Editing Sailing (section)

===Lift and drag on sails===
{{Main|Lift (force)|Lift-induced drag}}
[[File:Sail angles of attack and resulting flow patterns.jpg|thumb|right|Sail angles of attack (α) 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.]]

''Lift'' on a sail, acting as an [[airfoil]], occurs in a direction ''perpendicular'' to the incident airstream (the apparent wind velocity for the headsail) and is a result of pressure differences between the windward and leeward surfaces and depends on the angle of attack, sail shape, air density, and speed of the apparent wind. 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 |author-link=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 airflow. As air follows a curved path along the windward side of a sail, there is a pressure [[gradient]] perpendicular to the flow direction with higher pressure on the outside of the curve and lower pressure on the inside. To generate lift, a sail must present an "[[angle of attack]]" between the [[Chord (aeronautics)|chord line]] of the sail and the apparent wind velocity. The angle of attack is a function of both the craft's point of sail and how the sail is adjusted with respect to the apparent wind.<ref>Klaus Weltner ''A comparison of explanations of the aerodynamic lifting force'' Am. J. Phys. 55(1), January 1987 pg 52</ref>

As the lift generated by a sail increases, so does [[lift-induced drag]], which together with [[parasitic drag]] constitute total ''drag'', which acts in a direction ''parallel'' to the incident airstream. This occurs as the angle of attack increases with sail trim or change of course and causes the [[lift coefficient]] to increase up to the point of [[aerodynamic stall]] along with the lift-induced [[drag coefficient]]. At the onset of stall, lift is abruptly decreased, as is lift-induced drag. Sails with the apparent wind behind them (especially going downwind) operate in a stalled condition.<ref name = Clancy>
{{Citation
 | last = Clancy
 | first = L.J.
 | title = Aerodynamics
 | place = London
 | publisher = Pitman Publishing Limited
 | year = 1975
 | pages = 638
 | isbn = 978-0-273-01120-0 }}</ref>

Lift and drag are components of the total aerodynamic force on sail, which are resisted by forces in the water (for a boat) or on the traveled surface (for an iceboat or land sailing craft). Sails act in two basic modes; under the ''lift-predominant'' mode, the sail behaves in a manner analogous to a ''wing'' with airflow attached to both surfaces; under the ''drag-predominant'' mode, the sail acts in a manner analogous to a ''parachute'' with airflow in detached flow, eddying around the sail.

====Lift predominance (wing mode)====
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 (causing luffing) and sailing sufficiently off the wind that the sail cannot be oriented at a favorable angle of attack to prevent the sail from [[Stall (fluid dynamics)|stalling]] with [[flow separation]].

====Drag predominance (parachute mode)====
When sailing craft are on a course where the angle between the sail and the apparent wind (the angle of attack) exceeds the point of maximum lift, separation of flow occurs.<ref>{{Citation
 | last1 = Collie
 | first1 = S. J.
 | last2 = Jackson
 | first2 = P. S.
 | last3 = Jackson
 | first3 = M.
 | last4 = Gerritsen
 | first5 = J.B.
 | last5 = Fallow
 | title = Two-dimensional CFD-based parametric analysis of down-wind sail designs
 | journal = The University of Auckland
 | date = 2006
 | url = http://syr.stanford.edu/RINA_Steve.pdf
 | access-date = 4 April 2015
 | archive-date = 28 July 2010
 | archive-url = https://web.archive.org/web/20100728123723/http://syr.stanford.edu/RINA_Steve.pdf
 | url-status = dead
 }}</ref> Drag increases and lift decreases with increasing angle of attack as the separation becomes progressively pronounced until the sail is perpendicular to the apparent wind, when lift becomes negligible and drag predominates. In addition to the sails used upwind, [[spinnaker]]s provide area and curvature appropriate for sailing with separated flow on downwind points of sail, analogous to parachutes, which provide both lift and drag.<ref name=Textor>
{{cite book
 | last = Textor
 | first = Ken
 | title = The New Book of Sail Trim
 | publisher = Sheridan House, Inc.
 | date = 1995
 | url = https://books.google.com/books?id=2JIbS0c1XPwC&pg=PA50
 | page = 50
 | isbn = 978-0-924486-81-4 }}
</ref>
{{center|
;Downwind sailing with a spinnaker
}}
<gallery mode="packed" heights="175px">
File:Sailboat on broad reach with spinnaker.jpg|Spinnaker set for a broad reach, generating both lift, with separated flow, and drag.
File:Spinnaker trimmed for broad reach.jpg|Spinnaker cross-section trimmed for a broad reach showing transition from boundary layer to separated flow where vortex shedding commences.
File:Amante Choate 48 photo D Ramey Logan.jpg|Symmetric spinnaker while running downwind, primarily generating drag.
File:Symmetrical spinnaker with following apparent wind.jpg|Symmetric spinnaker cross-section with following apparent wind, showing vortex shedding.
</gallery>

====Wind variation with height and time====
{{Further|Wind gradient#Sailing}}
Wind speed increases with height above the surface; at the same time, wind speed may vary over short periods of time as gusts.

[[Wind shear]] affects sailing craft in motion by presenting a different wind speed and direction at different heights along the [[mast (sailing)|mast]]. Wind shear occurs because of friction above a water surface slowing the flow of air.<ref>
{{Citation
 | last1 = Deacon
 | first1 = E. L.
 | last2 = Sheppard
 | first2 = P. A.
 | last3 = Webb
 | first3 = E. K.
 | title = Wind Profiles over the Sea and the Drag at the Sea Surface
 | journal = Australian Journal of Physics
 | volume = 9
 | issue = 4
 | pages = 511
 | date = December 1956
 | doi = 10.1071/PH560511 | bibcode = 1956AuJPh...9..511D
 | doi-access = free
 }}
</ref> The ratio of wind at the surface to wind at a height above the surface varies by a power law with an exponent of 0.11-0.13 over the ocean. This means that a {{cvt|5|m/s|kn|adj=on}} wind at 3 m above the water would be approximately {{cvt|6|m/s|kn}} at {{cvt|15|m|ft|sigfig=1}} above the water. In hurricane-force winds with {{cvt|40|m/s|kn}} at the surface the speed at {{cvt|15|m|ft|sigfig=1}} would be {{cvt|49|m/s|kn}}<ref>
{{cite web
 |last        = Hsu
 |first       = S. A.
 |title       = Measurements of Overwater Gust Factor From NDBC Buoys During Hurricanes
 |publisher   = Louisiana State University
 |date        = January 2006
 |url         = http://www.nwas.org/ej/pdf/2006-EJ2.pdf
 |access-date  = 19 March 2015
 |url-status     = dead
 |archive-url  = https://web.archive.org/web/20160304190331/http://www.nwas.org/ej/pdf/2006-EJ2.pdf
 |archive-date = 4 March 2016
}}
</ref> This suggests that sails that reach higher above the surface can be subject to stronger wind forces that move the centre of effort on them higher above the surface and increase the heeling moment. Additionally, apparent wind direction moves aft with height above water, which may necessitate a corresponding [[Sail twist|twist in the shape of the sail]] to achieve attached flow with height.<ref>
{{Citation
 | first1 = A.
 | last1 = Zasso
 | first2 = F.
 | last2 = Fossati
 | first3 = I.
 | last3 = Viola
 | title = Twisted flow wind tunnel design for yacht aerodynamic studies
 | series = 4th European and African Conference on Wind Engineering
 | year = 2005
 | pages = 350–351
 | place = Prague
 | url = http://www.research.ed.ac.uk/portal/files/17535106/Zasso_etal_EACWE4.pdf }}</ref>

Gusts may be predicted by the same value that serves as an exponent for wind shear, serving as a gust factor. So, one can expect gusts to be about 1.5 times stronger than the prevailing wind speed (a 10-knot wind might gust up to 15 knots). This, combined with changes in wind direction suggest the degree to which a sailing craft must adjust sail angle to wind gusts on a given course.<ref name=Hsu>
{{cite web
 | last = Hsu
 | first = S. A.
 | title = An Overwater Relationship Between the Gust Factor and the Exponent of Power-Law Wind Profile
 | work = Mariners Weather Log
 | volume = 52
 | issue = 1
 | publisher = National Oceanic and Atmospheric Administration
 | date = April 2008
 | url = http://www.vos.noaa.gov/MWL/apr_08/overwater.shtml
 | access-date = 19 March 2015 }}
</ref>