An ekranoplan (Russian: экранопла́н, French: ecran screen + plan plane ) is a vehicle resembling an aircraft but which operates solely on the principle of ground effect (in Russian эффект экрана effekt ekrana – from which the name derived). Ground effect vehicles (GEV) fly above any flat surface, with the height above ground dependent upon the size of the vehicle. Ekranoplan design was conceived by revolutionary Soviet engineer Rostislav Alexeev.
During the Cold War, ekranoplans were sighted for years on the Caspian Sea as huge, fast-moving objects. The name Caspian Sea Monster was given by US intelligence operatives who had spotted the huge vehicle, which looked like an airplane with the outer halves of the wings removed. After the end of the Cold War, the “monster” was revealed to be one of several Soviet military designs meant to fly only a few meters above water, saving energy and staying below enemy radar.
The ekranoplan has a lifting power of 1,000 tonnes (984 long tons), among the largest ever achieved. The KM, as the Caspian Sea Monster was known in the top secret Soviet military development program, was over 100 metres (328 ft) long, weighed 540 t (531 long tons) fully loaded, and could travel over 400 kilometres per hour (249 mph), mere meters above the surface of the water. Another model was the Lun-class, entering service with the Black Sea Fleet in 1987; the Lun-class vehicles had a top speed of 297 knots (550 km/h).
The important design principle is that wing lift is reduced as operating altitude of the ekranoplan is increased (see ground effect). Thus it is dynamically stable in the vertical dimension. Once moving at speed, the ekranoplan was no longer in contact with the water, and could move over ice, snow, or level land with equal ease, though flight over land would have involved extreme risks unless the surface were very dependably flat.