Editing Lift (force) (section)

===Horseshoe vortex system===
[[Image:Wing horseshoe vortex.jpg|thumb|right|300px|Planview of a wing showing the horseshoe vortex system]]
The wingtip flow leaving the wing creates a tip vortex. As the main vortex sheet passes downstream from the trailing edge, it rolls up at its outer edges, merging with the tip vortices. The combination of the [[wingtip vortices]] and the vortex sheets feeding them is called the vortex wake.

In addition to the vorticity in the trailing vortex wake there is vorticity in the wing's boundary layer, called 'bound vorticity', which connects the trailing sheets from the two sides of the wing into a vortex system in the general form of a horseshoe. The horseshoe form of the vortex system was recognized by the British aeronautical pioneer Lanchester in 1907.<ref>Lanchester (1907)</ref>

Given the distribution of bound vorticity and the vorticity in the wake, the [[Biot–Savart law]] (a vector-calculus relation) can be used to calculate the velocity perturbation anywhere in the field, caused by the lift on the wing. Approximate theories for the lift distribution and lift-induced drag of three-dimensional wings are based on such analysis applied to the wing's horseshoe vortex system.<ref>Milne-Thomson (1966), Section 10.1</ref><ref>Clancy (1975), Section 8.9</ref> In these theories, the bound vorticity is usually idealized and assumed to reside at the camber surface inside the wing.

Because the velocity is deduced from the vorticity in such theories, some authors describe the situation to imply that the vorticity is the cause of the velocity perturbations, using terms such as "the velocity induced by the vortex", for example.<ref>Anderson (1991), Section 5.2</ref> But attributing mechanical cause-and-effect between the vorticity and the velocity in this way is not consistent with the physics.<ref>Batchelor (1967), Section 2.4</ref><ref>Milne-Thomson (1966), Section 9.3</ref><ref>Durand (1932), Section III.2</ref> The velocity perturbations in the flow around a wing are in fact produced by the pressure field.<ref>McLean (2012), Section 8.1</ref>