A blimp, or non-rigid airship, is a floating airship without an internal supporting framework, or keel. A non-rigid airship differs from a semi-rigid airship 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.
The term "blimp" refers only to free-flying aircraft. The term is sometimes erroneously used to refer to the tethered craft known as moored balloons. While often very similar in shape, moored balloons have no propulsion and are tethered to the ground.
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Since 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 its ability to be steered and top speed is 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 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 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 state heavier than air avoids the need to dump ballast at lift-off and also avoids the need to lose costly lifting gas on landing.
The term "blimp" is reportedly onomatopoeic, the sound the airship makes when one taps the envelope (balloon) with a finger.[1] Although there is some disagreement among historians, credit for coining the term is usually given to Lt. A.D. Conningham of the British Royal Navy in 1915.[2]
A 1943 etymology published in the New York Times confirms the 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.[3]
A 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 from a 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?"[4]
An oft-repeated, but false, etymology is that the United States military initially had two classes of aircraft: A-rigid and B-limp, hence "blimp". In fact,
... there was no American "A-class" of airships as such – all military aircraft, heavier or lighter-than-air were designated with "A" until the appearance of B-class airships in May 1917. There was an American B airship – but there seems to be no record of any official designation of non-rigids as "limp". Further, according to the Oxford Dictionary, the first appearance of the word in print was in 1916, in England, a year before the first B-class airship.—"Etymology of "Blimp" by Dr. A. D. Topping, AAHS Journal, Winter 1963.
The B class blimps were patrol airships operated by the United States Navy during and shortly after World War I. The Navy learned a great deal from the DN-1 fiasco. The result was the very successful B-type airships. Dr. Jerome Hunsaker was asked to develop a theory of airship design, Lt. John H. Towers returned from Europe having inspected British designs, the Navy sought bids for 16 blimps from American manufacturers. On 4 February 1917 the Secretary of the Navy directed that 16 nonrigid airships of Class B be procured. [1] Ultimately Goodyear built 9 envelopes, Goodrich built 5 and Curtiss built the gondolas for all of those 14 ships. Connecticut Aircraft contracted with U.S. Rubber for its two envelopes and with Pigeon Fraser for its gondolas. The Curtiss-built gondolas were modified JN-4 fuselages and were powered by OX-5 engines. The Connecticut Aircraft blimps were powered by Hall-Scott engines.
In 1930 a former German airship officer, Captain Anton Heinen, working in the US for the US Navy on it dirigible fleet attempted to design a build a four place blimp called the family air yacht for private fliers which if placed in production the inventor claimed would be priced below $10,000 and easier to fly than a fixed wing aircraft. It was unsuccessful. [5][6]
There are several blimps worldwide.[7] Some examples include:
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