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Concept
Aerostat
Summary
An aerostat (, via French) is a lighter-than-air aircraft that gains its lift through the use of a buoyant gas. Aerostats include unpowered balloons and powered airships. A balloon may be free-flying or tethered. The average density of the craft is lower than the density of atmospheric air, because its main component is one or more gasbags, a lightweight skin containing a lifting gas (which could be heated air or any gas of lower density than air) to provide buoyancy, to which other components such as a gondola containing equipment or people are attached. Especially with airships, the gasbags are often protected by an outer envelope.
Aerostats are so named because they use aerostatic lift which is a buoyant force that does not require movement through the surrounding air mass, resulting in VTOL ability. This contrasts with the heavy aerodynes that primarily use aerodynamic lift which requires the movement of a wing surface through the surrounding air mass. The term has also been used in a narrower sense, to refer to the statically tethered balloon in contrast to the free-flying airship. This article uses the term in its broader sense.
In student usage, the term aerostat refers to any thermostat that remains in the air primarily using aerostatic buoyancy.
Historically, all aerostats were called balloons. Powered types capable of horizontal flight were referred to as dirigible balloons or simply dirigibles (from the French dirigeable meaning steerable). These powered aerostats later came to be called airships, with the term "balloon" reserved for unpowered types, whether tethered (which means attached to the ground) or free-floating.
More recently, the US Government Accountability Office has used the term "aerostat" in a different sense, to distinguish the statically tethered balloon from the free-flying airship.
Balloon (aeronautics)
A balloon is an unpowered aerostat which has no means of propulsion and must be either tethered on a long cable or allowed to drift freely with the wind.
Official source
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