AIR-CUSHION MACHINE

also called Ground-effect Machine, or Hovercraft, vehicle that operates on land or water with its weight supported by air pressure created between the craft and the ground surface. There are two classes of air-cushion machines: those that generate their own pressure differential irrespective of forward speed, called aerostatic craft (sometimes denoted by the acronym ACV, for air-cushion vehicle), and those that require forward speed before the pressure differential can be achieved, called aerodynamic ground-effect machines (GEMs). Sir John Thornycroft of Great Britain was perhaps the first to conceive of an air-cushion vehicle. In the 1870s he theorized that if a ship had a plenum chamber (essentially an empty box, open at the bottom) for a hull and if that plenum chamber were pumped full of air, the ship would rise out of the water and move faster, since there would be reduced drag. He built models to test his idea and in 1877 took out a patent. Thornycroft discovered a problem with his concept, however, that for several decades hindered the development of a successful craft: how to keep the air cushion from escaping from under the craft. It is generally agreed that in the 1950s Christopher Cockerell, also of Great Britain, was the first to devise a method of containing the air cushion. Instead of having a plenum chamber for a hull, Cockerell designed a craft with a slot running around the entire circumference of its bottom from which air could be jetted. The peripheral jets used to pump the air were angled inward slightly, so that the jetted air would mass under the craft and lift it. It was theorized that the force of these jets would create a curtain of air that would keep the air cushion from escaping. In 1959 the world's first practical air-cushion vehicle, the SR.N1, was launched. This first model was able to carry only three passengers at relatively slow speeds and only over calm water or even ground. It differed from Cockerell's original plans in one major respect: it was found that the force of the peripheral jets was not enough to contain the air-cushion, and so a rubberized skirt was suspended around the craft's perimeter. The special advantage of the skirt was that it helped to maintain the air cushion over uneven ground or in choppy water. The skirt is one of the five main components of an air-cushion vehicle, the other four being the hull, engine, lift system, and propulsion system. In construction of the hull, aluminum skin is welded onto frames, also made of aluminum. The hull supports the gas turbine engine, which usually powers both the lift and propulsion systems. Proportionately far more of the engine's power is used to lift the vehicle than to propel it. For lift, high-speed centrifugal fans are used to drive the air through the jets under the craft. A modified aircraft propeller is used for propulsion. Situated at the rear of the craft, the propeller is often mounted on a pylon that can swivel to give the pilot greater control in maneuvering. Further directional control is provided by rudderlike fins at the rear of the craft. Control of these vehicles is difficult because of the air cushion. The skirt has gone through a great number of developments since the SR.N1 was launched. Originally the skirt hung like a curtain around the edge of the craft and was made of rubberized material that quickly wore out from friction with the surface (water or land) during high-speed travel. Skirts are now made of highly durable nylon and plastic, and instead of having the appearance of a curtain, the skirts, known as bag skirts, resemble a thick tube around the craft's edge. Air is jetted from holes along the inside of the inflated bag skirt under the craft to form the air cushion. Bag skirts also provide a means of support for the vehicle when it is at rest. Attached to the bottom edge of the bag skirt is a secondary skirt made of fingerlike segments that protect the bag skirt from friction damage. In the early 1960s it was thought that air-cushion machines would ply the world's oceans at high speeds, or open up the Earth's desert and arctic areas. As the problems of skirt design and engine maintenance (gas turbines are easily clogged and fouled by saltwater spray) arose, the early optimism faded. Although research and development of various craft were being conducted in several nations in the late 20th century, especially for military applications, Great Britain was the only nation in which there was large-scale production. The British built successively larger and faster vehicles and pioneered their commercial use as ferries across the English Channel. The Hovercrafts in use over that route travel at speeds of approximately 60 knots (about 70 miles per hour) and can carry some 420 passengers and 60 vehicles. also called ground-effect machine, or hovercraft, any of the machines characterized by movement in which a significant portion of the weight is supported by forces arising from air pressures developed around the craft, as a result of which they hover in close proximity to the Earth's surface. It is this proximity to the surface that chiefly distinguishes such craft from aircraft, which derive their lift from aerodynamic forces created by movement through the air. Two main classes of air-cushion vehicles exist: those that generate their own pressure differential irrespective of forward speed; and those, more closely related to true aircraft, that require forward speed before the pressure differential can be generated. The former are classed as aerostatic craft (ACVs); the latter are called aerodynamic ground-effect machines (GEMs). Additional reading Bill Gunston, Hydrofoils and Hovercraft (1969), is a comprehensive, easy-to-understand survey that is well illustrated and includes a glossary of terms and a table of major Hovercraft and ACVs. More technical, requiring some college-level mathematics, is G.H. Elsley and A.J. Devereux, Hovercraft Design and Construction (1968), an excellent textbook, based on lectures given at the Isle of Wight Technical College, which deals with the theory and practice of ACV building. Robert L. Trillo, Marine Hovercraft Technology (1971), covers operational problems to a greater extent. See also Jane's High-Speed Marine Craft (annual), a review of ACVs and hydrofoils, providing technical details of available craft, operators, and consultants; and Hoverfoil News (bimonthly), a review of the state of Hovercraft art, including the design, construction, and operation of ACVs of all sizes. John Beresford Bentley The Editors of the Encyclopdia Britannica