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There are three types of airships. In a nonrigid airship, also known as a blimp, the shape of the gas bag is maintained by the internal pressure of the enclosed gas. In a semirigid airship, also known as a keel-airship, internal gas pressure acts in conjunction with a longitudinal keel to maintain the form of the gas bag. In a rigid airship, the form of the gas bag is determined by a rigid framework, usually made of aluminum or a special aluminum alloy called Duraluminium; the framework is formed of longitudinal girders and cross-rings, also made of girders. The whole structure is covered with fabric for aerodynamic purposes.
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