Editing Lift (force) (section)

===The wider flow around the airfoil===
[[File:Karman trefftz.gif|frame|right|Flow around an airfoil: the dots move with the flow. The black dots are on [[Streamlines, streaklines, and pathlines|time slices]], which split into two – an upper and lower part – at the leading edge. A marked speed difference between the upper-and lower-surface streamlines is shown most clearly in the image animation, with the upper markers arriving at the trailing edge long before the lower ones. Colors of the dots indicate [[Streamlines, streaklines, and pathlines|streamlines]].]]

An airfoil affects the speed and direction of the flow over a wide area, producing a pattern called a ''velocity field''. When an airfoil produces lift, the flow ahead of the airfoil is deflected upward, the flow above and below the airfoil is deflected downward leaving the air far behind the airfoil in the same state as the oncoming flow far ahead. The flow above the upper surface is sped up, while the flow below the airfoil is slowed down. Together with the upward deflection of air in front and the downward deflection of the air immediately behind, this establishes a net circulatory component of the flow. The downward deflection and the changes in flow speed are pronounced and extend over a wide area, as can be seen in the flow animation on the right. These differences in the direction and speed of the flow are greatest close to the airfoil and decrease gradually far above and below. All of these features of the velocity field also appear in theoretical models for lifting flows.<ref name="Clancy 1975, Section 4.5">Clancy (1975), Section 4.5</ref><ref>Milne-Thomson (1966.), Section 5.31</ref>

The pressure is also affected over a wide area, in a pattern of non-uniform pressure called a ''pressure field''. When an airfoil produces lift, there is a diffuse region of low pressure above the airfoil, and usually a diffuse region of high pressure below, as illustrated by the isobars (curves of constant pressure) in the drawing. The pressure difference that acts on the surface is just part of this pressure field.<ref>McLean 2012, Section 7.3.3.7</ref>