Flaps

In general, the wings on most planes are designed to provide an appropriate amount of lift (along with minimal drag) while the plane is operating in its cruising mode (about 560 miles per hour, or 901 km per hour, for the Boeing 747-400). However, when these airplanes are taking off or landing, their speeds can be reduced to less than 200 miles per hour (322 kph). This dramatic change in the wing's working conditions means that a different airfoil shape would probably better serve the aircraft.

To accommodate both flight regimes (fast and high as well as slow and low), airplane wings have moveable sections called flaps. During takeoff and landing, the flaps are extended rearward and downward from the trailing edge of the wings. This effectively alters the shape of the wing, allowing the wing to turn more air, and thus create more lift. The downside of this alteration is that the drag on the wings also increases, so the flaps are put away for the rest of the flight.