airfoil, surface designed to develop a desired force by reaction with a fluid, especially air, that is flowing across the surface. For example, the fixed wing surfaces of an airplane produce lift, which opposes gravity. Airfoils that are manipulated to produce variable forces are called control surfaces. Ailerons, control surfaces hinged to the trailing edges of wings, can produce rolling, which is rotational motion of the aircraft about a line running through its fuselage, or yawing, which is rotational motion about a line running from the top to the bottom of an aircraft. Modern aircraft have fairly complex arrays of control surfaces, including elevators, a rudder, and flaps. Elevators, which are hinged to the rear of the horizontal airfoil of the tail assembly, are used to produce pitching, which occurs when an airplane in level flight points its nose upward or downward. The rudder, which is hinged to the rear of the vertical airfoil of the tail assembly, is used to produce yawing. Flaps are located near the ailerons to increase lift for takeoff and landing. Spoilers, which can be made to protrude from lifting surfaces to give controlled reduction of lift, often replace ailerons and elevators. In aircraft of the swing-wing type, in which the sweep of the wings is variable, the entire wing can be considered a control surface. Other airfoils include propeller blades and the blades utilized in turbojet engines.