Ground effect
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The term Ground effect (or Wing In Ground effect) refers to the increase in lift experienced by an aircraft as it approaches within roughly 1/4 of a wingspan's length of the ground or other level surface (such as the sea). It can present a hazard for inexperienced pilots who are not accustomed to take it into account on their approach to landing, but it has also been used to effectively enhance the performance of certain kinds of aircraft whose planform has been adapted to take advantage of it, such as the Russian ekranoplans. The term is also sometimes used in motorsport to refer to aerodynamic techniques for increasing downforce, such as wings and venturi tunnels, but strictly speaking they are not exploiting the same aerodynamic phenomena as the ground effect in fixed and rotary wing aircraft.
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[edit] Ground effect in aircraft
A conventional aircraft creates a lifting force. The leading edge (front) of the wing cuts through the air and the cross-sectional shape of the wing (airfoil) creates a net downward deflection of the incoming airstream, known as downwash. By Newton's third law of motion, the reaction force is upwards on the wing, that is, lift.
Near the wingtip the airstream is able to flow around the end of the wing and this produces curls of downward moving air off the ends of the wings as well. These downward curls of turbulence effectively slow down the air flowing across the top of the wing, and speed up the air flowing underneath, which reduces the lift generated by the wing.
When an aircraft is flying in "free air" these downward curls which spoil the efficiency of the wing simply dissipate as wasted energy; however, if the aircraft is flown less than about one wingspan away from the ground, these downward curls of air from the back of the wing and the wingtips begin to come into contact with the ground, which increases the lift produced by the wing. This is known as ground effect.
The physics which describe ground effect are still very much under debate.
It appears that one reason for the improvement in efficiency is the "cushion" of air formed beneath the wing, but the inability of the air below the wing to behave as "free air" also seems to affect the way in which the turbulence around the end of the wing (and to a lesser extent off the back of the wing) is able to develop into downward curls in the first place. With less turbulence generated behind and around the wing to impede the airflow, it is able to flow more quickly over the top surface and more slowly underneath, and the wing therefore becomes more efficient and produces better lift.
Within one wingspan of the ground, the ground effect begins to change the behaviour of the airflow around the end of the wings, and at closer distances (around one-tenth of the wingspan) it begins to affect the turbulence generated off the back of the wing as well.
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 skill 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.
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.
[edit] Ground effect in cars
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 although it is still permitted in Champ cars.
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.
Note that while such downforce-producing aerodynamic techniques are often referred to with the catch-all term "ground effect", they are not strictly speaking a result of the same aerodynamic phenomenon as the ground effect which is apparent in aircraft at very low altitudes.
[edit] External links
- Universal Hovercraft Hoverwing(tm) ACV/WIG
- SE-Technology, engineering explanation
- Photoessayist.com: The Chaparral 2J
- VintageRPM: Chaparral history
- 8W: Brabham-Alfa BT46B "fan car"
- Dennis David: Lotus 79
- Aerospaceweb.org | Ask Us - Ground Effect and WIG Vehicles
