Rigid airship

A rigid airship is a type of airship (or dirigible) in which the envelope is supported by an internal framework rather than by being kept in shape by the pressure of the lifting gas within the envelope, as in blimps (also called pressure airships) and semi-rigid airships. Rigid airships are often commonly called Zeppelins, though this technically refers only to airships built by the Luftschifbau Zeppelin company.

Rigid airships were produced and relatively successfully employed from the beginning of the 1900s to the end of the 1930s; their heyday ended when the Hindenburg caught fire on May 6, 1937.

Rigid airships consist of a structural framework usually covered in doped fabric containing a number of gasbags or cells containing a lifting gas. In the majority of airships constructed before World War II highly flammable hydrogen was used for this purpose, resulting in many airships such as the British R101 and the German Hindenburg being lost in catastrophic fires. The noble gas helium was used by American airships in the 1920s and '30s and is used in all modern airships.

Although airships rely on the difference in density between the lifting gas and the surrounding air to stay aloft they can also generate a certain amount of aerodynamic lift by using their elevators to fly in a nose-up attitude. Similarly, by flying nose-down down-force can be generated: this may be done to prevent the airship rising above its pressure height. Typically airships start a flight with their gasbags inflated to about 95% capacity: as the airship gains height the lifting gas expands as the surrounding atmospheric pressure reduces. The height at which the internal pressure of the gasbags equals external atmospheric pressure is called the pressure height: if the airship climbs beyond this it is necessary to vent gas in order to prevent the gasbags rupturing.

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