A ground-effect vehicle (GEV), ekranoplan (from ), wing-in-ground-effect (WIGE or WIG), ground-effect craft/machine (GEM), wingship, flarecraft, or surface effect vehicle, is a hybrid surface vehicle/aircraft that makes use of wings to generate minimal lift and ground effect, to reduce aerodynamic drag. Ground effect is the interaction between a moving wing and a stationary surface below it.
Typically, a GEV will glide just above a relatively level surface. Some vehicles can operate over any flat area such as a lake or flat plains.
Technically, GEVs do not include racecars that utilize ground-effect, for increasing downforce, as such vehicles are intended to remain in direct contact with a racetrack.
Ground effect
Takeoff Any airfoil passing through air increases air pressure on the underside, while decreasing pressure on the upper side, which generates lift. The high and low pressures are maintained until they flow off the ends of the wings, where they form vortices that are the major source of lift-induced drag—normally a significant portion of the total drag.
In GEV, the angle of attack is the angle between its chordline (a straight line from the leading edge to the trailing edge) and the ground. On takeoff, airplanes pitch their noses up to increase the angle of attack to reach the ideal of 12–20 degrees (depending on wing design and other factors).
Design
Placing the wing near a surface has the same effect as increasing the aspect ratio because the surface prevents wingtip vortices from expanding, but without the complications associated with a long, slender wing. The stubby wings on a GEV can produce as much lift as the much larger wing on a transport aircraft, though only while close to the earth's surface. Once sufficient speed has built up, some GEVs can function as conventional aircraft until approaching a destination. However, they are unable to land or take off without a significant amount of help from the ground effect, and cannot climb until they have reached a much higher speed. The greater the wingspan, the less drag created for each unit of lift and the greater the efficiency of the wing.
GEVs are not statically supported upon a cushion of pressurized air from a downward-directed fan. Some GEV designs, such as the Russian Lun and Dingo, blew air under the wing using auxiliary engines to assist takeoff; however they still require forward motion to generate sufficient lift to fly, unlike hovercraft, also lacking low-speed hover capability. GEVs also have no contact with the surface when in flight.
Straight wing Used by the Russian Rostislav Alexeyev for his ekranoplan. The wings are significantly shorter than those of comparable aircraft, and this configuration requires a high aft-placed horizontal tail to maintain stability. The pitch and altitude stability comes from the lift slopeCl/da, with Cl = lift coefficient, and a = angle of incidence. difference between a front low wing in ground-effect (commonly the main wing) and an aft, higher-located second wing nearly out of ground-effect (generally named a stabilizer). A design by REGENT uses a related design in the form of an approximately L-shaped wing attached to the top of the fuselage, with a pontoon at the end for water landings.
Reverse-delta wing Developed by Alexander Lippisch, this wing allows stable flight in ground-effect through self-stabilization. This is the main Class B form of GEV. Hanno Fischer later developed WIG craft based on the configuration, which were then transferred to multiple companies in Asia, thus becoming one of the "standards" in GEV design.
Tandem wings Tandem wings can have three configurations: A biplane-style type-1 utilising a shoulder-mounted main lift wing and belly-mounted sponsons similar to those on combat and transport helicopters. A canard-style type-2 with a mid-size horizontal wingNot a stabilizer because destabilizing. near the nose of the craft directing airflow under the main lift airfoil. This type-2 tandem design is a major improvement during takeoff, as it creates an air cushion to lift the craft above the water at a lower speed, thereby reducing water drag, which is the biggest obstacle to successful seaplane launches. Two stubby wings as in the tandem-airfoil flairboat produced by Günther Jörg in Germany. His particular design is self-stabilizing longitudinally.
Advantages and disadvantages
Given similar hull size and power, and depending on its specific design, the lower lift-induced drag of a GEV, as compared to an aircraft of similar capacity, will improve its fuel efficiency and, up to a point, its speed. GEVs are also much faster than surface vessels of similar power, because they avoid drag from the water.
On the water the aircraft-like construction of GEVs increases the risk of damage in collisions with surface objects. Furthermore, the limited number of egress points make it more difficult to evacuate the vehicle in an emergency. According to WST, the builders of the WIG craft WSH-500, GEVs furthermore have the advantage of avoiding conflict with ocean currents by flying over them.
Since most GEVs are designed to operate from water, accidents and engine failure typically are less hazardous than in a land-based aircraft, but the lack of altitude control leaves the pilot with fewer options for avoiding collision, and to some extent that negates such benefits. Low altitude brings high-speed craft into conflict with ships, buildings and rising land, which may not be sufficiently visible in poor conditions to avoid. GEVs may be unable to climb over or turn sharply enough to avoid collisions, while drastic, low-level maneuvers risk contact with solid or water hazards beneath. Aircraft can climb over most obstacles, but GEVs are more limited.
In high winds, take-off must be into the wind, which takes the craft across successive lines of waves, causing heavy pounding, stressing the craft and creating an uncomfortable ride. In light winds, waves may be in any direction, which can make control difficult as each wave causes the vehicle to both pitch and roll. The lighter construction of GEVs makes their ability to operate in higher sea states less than that of conventional ships, but greater than the ability of hovercraft or hydrofoils, which are closer to the water surface.
Like conventional aircraft, greater power is needed for takeoff, and, like seaplanes, ground-effect vehicles must get on the step before they can accelerate to flight speed. Careful design, usually with multiple redesigns of hullforms, is required to get this right, which increases engineering costs. This obstacle is more difficult for GEVs with short production runs to overcome. For the vehicle to work, its hull needs to be stable enough longitudinally to be controllable yet not so stable that it cannot lift off the water.
The bottom of the vehicle must be formed to avoid excessive pressures on landing and taking off without sacrificing too much lateral stability, and it must not create too much spray, which damages the airframe and the engines. The Russian ekranoplans show evidence of fixes for these problems in the form of multiple chines on the forward part of the hull undersides and in the forward location of the jet engines.
Finally, limited utility has kept production levels low enough that it has been impossible to amortize development costs sufficiently to make GEVs competitive with conventional aircraft.
A 2014 study by students at NASA's Ames Research Center claims that use of GEVs for passenger travel could lead to cheaper flights, increased accessibility and less pollution.
Classification One obstacle to GEV development is the classification and legislation to be applied. The International Maritime Organization has studied the application of rules based on the International Code of Safety for High-Speed Craft (HSC code) which was developed for fast ships such as hydrofoils, hovercraft, catamarans and the like. The Russian Rules for classification and construction of small type A ekranoplans is a document upon which most GEV design is based. However, in 2005, the IMO classified the WISE or GEV under the category of ships.
The International Maritime Organization recognizes three types of GEVs:
At the time of writing, those classes only applied to craft carrying 12 passengers or more, and (as of 2019) there was disagreement between national regulatory agencies about whether these vehicles should be classified, and regulated, as aircraft or as boats.
History
The term Ground-Effect Vehicle originally referred to any craft utilizing ground effect, including what later became known as hovercraft, in patent descriptions during the 1950s. However, this term came to exclude air-cushion vehicles or hovercraft.
By the 1920s, the ground effect phenomenon was well-known, as pilots found that their airplanes appeared to become more efficient as they neared the runway surface during landing. In 1934 the US National Advisory Committee for Aeronautics issued Technical Memorandum 771, Ground Effect on the Takeoff and Landing of Airplanes, which was a translation into English of a summary of French research on the subject. The French author Maurice Le Sueur had added a suggestion based on this phenomenon: "Here the imagination of inventors is offered a vast field. The ground interference reduces the power required for level flight in large proportions, so here is a means of rapid and at the same time economic locomotion: Design an airplane which is always within the ground-interference zone. At first glance this apparatus is dangerous because the ground is uneven and the altitude called skimming permits no freedom of maneuver. But on large-sized aircraft, over water, the question may be attempted ..."
By the 1960s, the technology started maturing, in large part due to the independent contributions of Rostislav Alexeyev in the Soviet Union and German Alexander Lippisch, working in the United States. Alexeyev worked from his background as a ship designer whereas Lippisch worked as an aeronautical engineer. The influence of Alexeyev and Lippisch remains noticeable in most GEVs seen today.
Initial efforts
Canada It is said that the research hydrofoil HD-4 by Alexander Graham Bell had part of its dynamic lift contributed by its pair of wings operating in ground effect. However it is dubious whether the designer was aware of its existence due to the relative infancy of aerodynamics.
Avro Canada investigated into aircraft with a Coanda-effect propulsion system. Such jets were supposed to create an air cushion below the airframe that will allow them to hover on the ground. In fact, of the only test aircraft built, this was the only mode they could possibly operate from due to stability issues when taking off. The designs were later further developed by the United States, while Convair could have possibly been inspired by them to create a preliminary design of a large ocean-going ground-effect ship called Hydroskimmer.
Soviet Union
Led by Alexeyev, the Soviet Central Hydrofoil Design Bureau () was the center of ground-effect craft development in the USSR. The vehicle came to be known as an ekranoplan (, экран screen + план plane, from , literally screen effect, or ground effect in English). The military potential for such a craft was soon recognized, and Alexeyev received support and financial resources from Soviet leader Nikita Khrushchev.