The flight characteristics of an airplane change as the distance from the ground is equal to its own wing span or less. Besides being able to fly at a lower-than-normal speed, the airplane can attain the same speed using less thrust than it would at atltitude. This increased performance many pilots encounter just before touch-down or just after take-off is known as the "ground effect" and should be taken into account especially when landing distance is limited or take-off is commenced. What happens is that the amount of induced drag decreases due to changes in the upwash patterns in front of the wing and downwash and wingtip vortices behind the wing (figure 1.1). As you might know induced drag is a by-product of the production of lift. Consequently, a decrease in induced drag means less thrust is required, allowing the airplane to become airborne at a lower-than-normal speed. Although this may sound favorable, making use of ground effect could get a pilot in trouble, either intentionally or unintentionally. Climbing out of ground effect at too low an airspeed, with induced drag values accumulating and consequently airspeed decreasing, may result in stalling the aircraft if no additional thrust is added.
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| Figure 1.1 - Induced drag and downwash patterns in ground effect |
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| Figure 1.2 - Induced drag and downwash patterns in normal flight |
The best way to describe ground effect and which many people, both pilots and passengers, have encountered is the floating effect during the landing flare. Since the wing is able to produce more lift at the same angle of attack, pitch angle should be reduced slightly to maintain a shallow descent while thrust should be decreased as well in order to continue slowing the airplane down for landing. As said before, ground effect may become a problem on short runways as it may cause the airplane to float so far down the runway that it may leave insufficient room to stop safely.
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