Editing Lift (force) (section)

===Explanation based on flow deflection and Newton's laws===
[[File:AirfoilDeflectionLift_W3C.svg|thumb|240px|right|When a wing generates lift, it deflects air downward, and to do this it must exert a downward force on the air. Newton's third law requires that the air must exert an equal upward force on the wing.]]

An airfoil generates lift by exerting a downward force on the air as it flows past. According to [[Newton's third law]], the air must exert an equal and opposite (upward) force on the airfoil, which is lift.<ref>"...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 Ed.|publisher=John Wiley & Sons|page=378}}</ref><ref name="Anderson and Eberhardt 2001">Anderson and Eberhardt (2001)</ref><ref name="Langewiesche 1944">Langewiesche (1944)</ref><ref>"When air flows over and under an airfoil inclined at a small angle to its direction, the air is turned from its course. Now, when a body is moving in a uniform speed in a straight line, it requires force to alter either its direction or speed. Therefore, the sails exert a force on the wind and, since action and reaction are equal and opposite, the wind exerts a force on the sails." In: {{citation|first1=John|last1=Morwood|title=Sailing Aerodynamics|publisher=Adlard Coles Limited|page=17}}</ref>

As the airflow approaches the airfoil it is curving upward, but as it passes the airfoil it changes direction and follows a path that is curved downward. According to Newton's second law, this change in flow direction requires a downward force applied to the air by the airfoil. Then Newton's third law requires the air to exert an upward force on the airfoil; thus a reaction force, lift, is generated opposite to the directional change. In the case of an airplane wing, the wing exerts a downward force on the air and the air exerts an upward force on the wing.<ref>a. {{cite web|publisher=NASA Glenn Research Center|title=Lift from Flow Turning|url=https://www.grc.nasa.gov/WWW/K-12/airplane/right2.html|date=May 27, 2000|access-date=June 27, 2021|archive-url=https://web.archive.org/web/20110705131653/http://www.grc.nasa.gov/WWW/K-12/airplane/right2.html|archive-date=July 5, 2011|quote=Lift is a force generated by turning a moving fluid... 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.}}<br />
b. {{cite web|quote=Essentially, due to the presence of the wing (its shape and inclination to the incoming flow, the so-called angle of attack), the flow is given a downward deflection. It is Newton's third law at work here, with the flow then exerting a reaction force on the wing in an upward direction, thus generating lift.|author=Vassilis Spathopoulos|title=Flight Physics for Beginners: Simple Examples of Applying Newton's Laws ''The Physics Teacher'' Vol. 49, September 2011 p. 373|url=http://tpt.aapt.org/resource/1/phteah/v49/i6/p373_s1|access-date=June 29, 2021|archive-date=June 18, 2013|archive-url=https://archive.today/20130618032326/http://tpt.aapt.org/resource/1/phteah/v49/i6/p373_s1|url-status=dead}}<br />
c. {{cite book|quote=The main fact of all heavier-than-air flight is this: ''the wing keeps the airplane up by pushing the air down.''|author=Langewiesche|title=Stick and Rudder, p. 6}}</ref><ref>
a. {{cite web |quote=Birds and aircraft fly because they are constantly pushing air downwards: L = Δp/Δt where L= lift force, and Δp/Δt is the rate at which downward momentum is imparted to the airflow.|title=Flight without Bernoulli|author= Chris Waltham |publisher=The Physics Teacher Vol. 36 Nov. 1998 |url=http://www.df.uba.ar/users/sgil/physics_paper_doc/papers_phys/fluids/fly_no_bernoulli.pdf|access-date=4 August 2011|url-status=live|archive-url=https://web.archive.org/web/20110928200519/http://www.df.uba.ar/users/sgil/physics_paper_doc/papers_phys/fluids/fly_no_bernoulli.pdf|archive-date=September 28, 2011}}<br />
b. {{cite book |author=Clancy, L. J.|title=Aerodynamics|publisher= Pitman 1975, p. 76|quote=This lift force has its reaction in the downward momentum which is imparted to the air as it flows over the wing. Thus the lift of the wing is equal to the rate of transport of downward momentum of this air.}}<br />
c. {{cite journal|quote=...if the air is to produce an upward force on the wing, the wing must produce a downward force on the air. Because under these circumstances air cannot sustain a force, it is deflected, or accelerated, downward. Newton's second law gives us the means for quantifying the lift force: F<sub>lift</sub> = m∆v/∆t = ∆(mv)/∆t. The lift force is equal to the time rate of change of momentum of the air. |last1=Smith|first1=Norman F.|year=1972|title=Bernoulli and Newton in Fluid Mechanics|journal=The Physics Teacher|volume=10|issue=8|page=451|doi=10.1119/1.2352317|bibcode=1972PhTea..10..451S}}</ref>
The downward turning of the flow is not produced solely by the lower surface of the airfoil, and the air flow above the airfoil accounts for much of the downward-turning action.<ref>"...when one considers the downwash produced by a lifting airfoil, the upper surface contributes more flow turning than the lower surface." ''Incorrect Theory #2'' Glenn Research Center NASA https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/foilw2/ {{Webarchive|url=https://web.archive.org/web/20230209111605/https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/foilw2/ |date=February 9, 2023 }}</ref><ref>" This happens to some extent on both the upper and lower surface of the airfoil, but it is much more pronounced on the forward portion of the upper surface, so the upper surface gets the credit for being the primary lift producer. " Charles N. Eastlake ''An Aerodynamicist's View of Lift, Bernoulli, and Newton'' ''The Physics Teacher'' Vol. 40, March 2002 [http://www.df.uba.ar/users/sgil/physics_paper_doc/papers_phys/fluids/Bernoulli_Newton_lift.pdf PDF] {{webarchive|url=https://web.archive.org/web/20090411055333/http://www.df.uba.ar/users/sgil/physics_paper_doc/papers_phys/fluids/Bernoulli_Newton_lift.pdf|date=April 11, 2009}}</ref><ref>"The pressure reaches its minimum value around 5 to 15% chord after the leading edge. As a result, about half of the lift is generated in the first 1/4 chord region of the airfoil. Looking at all three angles of attack, we observe a similar pressure change after the leading edge.
Additionally, in all three cases, the upper surface contributes more lift than the lower surface. As a result, it is critical to maintain a clean and rigid surface on the top of the wing. This is why most airplanes are cleared of any objects on the top of the wing." ''Airfoil Behavior: Pressure Distribution over a Clark Y-14 Wing'' David Guo, College of Engineering, Technology, and Aeronautics (CETA), Southern New Hampshire University https://www.jove.com/v/10453/airfoil-behavior-pressure-distribution-over-a-clark-y-14-wing {{Webarchive|url=https://web.archive.org/web/20210805181210/https://www.jove.com/v/10453/airfoil-behavior-pressure-distribution-over-a-clark-y-14-wing |date=August 5, 2021 }}</ref><ref>"There's always a tremendous amount of focus on the upper portion of the wing, but the lower surface also contributes to lift." ''Bernoulli Or Newton: Who's Right About Lift?'' David Ison Plane & Pilot Feb 2016</ref>

This explanation is correct but it is incomplete. It does not explain how the airfoil can impart downward turning to a much deeper swath of the flow than it actually touches. Furthermore, it does not mention that the lift force is exerted by [[#Pressure differences|pressure differences]], and does not explain how those pressure differences are sustained.<ref name="ReferenceA"/>

====Controversy regarding the Coandă effect====
{{Main|Coandă effect}}

Some versions of the flow-deflection explanation of lift cite the Coandă effect as the reason the flow is able to follow the convex upper surface of the airfoil.  The conventional definition in the aerodynamics field is that the ''Coandă effect'' refers to the tendency of a [[Jet (fluid)|fluid jet]] to stay attached to an adjacent surface that curves away from the flow, and the resultant [[Entrainment (hydrodynamics)|entrainment]] of ambient air into the flow.<ref name="auerbach">{{Citation|last=Auerbach|first=David|journal=Eur. J. Phys.|volume=21|issue=4|year=2000|page=289|title=Why Aircraft Fly|doi=10.1088/0143-0807/21/4/302|bibcode=2000EJPh...21..289A|s2cid=250821727 }}</ref><ref>{{Citation|url=http://www.av8n.com/how/htm/spins.html#sec-coanda-fallacy|last=Denker|first=JS|title=Fallacious Model of Lift Production|access-date=18 August 2008|url-status=dead|archive-url=https://web.archive.org/web/20090302153902/http://www.av8n.com/how/htm/spins.html#sec-coanda-fallacy|archive-date=March 2, 2009}}</ref><ref>{{Citation|title=Report on the first European Mechanics Colloquium, on the Coanda effect|last1=Wille|first1=R.|last2=Fernholz|first2=H.|journal=J. Fluid Mech.|year=1965|volume=23|issue=4|page=801|doi=10.1017/S0022112065001702|bibcode=1965JFM....23..801W|s2cid=121981660 }}</ref>

More broadly, some consider the effect to include the tendency of any fluid [[boundary layer]] to adhere to a curved surface, not just the boundary layer accompanying a fluid jet. It is in this broader sense that the Coandă effect is used by some popular references to explain why airflow remains attached to the top side of an airfoil.<ref name="scotteberhart">{{Citation|url=http://www.allstar.fiu.edu/AERO/airflylvl3.htm|last1=Anderson|first1=David|last2=Eberhart|first2=Scott|title=How Airplanes Fly: A Physical Description of Lift|year=1999|access-date=June 4, 2008|url-status=live|archive-url=https://web.archive.org/web/20160126200755/http://www.allstar.fiu.edu/aero/airflylvl3.htm|archive-date=January 26, 2016}}</ref><ref name="raskin">{{Citation|url=http://jef.raskincenter.org/published/coanda_effect.html|archive-url=https://web.archive.org/web/20070928072421/http://jef.raskincenter.org/published/coanda_effect.html|archive-date=September 28, 2007|title=Coanda Effect: Understanding Why Wings Work|last=Raskin|first=Jef|year=1994}}</ref> This is a controversial use of the term "Coandă effect"; the flow following the upper surface simply reflects an absence of boundary-layer separation, thus it is not an example of the Coandă effect.<ref>Auerbach (2000)</ref><ref>Denker (1996)</ref><ref>Wille and Fernholz(1965)</ref><ref>{{citation|last=White|first=Frank M.|title=Fluid Mechanics|year=2002|edition=5th|publisher=McGraw Hill}}</ref> Regardless of whether this broader definition of the "Coandă effect" is applicable, calling it the "Coandă effect" does not provide an explanation, it just gives the phenomenon a name.<ref>McLean, D. (2012), Section 7.3.2</ref>

The ability of a fluid flow to follow a curved path is not dependent on shear forces, viscosity of the fluid, or the presence of a boundary layer. Air flowing around an airfoil, adhering to both upper and lower surfaces, and generating lift, is accepted as a phenomenon in inviscid flow.<ref>McLean, D. (2012), Section 7.3.1.7</ref>