Aerostat

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The word aerostat was originally French and is derived from the greek aer (air) + statos (standing). An aerostat is a lighter than air object that can stay stationary in the air. Aerostats include free balloons, airships, moored balloons and tethered Helikites. Such a vehicle is lifted by buoyancy, containing a gas less dense than air within an envelope.

The gas is generally helium or hydrogen as these are lightest. Helium is safest as it is inert and so does not burn, but hydrogen has slightly more lift than helium.

Technically, aerostats are capable of providing "aerostatic" lift in that the force upwards arises without movement through the surrounding air mass. This contrasts with aerodynamic lift which requires the movement of at least some part of the aircraft through the surrounding air mass. However, in reality most aerostats (except spherical balloons) obtain lift from both aerodynamic lift and pure gas lift at some time or other.

Aerostats are generally tethered lighter-than-air objects. Types of tethered aerostat include spherical balloons, blimps and Helikites.

Spherical balloons have the lowest surface-area-to-volume ratio and they lift well in low or nil wind. However, unless they are very large, in most winds they quickly begin to be pushed down to the ground. In light winds very large rounded balloons are used to lift people for recreational flight, as in Bournemouth, England.

Blimp shaped balloons were originally designed as barrage balloons just before the first world war. The name blimp derived from the British Admiralty's description of them as "Balloon-Limp". Blimps are also officially known by the military as "Kite-Balloons" due to their stern fins. Thousands of Blimps were used in both world wars. Blimps have change little in design since World War One. The British L.Z. type of World War Two was based upon the French Caquot type of 1915. A British L.Z. barrage was sent to the USA in 1942 where it was copied and became the ZK Type made by Goodyear. Today most blimps are used for advertising in fair weather. Some massive blimps are used for lifting radar or surveillance cameras. Blimps are essentially sausage shaped to reduce frontal area and wind resistance. Blimps have stern fins to keep the blimp pointing into the wind. When they are correctly made they are more stable than spherical balloons, however their large surface area to volume ratio means they need to be large to lift a reasonable payload. Also, as a general rule blimps need to be large to cope with high winds. Blimps long thin shape necessitates a device to equalise pressure in the envelope called a ballonet if they are to go over 1000ft altitude, and to cope with large atmospheric temperature changes.

When set at an angle to the wind, blimps can produce aerodynamic lift especially from their stern fins. When blimps do this it is called "kiting". As the wind increases further this lift causes the stern to rise and the nose to lower. The low nose is further pushed down by the wind leading to an instability called "porpoising". To reduce porpoising the tethers are set to further raise the nose in high winds, however this increases the drag on the blimp causing the blimp to lose height and the tether to lay over to give "quatenary" problems. The handling and cost implications of the blimps large size means they are not commonly used by the general public. However, the military sometimes use large blimps for surveillance and radio relay due to their ability to stay in the air for long periods of time in reasonable weather.

Helikites are a combination of kite and aerostat. They are said to fly to greater altitude and in higher winds than comparably sized blimps. They are a tethered aerostat made of a combination of an oblate spheroid helium balloon and a kite. Helikites were designed, named and patented by Sandy Allsopp in 1993. They are made by Allsopp Helikites Ltd, in Damerham, Hampshire, England. Helikites are lighter than air and so will also fly in no wind to thousands of feet. However, they also utilise aerodynamic lift in a stable manner when wind is available. Due to their rounder shape, Helikites have a better surface-area-to-volume ratio than blimps so have greater aerostatic lift in no wind.

Aerostats are used for lifting military airborne radar equipment, parachute training, for advertising, lifting meterological equipment, raising antennas, gaining line of sight for ad hoc radio relay stations, lifting video equipment and digital cameras, for jungle marker balloon use and birdscaring.