Air-Cushion Vehicle, Ram-wing

(ram-wing ACV; also called ram-wing ground-effect machine [GEM]), an aircraft designed to fly close to the earth’s surface, usually at heights of one-tenth to one-fifth the width of the wing. A ram-wing ACV differs from an airplane in that the former uses the ground effect to produce the aerodynamic forces and differs from a marine GEM in that it uses the approaching airflow—that is, a velocity head—rather than the pressure of the air forced under a marine GEM by a special fan.

The main design features of a ram-wing ACV are as follows. The wing is mounted low to enhance the ground effect and has a low aspect ratio (seldom exceeding 1.5–2) to ensure safety in flight. End plates or floats are carried on the underside of the wing at the wing tips to reduce the flow of air from beneath the wing—that is, from the high-pressure area—over the wing tips. The tail unit, which is horizontal and extends above both the influence of the ground effect and the airflow swept past the wing, is set high to provide longitudinal stability. Takeoff devices—for example, flaps, spoilers, ailerons, water skis, and forcing engines—are used to reduce aerohydrodynamic drag when the vehicle accelerates over water or to reduce the frictional resistance of the ground when it moves over land.

The main advantage of a ram-wing ACV is its high lift-drag ratio, which may be as high as 20–25. The lift-drag ratio of a ram-wing ACV is 25–50 percent higher than that of modern transport airplanes. Such a high lift-drag ratio makes it possible to increase the payload or to reduce the thrust of the engine and, consequently, the fuel consumption.

Virtually all the ram-wing ACV’s that have been built are designed for takeoffs from water and landings on water. Ram-wing ACV’s that are intended for use only over land and do not have the required buoyancy for use over water have also been built. Some ram-wing ACV’s that have been developed can fly above the influence of the ground effect, that is, above a height ranging from eight-tenths the width of the wing to the width of the wing.

The first ram-wing ACV was built in 1935 by the Finnish engineer T. Kario. As of 1978, about 30 small experimental vehicles had been built in Finland, Sweden, the USA, Japan, the Federal Republic of Germany (FRG), Great Britain, and the USSR. As a rule, the performance characteristics of the experimental vehicles are modest. The vehicles weigh 0.3–4.3 tons, are powered by one or two engines that develop a total of 16–520 hp, have cruising speeds of 22–250 km/hr, and can seat one to six persons. Four of the experimental vehicles—the ESKA-1 (USSR), the X-112 (USA), and the X-113 and X-114 (FRG)—can fly above the influence of the ground effect. For example, the X-113 can reach an altitude of 800 m.