships have an outer structural framework which maintains the shape and carries all structural loads, while the lifting gas is contained in one or more internal gas bags or cells.
- Rigid airships were first flown by Count Zeppelin and the vast majority of rigid airships built were manufactured by the firm he founded. As a result, all rigid airships are sometimes called zeppelins.