A blimp, or non-rigid airship, is an airship without an internal supporting framework or keel. A non-rigid airship differs from a semi-rigid and a rigid airship (e.g. a Zeppelin) in that it does not have any rigid structure, neither a complete framework nor a partial keel, to help the airbag maintain its shape. Rather, these aircraft rely on both a higher pressure of the lifting gas (usually helium) inside the envelope and the strength of the envelope itself.
Blimps are also distinct from moored balloons. While often very similar in shape, moored balloons have no propulsion and are tethered to the ground in contrast to blimps which are free flying aircraft.
Because blimps keep their shape with internal overpressure, typically the only solid parts are the passenger car (gondola) and the tail fins. A non-rigid-airship that uses heated air instead of a light gas (such as Helium) as a lifting medium is called a hot air airship.
Volume changes of the lifting gas, due to temperature changes, is balanced using ballonets (air bags), in order to maintain the overpressure. Without sufficient overpressure the blimp loses steerability and top speed is also degraded. The propeller air stream can be used to inflate the hull. In some models, such as the Skyship 600 differential ballonet inflation can provide a measure of pitch trim control.
The engines driving the propellers are usually directly attached to the gondola, and in some models are partly steerable.
Blimps are the most commonly built airships, because they are relatively easy to build and easy to transport once deflated. However because of their unstable hull their size is limited. A blimp with too long a hull will kink in the middle when the overpressure is insufficient, or when maneuvered too fast (this has also happened with semi-rigid airships with weak keels). This leads to the development of semi-rigids and rigid airships.
Modern blimps launch somewhat heavier than air (overweight), in contrast to historic blimps. The missing lift is provided by lifting the nose and using engine power. Some types also use steerable propellers or ducted fans. Operating in a heavier than air state avoids the need to dump ballast at lift off and also avoids the need to lose costly lifting gas on landing.
An different derivation is given by Barnes & James in "Shorts Aircraft since 1900
"In February 1915 the need for anti-submarine patrol airships became urgent, and the Submarine Scout type was quickly improvised by hanging an obsolete B.E.2c fuselage froma spare Willows envelope; this was done by the R.N.A.S. at Kingsnorth, and on seeing the result for the first time, Horace Short, already noted for his very apt and original vocabulary, named it 'Blimp', adding, 'What else would you call it?'"
There is an often repeated, but false, alternative explanation for the term. The erroneous story is that at some time in the early 20th century, the United States military had two classes for airships: Type A-rigid and Type B-limp, hence "blimp". In fact,
The perpetuation of this erroneous explanation is an example of false etymology.