air-cushion vehicle

air-cushion vehicle (ACV), craft designed to travel close to but above ground or water. It is also called a ground-effect machine or Hovercraft. These vehicles are supported in various ways. Some of them have a specially designed wing that will lift them just off the surface over which they travel when they have reached a sufficient horizontal speed (the ground effect). Others are supported by fans that force air down under the vehicle to create lift. In a plenum chamber vehicle the rate of leakage of this air from underneath the vehicle is reduced by placing a skirt around the lower edge of the craft. In an annular jet vehicle the rate of leakage is reduced by directing the air downward and inward from the outer edges of the vehicle. Air propellers, water propellers, or water jets usually provide forward propulsion.

Air-cushion vehicles can attain higher speeds than can either ships or most land vehicles and use much less power than helicopters of the same weight. Air-cushion suspension has also been applied to other forms of transportation, in particular trains, such as the French Aerotrain and the British Hovertrain. A relatively smooth land or water surface, however, is a necessity; most of these vehicles cannot clear waves higher than 3 to 5 1⁄2 ft (1–1.67 m).

The first recorded design for an air-cushion vehicle was put forward by Emmanual Swedenborg, a Swedish designer and philosopher, in 1716. The project was short-lived and a craft was never built, for Swedenborg soon realized that to operate such a machine required a source of energy far greater than any available at that time. In the mid-1870s, the British engineer Sir John Thornycroft built a number of model craft to check the air-cushion effects and even filed patents involving air-lubricated hulls, although the technology required to implement the concept did not yet exist. From this time both American and European engineers continued work on the problems of designing a practical craft.

In the early 1950s the British inventor Christopher Cockerell began to experiment with such vehicles, and in 1955 he obtained a patent for a vehicle that was "neither an airplane, nor a boat, nor a wheeled land craft." He had a boat builder produce a two-foot prototype, which he demonstrated to the military in 1956 without arousing interest. Cockerell persevered, and in 1959 a commercially built one-person Hovercraft crossed the English Channel. In 1962 a British vehicle became the first to go into active service on a 19-mi (31-km) ferry run. The maximum size of air-cushion vehicles is now over 100 tons; some of them travel at over 100 mi (160 km) per hr. Although air-cushion vehicles of several thousand tons have been under development for many years, it is in small vehicles, usually called flarecraft, that the greatest current potential market exists; current flarecraft can carry one to eight people at 150 mi (240 km) per hr.

See J. R. Amyot, ed., Hovercraft Technology, Economics, and Applications (1990).

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