Editing Lift (force) (section)

===Circulation and the Kutta–Joukowski theorem===
[[Image:Circulation-around-aerofoil.svg|thumb|right|300px|Circulation component of the flow around an airfoil]]

When an airfoil generates lift, several components of the overall velocity field contribute to a net circulation of air around it: the upward flow ahead of the airfoil, the accelerated flow above, the decelerated flow below, and the downward flow behind.

The circulation can be understood as the total amount of "spinning" (or [[vorticity]]) of an inviscid fluid around the airfoil.

The [[Kutta–Joukowski theorem]] relates the lift per unit width of span of a two-dimensional airfoil to this circulation component of the flow.<ref name="Clancy 1975, Section 4.5"/><ref>von Mises (1959), Section VIII.2</ref><ref>Anderson(1991), Section 3.15</ref> It is a key element in an explanation of lift that follows the development of the flow around an airfoil as the airfoil starts its motion from rest and a [[starting vortex]] is formed and left behind, leading to the formation of circulation around the airfoil.<ref>Prandtl and Tietjens (1934)</ref><ref>Batchelor (1967), Section 6.7</ref><ref>Gentry (2006)</ref> Lift is then inferred from the Kutta-Joukowski theorem. This explanation is largely mathematical, and its general progression is based on logical inference, not physical cause-and-effect.<ref>McLean (2012), Section 7.2.1</ref>

The Kutta–Joukowski model does not predict how much circulation or lift a two-dimensional airfoil produces. Calculating the lift per unit span using Kutta–Joukowski requires a known value for the circulation. In particular, if the Kutta condition is met, in which the rear stagnation point moves to the airfoil trailing edge and attaches there for the duration of flight, the lift can be calculated theoretically through the conformal mapping method.

The lift generated by a conventional airfoil is dictated by both its design and the flight conditions, such as forward velocity, angle of attack and air density. Lift can be increased by artificially increasing the circulation, for example by boundary-layer blowing or the use of [[blown flap]]s. In the [[Flettner rotor]] the entire airfoil is circular and spins about a spanwise axis to create the circulation.