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A non-rigid airship, commonly called a blimp (/blɪmp/), is an airship (dirigible) without an internal structural framework or a keel. Unlike semi-rigid and (e.g. ), blimps rely on the pressure of their lifting gas (usually helium, rather than flammable hydrogen) and the strength of the envelope to maintain their shape. Blimps are known for their use in advertising, surveillance, and observation due to their maneuverability, slow speeds and steady flight capabilities.
Volume changes of the lifting gas due to temperature changes or to changes of altitude are compensated for by pumping air into internal (air bags) to maintain the overpressure. Without sufficient overpressure, the blimp loses its ability to be steered and is slowed due to increased drag and distortion. The propeller air stream can be used to inflate the ballonets and so the hull. In some models, such as the Skyship 600, differential ballonet inflation can provide a measure of Flight dynamics 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 may 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 led to the development of semi-rigids and .
Modern blimps are launched somewhat heavier than air (overweight), in contrast to historic blimps. The missing lift is provided by lifting the nose and using engine power, or by angling the engine thrust. Some types also use steerable propellers or . Operating in a state heavier than air avoids the need to dump ballast at lift-off and also avoids the need to lose costly helium lifting gas on landing (most of the achieved lift with very inexpensive hydrogen, which could be vented without concern to decrease altitude).
A 1943 etymology, published in The New York Times, supports a British origin during the First World War when the British were experimenting with lighter-than-air craft. The initial non-rigid aircraft was called the A-limp; and a second version called the B-limp was deemed more satisfactory.
Yet a third derivation is given by Barnes and James in Short Brothers Aircraft since 1900:
Dr. A. D. Topping researched the origins of the word and concluded that the British had never had a "Type B, limp" designation, and that Cunningham's coinage appeared to be the correct explanation.
The Oxford English Dictionary notes its use in print in 1916: "Visited the Blimps ... this afternoon at RNAS Capel". In 1918, the Illustrated London News said that it was "an onomatopœic name invented by that genius for apposite nomenclature, the late Horace Short".
In 1930, a former German airship officer, Captain Anton Heinen, working in the US for the US Navy on its dirigible fleet, attempted to design and build a four-place blimp called the "family air yacht" for private fliers which the inventor claimed would be priced below $10,000 and easier to fly than a fixed-wing aircraft if placed in production. It was unsuccessful.
In 2021, Reader's Digest said that "consensus is that there are about 25 blimps still in existence and only about half of them are still in use for advertising purposes". The Airsign Airship Group is the owner and operator of 8 of these active ships, including the Hood Blimp, DirecTV blimp, and the MetLife blimp.
Surveillance blimps known as have been used extensively in the Middle East by the United States military, the United Arab Emirates and Kuwait.
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