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Semi-rigid in this context refers to an airship that has a keel on the underside of the gasbag.

Contrast to non-rigid and rigid designs. An advantage of this design is that it can be deflated into a smaller transportable form than a fully framed rigid one.

Semi-rigid airships were first produced by the French, but were subsequently developed extensively by the Italians before, during, and after World War I when they attained a capacity of over a million cubic feet.
The semi-rigid resembles the non-rigid in that the form of the envelope is maintained by internal pressure, but differs in the method of distributing the load. In the semi-rigid a longitudinal keel girder fitted to the underside of the envelope provides a rigid and, in certain types, a slightly flexible, backbone. It is therefore possible for a semi-rigid to fly at a much lower gas pressure than a non-rigid. A lighter envelope fabric can be used, and sufficient weight saved to compensate for the extra weight of the keel girder. To fly a semi-rigid at speed requires a fully inflated envelope, but it can be brought in to land with a partially inflated envelope. This is made possible by dividing the gas container into compartments by means of transverse fabric bulkheads; it also reduces the risk of the airship losing lift through an accidental tear in the envelope.[1]

The 1907 British Army airship Nulli Secundus was of a semi-rigid design, which they then abandoned. Semi-rigid types were not used by British force in WWI.[2]

Sharper definitional information and historical examples are on en.wp.[3]


This wiki has 7 patents in category "Semi-rigid".

Patents in category Semi-rigid

Publications referring to Semi-rigid

Publications referring to semi-rigid

Enclosing categories LTA, Balloon, Dirigible
Keywords LTA, Balloon, Ballonet, Frame, Keel
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