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{{Main|Lift (force)|Lift-induced drag}}
Lift on a sail ('''L'''), acting as an [[airfoil]], occurs in a direction perpendicular to the incident airstream (the apparent wind velocity, '''V<sub>A</sub>''', for the head sail) and is a result of pressure differences between the windward and leeward surfaces and depends on angle of attack, sail shape, air density, and speed of the apparent wind. [[Pressure]] differences result from the [[Stress (mechanics)#Normal and shear stresses|normal force]] per unit area on the sail from the air passing around it. The lift force results from the average pressure on the windward surface of the sail being higher than the average pressure on the leeward side.<ref>{{Citation |first=G.K. |last=Batchelor |authorlink=George Batchelor |title=An Introduction to Fluid Dynamics |year=1967 |publisher=Cambridge University Press |isbn=978-0-521-66396-0 |pages=14–15 }}</ref> These pressure differences arise in conjunction with the curved air flow. As air follows a curved path along the windward side of a sail, there is a pressure [[gradient]] perpendicular to the flow direction with
As the lift generated by a sail increases, so does [[lift-induced drag]], which together with [[parasitic drag]] constitutes total drag, ('''D'''). This occurs when the angle of attack increases with sail trim or change of course to cause the [[lift coefficient]] to increase up to the point of [[Stall (flight)|aerodynamic stall]], so does the lift-induced [[drag coefficient]]. At the onset of stall, lift is abruptly decreased, as is lift-induced drag, but viscous pressure drag, a component of parasitic drag, increases due to the formation of separated flow on the surface of the sail. Sails with the apparent wind behind them (especially going downwind) operate in a stalled condition.<ref name = Clancy>
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