Editing Wing (section)

==Aerodynamics==
[[File:Cloud over A340 wing.JPG|thumb|right|Condensation in the low pressure region over the wing of an [[Airbus A340]], passing through humid air]]
[[File:Aircraft flaps.svg|thumb|right|[[flap (aircraft)|Flaps]] (green) are used in various configurations to increase the wing area and to increase the lift. In conjunction with [[spoiler (aeronautics)|spoilers]] (red), flaps maximize drag and minimize lift during the landing roll.]]
{{Main|Lift (force)}}
The design and analysis of the wings of aircraft is one of the principal applications of the science of [[aerodynamics]], which is a branch of [[fluid mechanics]]. The properties of the airflow around any moving object can be found by solving the [[Navier-Stokes equations]] of [[fluid dynamics]]. Except for simple geometries, these equations are difficult to solve.<ref name=Nasa_NS>{{cite web|url=http://www.grc.nasa.gov/WWW/K-12/airplane/nseqs.html|title=Navier-Stokes Equations|publisher=[[Glenn Research Center|Grc.nasa.gov]]|date=2012-04-16|accessdate=2012-04-25}}</ref> Simpler explanations can be given.

For a wing to produce "lift", it must be oriented at a suitable [[angle of attack]] relative to the flow of air past the wing. When this occurs, the wing deflects the airflow downwards, "turning" the air as it passes the wing. Since the wing exerts a force on the air to change its direction, the air must exert a force on the wing, equal in size but opposite in direction. This force arises from different air pressures that exist on the upper and lower surfaces of the wing.<ref name=HR_378a>"...the effect of the wing is to give the air stream a downward velocity component. The reaction force of the deflected air mass must then act on the wing to give it an equal and opposite upward component." In: {{citation|first1=David|last1=Halliday|first2=Robert|last2=Resnick|title=Fundamentals of Physics 3rd Edition|publisher=John Wiley & Sons|page=378}}</ref><ref>"If the body is shaped, moved, or inclined in such a way as to produce a net deflection or turning of the flow, the local velocity is changed in magnitude, direction, or both. Changing the velocity creates a net force on the body" {{cite web|publisher=NASA Glenn Research Center|title=Lift from Flow Turning|url=http://www.grc.nasa.gov/WWW/K-12/airplane/right2.html|accessdate=2011-06-29}}</ref><ref name="Weltner_Physics_of_Flight_Reviewed">"The cause of the aerodynamic lifting force is the downward acceleration of air by the airfoil..." {{Citation|last1=Weltner|first1=Klaus|last2=Ingelman-Sundberg|first2=Martin|title=Physics of Flight – reviewed|url=http://user.uni-frankfurt.de/~weltner/Flight/PHYSIC4.htm|url-status=dead|archiveurl=https://web.archive.org/web/20110719102847/http://user.uni-frankfurt.de/~weltner/Flight/PHYSIC4.htm|archivedate=2011-07-19}}</ref>

Lower-than-ambient air pressure is generated on the top surface of the wing, with a higher-than ambient-pressure on the bottom of the wing. (See: [[airfoil]]) These air pressure differences can be either measured using a pressure-measuring device, or can be calculated from the airspeed using [[physics|physical principles]] {{Ndash}}including [[Bernoulli's principle]], which relates changes in air speed to changes in air pressure.

The lower air pressure on the top of the wing generates a smaller downward force on the top of the wing than the upward force generated by the higher air pressure on the bottom of the wing. This gives an upward force on the wing. This force is called the lift generated by the wing.

The different velocities of the air passing by the wing, the air pressure differences, the change in direction of the airflow, and the lift on the wing are different ways of describing how lift is produced so it is possible to calculate lift from any one of the other three. For example, the lift can be calculated from the pressure differences, or from different velocities of the air above and below the wing, or from the total momentum change of the deflected air. Fluid dynamics offers other approaches to solving these problems {{Ndash}}all which methods produce the same answer if correctly calculated. Given a particular wing and its velocity through the air, debates over which mathematical approach is the ''most convenient''{{citation needed|date=January 2025}} to use can be mistaken by those not familiar with the study of aerodynamics as differences of opinion about the basic principles of flight.<ref>{{Cite web |title=Equal Transit Theory Interactive {{!}} Glenn Research Center {{!}} NASA |url=https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/foilw1/ |url-status=live |archive-url=https://web.archive.org/web/20240927024228/https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/foilw1/ |archive-date=27 September 2024 |access-date=17 November 2024 |website=[[Glenn Research Center]]}}</ref>

===Cross-sectional shape===
Wings with an asymmetrical cross-section are the norm in [[subsonic flight]]. Wings with a symmetrical cross-section can also generate lift by using a positive [[angle of attack]] to deflect air downward. Symmetrical airfoils have higher [[Stall (flight)|stalling]] speeds than [[Camber (aerodynamics)|cambered airfoils]] of the same wing area<ref>E. V. Laitone, Wind tunnel tests of wings at Reynolds numbers below 70 000, ''Experiments in Fluids'' '''23''', ''405'' (1997). {{doi|10.1007/s003480050128}}</ref> but are used in [[aerobatic]] aircraft as they provide the same flight characteristics whether the aircraft is upright or inverted.<ref>The Design Of The Aeroplane,Darrol Stinton,{{ISBN|0 632 01877 1}},p.586</ref> Another example comes from sailboats, where the sail is a thin sheet.<ref name=Babinsky>"...consider a sail that is nothing but a vertical wing (generating side-force to propel a yacht). ...it is obvious that the distance between the stagnation point and the trailing edge is more or less the same on both sides. This becomes exactly true in the absence of a mast—and clearly the presence of the mast is of no consequence in the generation of lift. ''Thus, the generation of lift does not require different distances around the upper and lower surfaces.''" Holger Babinsky ''How do Wings Work?'' Physics Education November 2003, [http://iopscience.iop.org/0031-9120/38/6/001/pdf/pe3_6_001.pdf PDF]</ref>

For flight speeds near the speed of sound ([[transonic flight]]), specific asymmetrical airfoil sections are used to minimize the very pronounced increase in drag associated with airflow near the speed of sound.<ref>John D. Anderson, Jr. ''Introduction to Flight'' 4th ed page 271.</ref> These airfoils, called [[supercritical airfoil]]s, are flat on top and curved on the bottom.<ref>'Supercritical wings have a flat-on-top "upside down" look.' NASA Dryden Flight Research Center http://www.nasa.gov/centers/dryden/about/Organizations/Technology/Facts/TF-2004-13-DFRC.html</ref>