Airfoil

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For the kite, see foil kite.

Various components of the airfoil.

An airfoil (in American English, or aerofoil in British English) is the shape of a wing or blade (of a propeller or ship's screw or sail) as seen in cross-section. It is passed through a fluid in order to provide either lift or downforce, depending on its application. Subsonic-flight airfoils have a characteristic shape with a rounded leading edge, followed by a sharp trailing edge, and often with camber.

The historical evolution of airfoil sections, 1908 - 1944, NASA

Lift and Drag curves for a typical airfoil

To understand lift itself, see lift. As well as the wing, an aircraft's horizontal and vertical stabilizers are airfoils. Airfoils are also found in propellers, fans, compressors and turbines. Sails are also airfoils, and the underwater fins of sailboats, such as centerboards, 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; common examples being bird wings, the bodies of fishes, and the shape of sand dollars.

An inverted airfoil will create a downward pressure on an automobile or other motor vehicle, improving its traction and keeping it on the ground. The term "lift" can mean a force generated in any direction in any medium. Any thin object with a positive angle of attack, such as a flat plate or the deck of a bridge, will generate lift. Airfoils are more efficient, generating lift with the least drag and maintaining lift at higher angle of attack. A lift and drag curve obtained in wind tunnel testing is shown on the right.

Airfoil design is a major facet of aerodynamics. Various airfoils serve different flight regimes. A supercritical airfoil, with its low camber, reduces transonic drag divergence, while a symmetric airfoil may better suit frequent inverted flight. Supersonic airfoils are much more angular in shape and can have a very sharp leading edge. Moveable high-lift devices, flaps and slats are fitted to airfoils on most aircraft. New airfoil design techniques continue to develop.

Various systems have been devised to describe and characterise airfoils — the most common and prevalent is the NACA system. Before this, various ad-hoc systems were used. An example of a general purpose airfoil that finds wide application, and predates the NACA system, is the Clark-Y.