Wednesday, October 31, 2012

Air-Cushion Vehicle

This British hovercraft is held above the surface of the water by blown air. Once the craft is above the surface, it moves much more efficiently than a boat plowing through the water. The propellers on the rear steer the hovercraft.

Air-Cushion Vehicle, also hovercraft, craft that operates above the surface of water or land. The vehicle is supported on a cushion of air 1.2 to 2.4 m (4 to 8 ft) thick. The air cushion is provided by a large fan that pushes air downward within a flexible skirt attached to the perimeter of the vehicle. The skirt maintains the cushion by restraining the air. It makes the vehicle appear to be operating only a few inches above the surface. The vehicle is moved forward by propellers mounted above the vehicle or by control of the air exhaust through small openings around the skirt. Braking is controlled by reversing the pitch of the propeller or by changing the direction of airflow through the skirt vents. This phenomenon is also known in aerodynamics as ground effect.
The operating controls are essentially like those found in an airplane. A control wheel adjusts the pitch of the vehicle, and rudder pedals control yaw (side-to-side movement). The speed over water is limited by wave height and wind speed. Operation of the vehicle is much the same when moving from water to land or vice versa; the air cushion prevents the hull from striking the ground when the terrain changes.
Some air-cushion vehicles, such as the Bertin Hovertruck, have conventional wheel systems for travel over highways and solid ground, but for movement over marshland a built-in air-cushion system supports three-fourths of the weight of the truck. Most air-cushion vehicles, however, are designed for amphibious operation over water, marshy ground, and beaches.
An experimental air-cushion vehicle, designed to run on a track similar to a monorail, reached a speed of 346 km/h (215 mph) on the 6.7-km (4.2-mi) concrete test track in Gometz-la-Ville, France. This so-called aerotrain rode on a cushion of air 0.25 cm (0.1 in) thick.
The vehicles have both commercial and military applications, and a number of 15- to 30-passenger craft have been placed in operation around the world. One of the first commercial air-cushion vehicles began service in 1968. Designed for ferry and passenger service across the English Channel, the craft could carry 30 automobiles and 250 passengers.

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