Movable surfaces on aircraft wings that control lift and drag, or the action of moving something back and forth loosely.
From Middle English 'flappe', imitative of the sound of something loose hitting against a surface. The aviation sense developed in the early 20th century as aircraft technology advanced.
Aircraft flaps are engineering marvels that literally bend the laws of physics to their will - by changing the wing's shape, they can make a heavy metal tube weighing hundreds of tons gently kiss the runway instead of crashing into it. The word beautifully captures both the mechanical precision of flight and the simple sound of fabric in the wind.
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