An airfoil or aerofoil is the shape of a wing, blade, or sail. An airfoil-shaped body moved through a fluid produces an aerodynamic force, the component of this force perpendicular to the direction of motion is called lift. The component parallel to the direction of motion is called drag, subsonic flight airfoils have a characteristic shape with a rounded leading edge, followed by a sharp trailing edge, often with a symmetric curvature of upper and lower surfaces. Foils of similar function designed with water as the fluid are called hydrofoils. The lift on an airfoil is primarily the result of its angle of attack, when oriented at a suitable angle, the airfoil deflects the oncoming air, resulting in a force on the airfoil in the direction opposite to the deflection. This force is known as force and can be resolved into two components, lift and drag. Most foil shapes require an angle of attack to generate lift. This turning of the air in the vicinity of the airfoil creates curved streamlines, resulting in pressure on one side. The lift force can be related directly to the average top/bottom velocity difference without computing the pressure by using the concept of circulation, a fixed-wing aircrafts wings, horizontal, and vertical stabilizers are built with airfoil-shaped cross sections, as are helicopter rotor blades. Airfoils are also found in propellers, fans, compressors and turbines, sails are also airfoils, and the underwater surfaces of sailboats, such as the centerboard and keel, are similar in cross-section and operate on the same principles as airfoils. Swimming and flying creatures and even many plants and sessile organisms employ airfoils/hydrofoils, common examples being bird wings, the bodies of fish, an airfoil-shaped wing can create downforce on an automobile or other motor vehicle, improving traction. Any object with an angle of attack in a fluid, such as a flat plate. Airfoils are more efficient lifting shapes, able to more lift. A lift and drag curve obtained in wind tunnel testing is shown on the right, the curve represents an airfoil with a positive camber so some lift is produced at zero angle of attack. With increased angle of attack, lift increases in a linear relation. At about 18 degrees this airfoil stalls, and lift falls off quickly beyond that, the drop in lift can be explained by the action of the upper-surface boundary layer, which separates and greatly thickens over the upper surface at and past the stall angle. The thicker boundary layer also causes an increase in pressure drag, so that the overall drag increases sharply near. Airfoil design is a facet of aerodynamics
Lift and Drag curves for a typical airfoil
An airfoil section is displayed at the tip of this Denney Kitfox aircraft, built in 1991.