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Flight control mechanism for an airplane, comprising a pivotally supported control surface with a hinged boost tab thereon, means for swinging the control surface for airplane manoeuvring purposes, comprising an element mounted to rotate about an axis in coaxial relation with the pivot axis of the control surface, and an elongated torsional spring in concentric relation with said pivot axis of the control surface, having one end thereof fixedly connected to the element and its other end fixedly connected to said control surface, and adapted when the element is rotated while the control surface is subjected to low air resistance to swing said control surface conjointly with the element and further adapted when the element is rotated while the control surface is subjected to high air resistance to permit said element to rotate relatively to the control surface, and means operative in response to rotation of the element relatively to the control surface to tilt the tab into a position wherein it serves to assist in swinging the control surface.