Ground effect in aircraft

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

The most significant of these effects is known as the wing in ground (WIG) effect, which refers to the reduction in drag experienced by an aircraft as it approaches a height approximately equal to the aircraft's wingspan above 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 an altitude of one half the wingspan. 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.


Principle of ground effect

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. In normal flight, these air pressure differences also accelerate the mass of air downwards balancing momentum. However kinetic energy is proportional to the square of the velocity so designers aim to minimize the accelerated air velocities to reduce wasted energy in both the downward mass acceleration and wingtip vortices. Flying close to a surface increases air pressure on the lower wing surface (the ram or cushion effect) and decreases air acceleration so the ground effect improves the aircraft lift to drag ratios in two ways. Momentum is still balanced because the air pressure beneath the wing is pressing on the underlying surface—the water or flat land.

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.

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