Editing Blimp (section)

==Principle==
[[File:GR SK Propeller.jpg|thumb|left|Steerable ducted fans on a [[Airship Industries Skyship 600|Skyship 600]] provide thrust, limited direction control, and also serve to inflate the ballonets to maintain the necessary overpressure.]]

Since blimps keep their shape with internal overpressure, typically the only solid parts are the passenger car (gondola) and the [[empennage|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 [[thermal airship|hot-air airship]] (sometimes there are [[batten]]s near the bow, which assist with higher forces there from a mooring attachment or from the greater aerodynamic pressures there).

Volume changes of the lifting gas due to temperature changes or to changes of altitude are compensated for by pumping air into internal [[ballonet]]s (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|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 [[Hogging and sagging|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-rigid airship|semi-rigid]]s and [[rigid airship]]s.

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 [[ducted fan]]s. Operating in a state heavier than air avoids the need to dump [[buoyancy compensator (aviation)|ballast]] at lift-off and also avoids the need to lose costly helium lifting gas on landing (most of the [[Zeppelin]]s achieved lift with very inexpensive hydrogen, which could be vented without concern to decrease altitude).
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