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Categories of Aircraft
Supported by Lighter-Than-Air Gases (aerostats)
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Supported by LTA Gases + Aerodynamic Lift
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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 consists of a lightweight skin filled with a lifting gas to create buoyancy.

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 no wind. However, unless they are very large, in most winds they quickly begin to be pushed 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. Thousands of blimps were used in both world wars, but they have changed little in design since the First World War. The British L.Z. type of the Second World War 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 the Goodyear Tire & Rubber Company. Today most blimps are used for advertising in fair weather. Some massive blimps are used for lifting radar or surveillance cameras.

Blimps are more or less sausage-shaped, to reduce frontal area and wind resistance. They have stern fins to keep the balloon pointing into the wind. When they are correctly made, blimps are more stable than spherical balloons; however, their large ratio of surface area to volume requires blimps to be large so that they can lift a sufficient payload to be efficient. Generally, blimps must be large also to cope with high winds. Their long, thin shape necessitates a device to equalize pressure in the envelope, called a ballonet, if they are to rise over 900 feet in 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 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 can be very small or large as required, because even the smallest are very stable in high winds and fly thousands of feet high. 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. Unlike all other aerostats, Helikites are so stable that they require no adjustment whatever the wind conditions. Helikites are a semi-rigid design, with solid carbon-fibre hard-points for securely attaching equipment. This makes them useful for time-critical situations such as military use and for stable photography.

Aerostats are used for lifting military airborne radar equipment, parachute training, for advertising, lifting meteorological 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.


Types of aerostats:

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Published in July 2009.

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