Aerostat

An aerostat (from Ancient Greek ἀήρ (aḗr) 'air' and στατός (statós) 'standing', via French) or lighter-than-air aircraft is an aircraft that relies on buoyancy to maintain flight. Aerostats include unpowered balloons (free-flying or tethered) and powered airships. The relative density of an aerostat as a whole is lower than that of the surrounding atmospheric air (hence the name "lighter-than-air"). Its main component is one or more gas capsules made of lightweight skins, containing a lifting gas (hot air, or any gas with lower density than air, typically hydrogen or helium) that displaces a large volume of air to generate enough buoyancy to overcome its own weight. Payload (passengers and cargo) can then be carried on attached components such as a basket, a gondola, a cabin or various hardpoints. With airships, which need to be able to fly against wind, the lifting gas capsules are often protected by a more rigid outer envelope or an airframe, with other gasbags such as ballonets to help modulate buoyancy. Aerostats are so named because they use aerostatic buoyant force that does not require any forward movement through the surrounding air mass, resulting in the inherent ability to levitate and perform vertical takeoff and landing. This contrasts with the heavier-than-air aerodynes that primarily use aerodynamic lift, which must have consistent airflow over an aerofoil (wing) surface to stay airborne. 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.