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.

Contents 1 Ground effect in aircraft 2 Ground effect in cars 3 External links 4 See also

[edit] Ground effect in aircraft

Wings create lift through the generation of a high(er) pressure area below the wing and a low(er) pressure area above the wing; this is what contributes to the lifting force. The high-pressure air under the wing will seek to flow to the low-pressure area on top of the wing, and this mostly happens at the wingtips, creating what are known as wingtip vortices. Wingtip vortices cause downwash, which reduces the amount of lift produced by the wing, reducing its effective angle of attack and increasing its induced drag. It is thought that the phenomenon of ground effect is caused by the ground 'interrupting' the wingtip vortices, reducing downwash and thus increasing lift.

Ground effect is much misunderstood. During landing, a small plane may 'float' some distance down the runway beyond the pilot's intended touch-down point, but this is mostly due to the airplane's change in attitude as the pilot 'flares' for landing. During the flare, the pilot is arresting the steady rate of descent he/she has maintained during the final approach by rotating the aircraft about its pitch axis. This increases the wing's angle of attack and increases lift, ideally resulting in a smooth landing.

Ground effect, often described as a 'cushion', is thought to be an INCREASE in air pressure which occurs below a wing when it comes into close proximity with the ground. Generally this is imagined to occur during the flare for landing. But while apparently a real phenomenon, ground effect is probably much more subtle than is commonly imagined. What is typically thought of as ground effect is probably created when the pilot misjudges the flare maneuver and floats before landing. This can be compounded by varying winds and unpredictable airflows as they interact with local structures and topography. Real ground effect probably occurs only within a few feet of the ground in a small airplane, or perhaps within a quarter of the wingspan for a large airliner.

Issues affecting ground effect are numerous, and may include the wing's area, its chord length, and its angle-of-attack as it nears the surface in a landing attitude, as well as the weight, speed, and configuration of the aircraft, and wing loading(aircraft weight per unit-area of wing).

Consider a heavy, high-speed fighter jet, with a comparatively small wing area. The wing is highly loaded, and twenty-thousand pounds landing at one hundred and twenty knots will probably not experience much ground effect. A light, slow Piper Cub however, will likely be much effected. A low-wing airplane of the same size even more so. A sailplane, however, may be less so due to the short chord and very long wingspan for weight. Also, the sailplane pilot will less radically alter his angle-of-attack during flare due to the very high-efficiency wing and low weight of his aircraft.

Ground effect during take-off is also thought to be a cause of many small aircraft accidents. A small plane loaded beyond gross weight capabilities may be able to take off under ground effect, but it may not be able to climb. The pilot may be forced to put-down beyond the runway, or crash into obstacles.

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. It's probably true the the spruce goose was underpowered in its current configuration - development of the engines the plane was designed to carry was canceled before completion, and as a result, the goose was running at probably 60% of its designed power. Nevertheless the power necessary to bring a seaplane to flight speeds is greater than land-planes, and the height the goose reached was probably well beyond the ground effect for such a heavy-lifter.

[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

[edit] See also

• Coandă effect
• Hovercraft
• Ekranoplane
• Boeing Pelican
• Ground effect vehicle