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Flaps exist on the trailing edge of an airplane wing, and are used to increase the airplane's lift. When extended, they change the shape of the wing, creating more area for lift to be produced. However, this has the downside of also creating more surface area for drag to occur. Friction drag will rub against the flap and slow the plane down, but only slightly: since there is a lot more lift being produced than drag, the plane will be fine.