As it pertains to fixed wing aircraft, "ground effect" refers to the increased lift and decreased drag that an aircraft airfoil or wing generates when an aircraft is about one wingspan's length or less over the ground (or surface).[1] Ground effect often gives pilots and/or passengers of light aircraft the feeling that the aircraft is "floating", especially when landing.[2]
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When an aircraft is flying at an altitude that is approximately at or below the same distance as the aircraft's wingspan there is, depending on airfoil and aircraft design, an often noticeable ground effect. This is caused primarily by the ground interrupting the wingtip vortices and downwash behind the wing. When a wing is flown very close to the ground, wingtip vortices are unable to form effectively due to the obstruction of the ground. The result is lower induced drag, which increases the speed and lift of the aircraft while it is in the ground effect.[3][4]
A wing generates lift, in part, due to the difference in air pressure gradients on the wing surfaces: both upper and lower. During normal flight, the upper wing surface experiences reduced static air pressure and the lower surface comparatively higher static pressure, these air pressure differences also accelerate the mass of air downwards. Flying close to a surface increases air pressure on the lower wing surface, (the ram or cushion effect) improving the aircraft lift to drag ratio. As the wing gets lower the ground effect becomes more pronounced. While in the ground effect, the wing will require a lower angle of attack to produce the same amount of lift. If the angle of attack and velocity remain constant, an increase in the lift coefficient will result,[5] accounting for the "floating" effect. Ground effect will also alter thrust versus velocity in that reducing induced drag will require less thrust to maintain velocity.[5]
Low winged aircraft are more affected by ground effect than high wing aircraft.[6] Due to the change in up-wash, down-wash and wingtip vortices there may be errors in the airspeed system while in ground effect due to changes in the local pressure at the static source.[5]
The wing in ground effect is affected by numerous factors, including the wing's area, its chord length, and its angle of attack as it nears the surface, as well as the weight, speed, and configuration of the aircraft, and wing loading (aircraft weight per unit-area of wing).
The wing in ground effect, often described as a 'cushion', is thought to be an increase in air pressure which occurs below a wing when it gets close to the ground. The effect begins to be noticeable when the aircraft's altitude is within 1–1.5 times the length of its own wingspan and, when the altitude is within about half the wing chord, the effect can increase lift by as much as 10%. Due to the effect of spoilers and high wing loading, this effect is only dramatically noticed in smaller, less complex aircraft, usually weighing less than 12,500 lbs (5,670 kg). Ground effect is a major factor in aircraft "floating" down the runway, and is the reason that low-wing aircraft have a tendency to float more than the high-wing varieties.
Wing in ground effect during take-off is thought to be a cause of many aircraft accidents. A small plane loaded beyond gross weight capabilities may be able to take off under ground effect, due to the 'artificially' low stall speed due to the decreased induced drag. However, once the aircraft climbs to a height at which wingtip vortices can form, the wings will stall, and the aircraft will suddenly descend — usually resulting in a crash. (Note that the ground effect cushion does not of itself reduce wing vortices; rather, on leaving ground effect the pilot of an overloaded aircraft must increase the angle of attack to keep flying. This action may result in the wing exceeding the critical angle of attack, thus inducing a stall at low altitude.).
Gliders may be less affected by wing in ground effect due to the short chord and very long wingspan (in other words, high aspect ratio) for weight, which minimizes the effect of induced drag caused by wingtip vortices. On the other hand, since gliders generally are built to minimize all form drag and parasitic drag as well, the reduction in the induced drag caused by ground effect can in fact effectively increase flight performance, resulting in an enhanced glide ratio. Pilots of gliders who seek to exploit this phenomenon on landing are said to be performing a "penetration approach." A successful penetration approach would involve diving at a speed higher than the usual optimal glide speed for a given glider (which would result in a sub-optimal glide ratio on the descent), and then flaring and holding the lowest possible altitude above the ground, at this high speed. Theoretically the positive effect of wing in ground effect — decreased drag — could result in a final stopping place farther than would have been achieved, had the pilot simply flown the speed resulting in the best glide ratio. This approach is risky, and it is not a sure bet that performance will be increased; thus it is not a generally recommended means of improving glide distance. This should also not be confused with a glider performing a penetration approach into wind; the fact that the wind generally is less strong near the ground improves the achieved groundspeed enough to offset the higher airspeed drag penalties.
Wingtip vortices are a major cause of induced drag, which refers to any drag created as a side effect of generating lift. Reducing this form of drag leads to a number of widely-used design considerations found on many aircraft. Gliders, for instance, use very long wings with a high aspect ratio to reduce the development of spanwise flow. As the wing has a smaller chord length over wing length, spanwise flow has less time to develop and therefore the angle at which the upper and lower airflows converge is reduced. This smaller angle creates vortices of less magnitude and therefore produce less induced drag. Other aircraft sometimes include winglets or end-plates to decrease the pressure differential between the upper and lower wing. This barrier method increases the distance air has to flow from HIGH to LOW thereby reducing the speed at which this air flows. To grasp how it does this think of weather maps and isobars. When isobars (regions of similar pressure) are closer together, the pressure differential is greater and the wind speed will be higher and vice versa. This reduced pressure differential results in a reduction in spanwise flow, the angles at which the airflows meet and ultimately induced drag.
Some critics of Howard Hughes' massive Spruce Goose claim that the famous flying boat's first (and only) flight was due entirely to wing in ground effect and that the craft was incapable of sustaining flight above a very low altitude. It is probably true that 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.
Many aircraft have a design that makes use of the wing in ground effect.
Although all airplanes fly through ground effect at some point, craft that do so in a dedicated manner must be designed in such a way that their wings are either unsuitable or unable to take them into flight out of ground effect (free flight). Those that can fly out of ground effect are so out of their element that they are only capable of short distance hops into free flight. Because of this, these craft do not meet the required legal criteria to be referred to as airplanes and the operators of such craft need not possess a pilot's license to fly them.
These specially designed craft include airplanes with inverse delta wing, ekranoplan wing or tandem wing.
Pilots of Rotary wing aircraft also feel changes associated with being close to a flat surface such as the ground. Ground effect is often called a 'cushion of air', which although it may cause that sensation, it is not. Ground effect as it pertains to helicopters occurs when the helicopter is within a rotor diameter of the surface or similar object such as an elevated heli-pad due to the interference of the downflow of air created by the rotor system. This interference reduces the size of the vortice encompassing the tips of the rotor blades which in turn increases the effect area of the rotor blade producing lift. The overall result is that less power is required to maintain a given height above the surface. Another important issue regarding 'ground effect' is that the makeup of the surface directly effects the intensity; this is to say that a concrete or other hard surface will produce more interference than a grass or water surface.
Airships can benefit from the wing in ground effect, too. Feasibility studies have been made of specially designed airships taking advantage of the effect.[7][8][9]