Abstract: An airfoil of an aircraft is provided with one or more control flaps which are pivotally mounted on the airfoil in an aeromechanically unstable manner. The trailing edges of the control flaps are provided with one or more control tab zones which include trailing portions that deform in response to application of electrical current, heat or pressure thereto. The pitch of the airfoil is constantly controlled by selectively deforming the control tab zones in response to measured signals indicative of variations in fluid pressure acting upon, or motion of the unstable control flaps. The impetus for moving the control system is thus derived from the air-stream through which the aircraft moves. The control system can also be utilized in watercraft, with the operating impetus being derived from the water stream through which the craft moves.