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Ground EffectThe phenomenon of ground effect is associated with the reduction of induced drag. During takeoffs and landings, when you are flying very close to the ground, the earth's surface actually alters the three-dimensional airflow pattern around the airplane. Theis causes a reduction in wingtip vorticies and a decrease in upwash and downwash. Since ground effect restricts the downward deflection of the airstream, induced drag decreases. When the wing is at a height equal to its span, the decline in induced drag is only about 1.4% ; when the wing is at a height equal to one-fourth its span, the loss of induced drag is about 24%. With the reduction of induced drag in ground effect, the amount of thrust required to produce lift is reduced. What this means is that your airplane is capable of lifting off at a lower-than-normal speed. Although you might initially think that this is desirable, consider what happens as you climb out of the ground effect. The power (thrust) required to sustain flight increases significantly as the normal airflow around the wing returns and induced drag is suddenly increased. If you attempt to climb out of ground effect before reaching the speed for normal climb, the airplane might sink back to the surface. Ground effect is noticeable in the landing phase of flight, too, just before touchdown. Within one wingspan above the ground, the decrease in induced drag makes your airplane seem to float on the cushion of air beneath it. Because of this, power reduction usually is required during the flare to help the airplane land. Although all airplanes may experience ground effect, it is more noticeable in low-wing airplanes, simply because the wings are closer to the ground. |
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This page was last modified June 11, 2000 |
