Ground effect in aircraft

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Antonov An-2 converted into a wing in ground effect vehicle.
Antonov An-2 converted into a wing in ground effect vehicle.

Aircraft may be affected by a number of ground effects, aerodynamic effects due to a flying body's proximity to the ground.[1]

One of the most important of these effects is the Wing In Ground effect, which refers to the reduction in drag experienced by an aircraft as it approaches a height approximately twice a wingspan's length off the ground or other level surface (such as the sea). The effect increases as the wing descends closer to the ground, with the most significant effects occurring at a height of one half the wingspan length above the ground. It can present a hazard for inexperienced pilots who are not accustomed to correcting for it 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 first to give scientific description of the ground effect and to provide theoretical methods of calculation of air cushion vehicles was Konstantin Tsiolkovsky in his 1927 paper "Air Resistance and the Express Train".[2][3]

Contents

[edit] Principle of Ground Effect

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 in order to reduce the size of the wingtip in relation to the size of the wing as a whole, thereby reducing the contribution of induced drag. Other aircraft sometimes include winglets to actively disrupt the airflow over the tip, to the same end.

The phenomenon of 'wing in ground effect' is caused by the ground 'interrupting' the wingtip vortices and downwash behind the wing.[4] 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 performance of the aircraft while it is experiencing the ground effect.

[edit] Factors affecting Wing in Ground effect

Factors affecting Wing in Ground effect are numerous, and may include 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).

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 comes into close proximity with the ground. Wing in Ground effect begins to be noticeable (to both the pilot and an onlooker) when the aircraft is within 1-1.5 times the length of its own wingspan from the ground. Wing in Ground effect, however, only becomes extremely pronounced, where lift can momentarily increase by as much as 40%, when the plane is within about half its wingspan distance from the ground.[5] 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. 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, thanks to the 'artificially' low stall speed due to the decreased induced drag. But it may not be able to climb beyond a certain point. Once the pilot climbs out of ground effect wingtip vortices will 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 will increase drag to the point where ultimately a crash may occur).

Sailplanes 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 salience of induced drag caused by wingtip vortices. On the other hand, since sailplanes 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 sailplanes 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 sailplane (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 relatively 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. Obviously, a penetration approach is fraught with risks, 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 sailplane 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.

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.[citation needed]

[edit] Wing in Ground effect and helicopters

Rotary wing aircraft experience performance changes associated with Wing in Ground effect just like other aircraft. A helicopter hovering close to the ground will require less power than when hovering out of 'wing in ground effect'. To hover in 'wing in ground effect' you must be within one blade length of the surface and the surface must be a hard surface.

Following an engine failure (depending on weight and environmental conditions), a multi engine helicopter may be able to hover in 'wing in ground effect' but not outside it.

This effect can be described at three different levels of understanding: basic, intermediate and advanced.

[edit] Basic

As with a fixed wing surface a helicopter's blades produce a cushion of air when close to the ground which helps to support the aircraft.

Wind, surface slope and surface texture will all have an effect on how effectively the high pressure cushion is maintained under the rotor.

[edit] Intermediate

When hovering in free air a rotor disc produces a duct effect with an induced downwards movement of air above the rotor. Rather than being stationary, when the air hits the rotor it already has a downward vector as it attempts to fill the region of low pressure left by air below it which has already been forced downwards by the rotor. To hover, the rotor disc must produce a vertical force in the manner of Force = Mass x Acceleration where mass is dependent on number of rotors, RPM and rotor diameter and acceleration by the difference in the initial air vector and its resulting vector. Assuming mass moved is constant then a helicopter hovering in free air will be produce a force equivalent to V2 (speed after passing through the rotor) - V1 (starting speed of air) where V1 is some value above 0 given that the air is already moving downwards before reaching the rotor.

When close to the surface (generally considered 1/3-2/3 of the rotor diameter), air forced downwards through the rotor disc is restricted in its flow by the ground. This produces an area of high pressure below the disc and in turn, reduces the duct effect and hence the downwards velocity of the air above the disc. This reduces V1 while V2 remains relatively static, so the value of V2 - V1 increases. From the equation F = M x A we can see that the 'lift' of the rotor disc is greater when in ground effect.

[edit] Advanced

The relative air flow meeting the advancing face of a rotor blade in a hovering helicopter is determined by the speed of rotation (the blade moving forward into the air) and the induced flow (the induced downwards movement of air above the rotor). Rotational air flow is taken as horizontal, induced flow as vertically down and the relative airflow as the resulting vector. Through geometry the angle of attack(AoA) of the rotor blade will decrease as the inflow increases and the relative airflow direction moves closer to the chordline.

When the helicopter is in 'wing in ground effect' the induced flow is decreased as described above. This moves the relative airflow vector closer to the horizontal and increases the AoA for a given blade pitch. This increases the lift produced by the rotor disc and the helicopter will start to accelerate vertically. The vertical movement will induce its own inflow reducing the AoA again until a point of equilibrium is reached.

In reality a pilot will lower the collective slightly -> reducing rotor pitch -> reducing AoA -> reducing lift and the helicopter will hover in ground effect(IGE) with a lower power setting than that required out of ground effect (OGE).

[edit] References

 This article needs additional citations for verification.
Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (January 2007)
  1. ^ See also vortex ring
  2. ^ Gillispie, Charles Coulston. Dictionary of Scientific Biography, Charles Scribner's Sons, 1980, p.484, ISBN 0684129256
  3. ^ (Russian) Air cushion vehicle history
  4. ^ Aerodynamics Aviation Theory Centre, July 2005. Cheltenham, VIC. Australia.
  5. ^ This effect is noted in the article about the Lockheed U-2

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