The static buoyancy of airships in flight is not constant. It is therefore necessary to control the altitude of an airship by controlling its buoyancy: buoyancy compensation.
On the Macon, the exhaust water recovery condensers appear as dark vertical strips above each engine. The Akron and LZ 130 Graf Zeppelin had similar systems.
Blimp
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A blimp, less commonly called a non-rigid airship, is an airship without an internal structural framework or a keel. Unlike semi-rigid and rigid airships, blimps rely on the pressure of their lifting
The Spirit of Goodyear, one of the iconic Goodyear Blimps
Steerable ducted fans on a Skyship 600 provide thrust, limited direction control, and also serve to inflate the ballonets to maintain the necessary overpressure.
A modern blimp from Airship Management Services showing a strengthened nose, ducted fans attached to the gondola under the hull, and cable-braced fins at the tail
Advertising blimp landing at local airport in New Jersey