Airfoil

Streamlines on an airfoil visualised with a smoke wind tunnel

An airfoil (American English) or aerofoil (British English) is a streamlined body that is capable of generating significantly more lift than drag.^[1] Wings, sails and propeller blades are examples of airfoils. Foils of similar function designed with water as the working fluid are called hydrofoils.

When oriented at a suitable angle, a solid body moving through a fluid deflects the oncoming fluid (for fixed-wing aircraft, a downward force), resulting in a force on the airfoil in the direction opposite to the deflection.^[2]^[3] This force is known as aerodynamic force and can be resolved into two components: lift (perpendicular to the remote freestream velocity) and drag (parallel to the freestream velocity).

The lift on an airfoil is primarily the result of its angle of attack. Most foil shapes require a positive angle of attack to generate lift, but cambered airfoils can generate lift at zero angle of attack. Airfoils can be designed for use at different speeds by modifying their geometry: those for subsonic flight generally have a rounded leading edge, while those designed for supersonic flight tend to be slimmer with a sharp leading edge. All have a sharp trailing edge.^[4]

The air deflected by an airfoil causes it to generate a lower-pressure "shadow" above and behind itself. This pressure difference is accompanied by a velocity difference, via Bernoulli's principle, so the resulting flowfield about the airfoil has a higher average velocity on the upper surface than on the lower surface.^[5] In some situations (e.g. inviscid potential flow) the lift force can be related directly to the average top/bottom velocity difference without computing the pressure by using the concept of circulation and the Kutta–Joukowski theorem.^[6]

^ Clancy 1975, §5.2.
^ Halliday & Resnick 1988, p. 378: "The effect of the wing is to give the air stream a downward velocity component. The reaction force of the deflected air mass must then act on the wing to give it an equal and opposite upward component."
^ Hall, Nancy R. "Lift from Flow Turning". NASA Glenn Research Center. Archived from the original on 5 July 2011. Retrieved 2011-06-29. If the body is shaped, moved, or inclined in such a way as to produce a net deflection or turning of the flow, the local velocity is changed in magnitude, direction, or both. Changing the velocity creates a net force on the body.
^ “It has been known from the very beginning of flight that wings with a sharp trailing edge must be used in order to obtain a well-defined lift.” von Mises, Richard (1945), Theory of Flight, Section VIII.2, p.179, Dover Publications Inc. ISBN 0-486-60541-8
^ Weltner & Ingelman-Sundberg 1999.
^ Babinsky 2003, pp. 497–503: "If a streamline is curved, there must be a pressure gradient across the streamline."

[FOOTNOTEClancy1975§5.2-1] Clancy 1975, §5.2.

[2] Halliday & Resnick 1988, p. 378: "The effect of the wing is to give the air stream a downward velocity component. The reaction force of the deflected air mass must then act on the wing to give it an equal and opposite upward component."

[3] Hall, Nancy R. "Lift from Flow Turning". NASA Glenn Research Center. Archived from the original on 5 July 2011. Retrieved 2011-06-29. If the body is shaped, moved, or inclined in such a way as to produce a net deflection or turning of the flow, the local velocity is changed in magnitude, direction, or both. Changing the velocity creates a net force on the body.

[4] “It has been known from the very beginning of flight that wings with a sharp trailing edge must be used in order to obtain a well-defined lift.” von Mises, Richard (1945), Theory of Flight, Section VIII.2, p.179, Dover Publications Inc. ISBN 0-486-60541-8

[FOOTNOTEWeltnerIngelman-Sundberg1999-5] Weltner & Ingelman-Sundberg 1999.

[6] Babinsky 2003, pp. 497–503: "If a streamline is curved, there must be a pressure gradient across the streamline."

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