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As an airplane approaches the ground to land, on a runway or even in a field, it experiences an effect called "ground effect." This phenomenon reduces the drag on the aircraft considerably, sometimes causing the airplane to "float" past the point of intended landing. Ground effect in airplanes can be extremely beneficial - a pilot who knows how his or her aircraft will react during the final stages of landing can use ground effect to make extraordinarily smooth touchdowns. Pilots in the United States are trained in using ground effect to aid soft-field takeoffs. Ground effect can also be dangerous, allowing an aircraft to become airborne before it is moving fast enough to truly fly. Read on to find out more about ground effect.
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Why and Where?
Ground effect generally occurs at half the aircraft's wingspan above the ground. If an aircraft has a 40-foot wingspan, the pilot should expect ground effect to cause the greatest reduction in drag at around 20 feet above the ground.
Why does this happen? One of the major causes of drag on an aircraft is wingtip vortices, the swirling of air caused by the wingtips of an aircraft. As an aircraft gets close to the ground, these vortices do not have room to fully occur, and so drag is lessened. As drag decreases, the aircraft's speed will increase, allowing air to flow over the airfoil at a greater rate. This allows the wings to generate more lift.
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Benefits of Ground Effect: Smooth Landings
Landing any aircraft smoothly takes lots of practice, and sometimes a little luck. Ground effect is something that all pilots must take into account in order to make their landing smooth. As a general aviation aircraft approaches the runway, it is generally descending anywhere from 300 to 800 feet per minute. This rate of descent can vary wildly depending on how the traffic pattern has been flown. When the aircraft reaches ground effect, the wings will begin to generate less drag, causing the aircraft's speed to increase and therefore, generate more lift. The aircraft's rate of descent will decrease, and depending on the speed during the flare, the aircraft may even begin climbing instead of settling to the runway. A pilot who is familiar with his or her aircraft can set an appropriate rate of descent with ground effect in mind, and allow ground effect to "soften" the landing by reducing the rate of descent just before touchdown.
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Dangers of Ground Effect: Preemptive Takeoff
As noted, ground effect can allow an aircraft to become airborne before it is really ready to fly. If a pilot allows the aircraft to liftoff into ground effect and then initiates a climb, disaster can occur. The aircraft will climb out of the relatively small area of ground effect, and drag will increase rapidly. This can cause the aircraft to stall - and because ground effect is only present around half the wingspan of the plane (generally around 20 feet above the ground), the pilot may not have room (or time) to recover.
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Dangers of Ground Effect: Floating
Ground effect can be used by a skillful pilot to smooth out landings, but if a pilot is surprised by ground effect, he or she may accidentally fly the plane off the end of the runway. If a pilot is not prepared for the reduction in drag caused by ground effect, he or she may accidentally "float" the aircraft. This occurs when the plane is still moving fast enough that "flaring" will cause the airplane to level out or even climb instead of sink to the runway. On a short runway, an accidental float can have the aircraft touching down halfway or even 3/4 of the way down the runway, with not enough room to stop. If a float occurs, the safest action is to add full power, retract flaps, and go-around.
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