Ground effect
Ground effect (or
Wing In Ground effect) is a phenomenon of
aerodynamics where the flow of air around part of an
aircraft or a
racing car is interrupted by the ground.
A conventional aircraft creates a
lifting force between the wing and the air, and this produces wake vortices composed of downward moving air. An aircraft with infinite wingspan works differently: it does not deflect the oncoming air, instead it gains lift from the pressure pattern of the bound vortex surrounding the wing, and this vortex applies a downward force to the "floor" (or "ceiling") no matter how distant. A similar effect occurs if an aircraft flies much less than one wingspan above the ground. During such low altitude flight there is no wake of descending air left behind the aircraft, so no fuel is needed to perform the extra work upon the air. For this reason ground-effect flight is highly efficient. Conventional aircraft experience ground-effect flight during takeoff and landing.
Most pilots, especially of small aircraft, will experience ground effects on landing; in fact the art of landing largely comes down to understanding when these effects need to be taken into account. As the aircraft descends towards the
runway, it will not be influenced by ground effect, but as the aircraft flares and descends within one wingspan of the strip, ground effect will cause a pronounced increase in lift. If not anticipated by the pilot this can cause the aircraft to rise suddenly and significantly — an effect known as a "balloon". Left uncorrected, a balloon can lead to a dangerous situation where the aircraft is rising yet decelerating, a condition which can rapidly lead to a
stall, especially when it is considered that landing speeds are generally only a very small margin above the stall speed. A stall even from a few tens of feet above the ground can cause a major, possibly fatal, crash. A "balloon" may be corrected given sufficient runway remaining, but for novice pilots a better option is to
go around. A good landing approach allows for ground effect such that the aircraft flares and is
held off in ground effect until it gently descends onto the runway.For
helicopters, the ability to hover
in-ground-effect or IGE is much improved, on the order of 1200 to 1500 m higher in altitude when compared to
out-of-ground-effect or OGE hover capability.
The
physics which describe ground effect are still very much under debate. A common belief is that ground effect is caused by a "cushion" of compressed air between the wing and the ground. However, wind tunnel testing and experiments have indicated that while a "cushion" effect is present, ground effect is almost solely due to the ground interrupting the formation of downwash or
wingtip vortices. Wingtip vortices destroy great amounts of the lift generated by the wing through increased downwash behind it, thereby decreasing the aircraft's theoretical
angle of attack. Its effective diminishment needs to be compensated for by pulling back on the control wheel, increasing the actual angle of attack and consequently increasing drag. This is sometimes referred to as
induced drag, and the tendency decreases an aircraft's efficiency greatly. When very close to the ground, the
wingtip vortices are interrupted, and this results in greater efficiency.
Some critics of
Howard Hughes's massive
Spruce Goose claim that the famous
flying boat's first (and only) flight was due entirely to ground effect and that the craft was incapable of sustaining flight above a very low altitude.
A WIG craft is an aircraft that normally operates in ground effect. Craft of this type can be grouped into one of three Classes which were established by the International Maritime Organization (IMO):
Class A craft are unable to operate out of ground effect.
Class B craft are capable of "jumping" out of ground effect in order to clear potential obstacles but cannot sustain true free flight and will return to ground effect flight after jumping.
Class C craft are able to enter true free flight and operate out of ground effect.
Since Class A and B craft cannot sustain free flight and therefore are not considered true aircraft, they do not fall under the jurisdiction of the Federal Aiviation Administration. Because of this, a pilot's license is not required for their operation. This exception does not apply to Class C craft, and there are no real differences between them and normal seaplanes.
Class A and B craft normally use special wing configurations and airfoils which are either not suitable for free flight, or are so specialized for ground effect that they are much less efficient than normal airfoils in free flight. Common type A configurations are the tandem, ram-wing and ekranoplane. A good example of the Type B configuration is the reverse delta wing used in the Airfisch craft. This configuration was pioneered by the German glider designer Alexander Lippisch and can also be referred to by his last name..
The USSR invested heavily in the design of ground effect craft producing a multitude of designs over the years, which culminated in the KM, the heaviest ground effect craft ever made. It was referred to by the CIA as the Caspian Sea Monster because of its enormous size. The KM used
PAR thrust coming from eight of its ten turbojets to assist in takeoff, a feature that has been used on various model and full sized WIG craft. Another notable design was the Orlyonok, an ekronoplane design which but for the fall of the Soviet union would have been the only ekranoplane to go into mass production. The Orlyonok was powered by a massive turboprop in the tail assembly with two turbojets providing PAR thrust on takeoff and later shut down for normal flight, its main use would have been as a high speed military assault transport, all production was stopped after the fall of te Soviet union. Only one ekranoplane remains in a close to operational condition, one of the Lun missile boats has been completed and is awaiting money for finishing touches after which it will be used as a fast response search and rescue craft.
In racing cars, a designer's aim is not for increased lift but for increased
downforce, allowing greater cornering speeds. (By the
1970s 'wings', or inverted
aerofoils, were routinely used in the design of racing cars to increase downforce, but this is
not ground effect.) This kind of ground effect is easily illustrated by taking a tarpaulin out on a windy day and holding it close to the ground, it can be observed that when close enough to the ground the tarp will suddenly be sucked towards the ground.
However, substantial further downforce is available by understanding the ground to be part of the aerodynamic system in question. The basic idea is to create an area of low
pressure underneath the car, so that the higher pressure above the car will apply a downward force. Naturally, to maximize the force one wants the maximal area at the minimal pressure. Racing car designers have achieved low pressure in two ways: first, by using a fan to push air out of the cavity; second, to design the underside of the car so that large amounts of incoming air are accelerated through a narrow slot between the car and the ground, lowering pressure by
Bernoulli's principle. Official regulations
as of 2006 disallow ground effects in many types of racing, such as
Formula One.
Jim Hall, the first car aerodynamicist to harness downforce, built
Chaparral cars to both these principles. His 1961 car attempted to use the shaped underside method but there were too many other aerodynamic problems with the car for it to work properly. His 1966 cars used a dramatic high wing for their downforce. His Chaparral 2J "sucker car" of 1970 was revolutionary. It had two fans at the rear of the car driven by a dedicated
two-stroke engine; it also had "skirts", which left only a minimal gap between car and ground, so as to seal the cavity from the atmosphere. Although it did not quite win a race, the competition lobbied for its ban, which came into place at the end of that year. Movable aerodynamic devices were banned from most branches of the sport.
Formula One in the late 1970s was the next setting for ground effect in racing cars. In 1977
Lotus brought out their "Wing Car", the
Lotus 78, designed by
Peter Wright,
Colin Chapman, and
Tony Rudd. Its sidepods, bulky constructions between front and rear wheels, were shaped as inverted aerofoils and sealed with flexible "skirts" to the ground. The team won 5 races that year, and 2 in 1978 while they developed the much improved
Lotus 79. The most notable contender in 1978 was the
Brabham BT46B Fancar, designed by
Gordon Murray. Its fan, spinning on a horizontal, longitudinal axis at the back of the car, took its power from the main gearbox. The car avoided the sporting ban by claims that the fan's main purpose was for engine cooling as less than 50% of the airflow was used to create a depression under the car . It raced just once, with
Niki Lauda winning at the Swedish Grand Prix. However, the team, led by
Bernie Ecclestone who had recently become president of the
Formula One Constructors Association, withdrew the car before it had a chance to be banned. The Lotus 79, on the other hand, went on to win 6 races and the world championship for
Mario Andretti. In following years other teams copied and improved on the Lotus until, cornering speeds became dangerously high, resulting in several severe accidents in 1982 (most notably the death of
Gilles Villeneuve), flat undersides became mandatory for 1983. Part of the danger of relying on ground effects to corner at high speeds is the possibility of the sudden removal of this force; if the belly of the car contacts the ground, the flow is constricted too much, resulting in almost total loss of any ground effects. If this occurs in a corner where the driver is relying on this force to stay on the track, its sudden removal can cause the car to abruptly lose most of its traction and skid off the track.
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SE-Technology, engineering explanation*
Photoessayist.com: The Chaparral 2J*
VintageRPM: Chaparral history*
8W: Brabham-Alfa BT46B "fan car"*
Dennis David: Lotus 79*
Coandă effect*
Hovercraft*
Ekranoplane*
Boeing Pelican
