Ekranoplans

A ground effect vehicle (GEV) is a vehicle that is designed to attain sustained flight over a level surface (usually over the sea), by making use of ground effect, the aerodynamic interaction between the wings and the surface. Among the best known are the Soviet ekranoplans, but names like wing-in-ground-effect (WIG), flarecraft, sea skimmer, or wing-in-surface-effect ship (WISE) are also used.

In recent years a large number of different GEV types have been developed for both civilian and military use. However, these craft have yet to enter widespread use. The German Tandem Airfoil Flairboats or Skimmerfoils constructed by Gunther Jörg differ in their use of self-stabilizing wings.

Although they may look and have related technical characteristics similar to seaplanes, ground effect vehicles are not aircraft, as they are unable to fly freely in the air. They are also dissimilar from hovercraft or hydrofoils, as they do not have any contact with the surface of the water. Ground effect vehicles constitute a completely unique class of transportation.

A ground effect vehicle needs some forward velocity to produce lift dynamically and the principal benefit of operating a wing in ground effect is to reduce its lift-dependent drag. The basic design principle is that the closer the wing operates to an external surface such as the ground, said to be in ground effect, the more efficient it becomes.

An airfoil passing through air increases air pressure on the underside, while decreasing pressure across the top. The high and low pressures are maintained until they flow off the ends of the wings, where they form vortices which in turn are the major cause of lift-induced drag—normally a large portion of the drag affecting an aircraft. The higher the aspect ratio of the wing (that is, the longer and skinnier it is), the less induced drag created for each unit of lift and the greater the efficiency of the particular wing. This is the primary reason gliders have long and skinny wings.

Placing the same wing near a surface such as the water or the ground has the effect of greatly increasing the aspect ratio, but without having the complications associated with a long and slender wing, so that the short stubs on an Ekranoplan can produce just as much lift as the much larger wing on a transport aircraft, though it can only do this when close to the earth's surface. Once sufficient speed has built up, some GEVs may be capable of leaving ground effect and functioning as normal aircraft until they approach their destination. The distinguishing characteristic is that they are unable to land or take off without a significant amount of help from the ground effect cushion, and cannot climb until they have reached a much higher speed.

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