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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.
The cross section of a body that is placed in an airstream in order to produce a useful aerodynamic force in the most efficient manner possible. The cross sections of wings, propeller blades, windmill blades, compressor and turbine blades in a jet engine, and hydrofoils on a high-speed ship are examples of airfoils.
The mean camber line of an airfoil (see illustration) is the locus of points halfway between the upper and lower surfaces as measured perpendicular to the mean camber line itself. The most forward and rearward points of the mean camber line are the leading and trailing edges, respectively. The straight line connecting the leading and trailing edges is the chord line of the airfoil, and the distance from the leading to the trailing edge measured along the chord line is simply designated the chord of the airfoil, represented by c. The thickness of the airfoil is the distance from the upper to the lower surface, measured perpendicular to the chord line, and varies with distance along the chord. The maximum thickness, and where it occurs along the chord, is an important design feature of the airfoil. The camber is the maximum distance between the mean camber line and the chord line, measured perpendicular to the chord line. Both the maximum thickness and the camber are usually expressed in terms of a percentage of the chord length; for example, a 12% thick airfoil has a maximum thickness equal to 0.12c.
The airfoil may be imagined as part of a wing which projects into and out of the page, stretching to plus and minus infinity. Such a wing, with an infinite span perpendicular to the page, is called an infinite wing. The aerodynamic force on the airfoil, by definition, is the force exerted on a unit span of the infinite wing. For this reason, airfoil data are frequently identified as infinite wing data.
The flow of air (or any fluid) over the airfoil results in an aerodynamic force (per unit span) on the airfoil, denoted by R. The relative wind is the magnitude and direction of the free-stream velocity far ahead of the airfoil. The angle between the chord line and relative wind is defined as the angle of attack of the airfoil, denoted by α. By definition, the component of R perpendicular to the relative wind is the lift, L; similarly, the component of R parallel to the relative wind is the drag, D.
The airfoil may be visualized as being supported by an axis perpendicular to the airfoil, and taken through any point on the airfoil. The airfoil has a tendency to twist about this axis; that is, there is an aerodynamic moment exerted on the airfoil. By definition, the moment is positive or negative if it tends to increase or decrease respectively the angle of attack (that is, if it tends to pitch the airfoil up or down, respectively).