Ground Effect
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.

Figure 1.1 - Induced drag and downwash patterns in ground effect
 
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.

 

Ground effect during take-off
As an airplane climbs out of ground effect on take-off the lifting ability of the wing decreases for the same angle of attack. Consequently, induced drag increases due to greater wingtip vortices meaning that the aircraft performance is affected and it will not perform as well in free air as it will in ground effect. This means that the pilot will experience a sagging in climb performance as ground effect diminishes quite rapidly. To overcome this, both the angle-of-attack should be increased in order to generate more lift while an increase of thrust makes up for the increase in induced drag. The pilot either decides to take such action or accept a reduced climb performance.

It is obvious that ground effect should be taken into account when a take-off from a short runway is planned. Although ground effect may allow the airplane to become airborne at a speed that is below the recommended take-off speed, climb performance would be less and should be a good reason to avoid this. It is also important to remember that an overloaded airplane may fly in ground effect but hasn't got the ability to climb out of it and eventually will come to stop if it hits something. The A330 to the right demonstrates the point where ground effect is greatest.

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Ground effect during landing
As the airplane descends on approach and enters ground effect, the pilot experiences a floating sensation which is a result from the increased lift and decreased induced drag value. Less drag also means a lack of deceleration and could become a problem on short runways were roll-out distance is limited. It is therefore important that power is throttled back as soon as the airplane is flared over the runway and the weight of the airplane is transferred from the wings to the wheels as soon as possible.

The Boeing 737 to the right illustrates the point where many pilots experience a floating feeling.



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