Forces on sails: Difference between revisions

Revision as of 21:13, 3 June 2020

Revision as of 07:18, 26 July 2020

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===Lift predominant (attached flow)===

[[File:Sail angles of attack and resulting flow patterns.jpg|thumb|right|300 px|Sail angles of attack and resulting (idealized) flow patterns for attached flow, maximum lift, and stalled for a hypothetical sail. The stagnation streamlines (red) delineate air passing to the leeward side (top) from that passing to the windward (bottom) side of the sail.]]

Sails allow progress of a sailing craft to windward, thanks to their ability to generate lift (and the craft's ability to resist the lateral forces that result). Each sail configuration has a characteristic coefficient of lift and attendant coefficient of drag, which can be determined experimentally and calculated theoretically. Sailing craft orient their sails with a favorable angle of attack between the entry point of the sail and the apparent wind even as their course changes. The ability to generate lift is limited by sailing too close to the wind when no effective angle of attack is available to generate lift (luffing) and sailing sufficiently off the wind that the sail cannot be oriented at a favorable angle of attack (running downwind). Instead, past a [[Angle of attack#Critical angle of attack|critical angle of attack]], the sail [[Stall (fluid mechanics)|stalls]] and promotes [[flow separation]].

====Effect of angle of attack on coefficients of lift and drag====