Editing Lift (force) (section)

====False explanation based on equal transit-time<span class="anchor" id="False explanation based on equal transit-time"></span>====

The "equal transit time" explanation starts by arguing that the flow over the upper surface is faster than the flow over the lower surface because the path length over the upper surface is longer and must be traversed in equal transit time.<ref>Burge, Cyril Gordon (1936). Encyclopædia of aviation. London: Pitman. p. 441.  "… the fact that the air passing over the hump on the top of the wing has to speed up more than that flowing beneath the wing, in order to arrive at the trailing edge in the same time."</ref><ref>Illman, Paul (2000). The Pilot's Handbook of Aeronautical Knowledge. New York: McGraw-Hill. pp. 15–16. ISBN 0071345191. When air flows along the upper wing surface, it travels a greater distance in the same period of time as the airflow along the lower wing surface."</ref><ref>Dingle, Lloyd; Tooley, Michael H. (2005). Aircraft engineering principles. Boston: Elsevier Butterworth-Heinemann. p. 548. ISBN 0-7506-5015-X. The air travelling over the cambered top surface of the aerofoil shown in Figure 7.6, which is split as it passes around the aerofoil, will speed up, because it must reach the trailing edge of the aerofoil at the same time as the air that flows underneath the section."</ref>  [[Bernoulli's principle]] states that under certain conditions increased flow speed is associated with reduced pressure.  It is concluded that the reduced pressure over the upper surface results in upward lift.<ref>"The airfoil of the airplane wing, according to the textbook explanation that is more or less standard in the United States, has a special shape with more curvature on top than on the bottom; consequently, the air must travel farther over the top surface than over the bottom surface. Because the air must make the trip over the top and bottom surfaces in the same elapsed time ..., the velocity over the top surface will be greater than over the bottom. According to Bernoulli's theorem, this velocity difference produces a pressure difference which is lift." ''Bernoulli and Newton in Fluid Mechanics'' Norman F. Smith ''The Physics Teacher'' November 1972 Volume 10, Issue 8, p. 451 [http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=PHTEAH000010000008000451000001&idtype=cvips&doi=10.1119/1.2352317&prog=normal] {{dead link|date=January 2018|bot=InternetArchiveBot|fix-attempted=yes}}</ref>

While it is true that the flow speeds up, a serious flaw in this explanation is that it does not correctly explain what causes the flow to speed up.<ref name="ReferenceA"/>  The longer-path-length explanation is incorrect.  No difference in path length is needed, and even when there is a difference, it is typically much too small to explain the observed speed difference.<ref>Craig G.M. (1997), ''Stop Abusing Bernoulli''</ref>  This is because the assumption of equal transit time is wrong when applied to a body generating lift. There is no physical principle that requires equal transit time in all situations and experimental results confirm that for a body generating lift the transit times are not equal.<ref>"Unfortunately, this explanation [fails] on three counts. First, an airfoil need not have more curvature on its top than on its bottom. Airplanes can and do fly with perfectly symmetrical airfoils; that is with airfoils that have the ''same'' curvature top and bottom. Second, even if a humped-up (cambered) shape is used, the claim that the air must traverse the curved top surface in the same time as it does the flat bottom surface...is fictional. We can quote no physical law that tells us this. Third—and this is the most serious—the common textbook explanation, and the diagrams that accompany it, describe a force on the wing with no net disturbance to the airstream. This constitutes a violation of Newton's third law." ''Bernoulli and Newton in Fluid Mechanics'' Norman F. Smith ''The Physics Teacher'' November 1972 Volume 10, Issue 8, p. 451 {{cite web|url=http://tpt.aapt.org/resource/1/phteah/v10/i8|title=Browse - the Physics Teacher|access-date=4 August 2011|url-status=dead|archive-url=https://web.archive.org/web/20120317075304/http://tpt.aapt.org/resource/1/phteah/v10/i8|archive-date=March 17, 2012}}</ref><ref>
{{Citation|last=Anderson|first=David|title=Understanding Flight|publisher=McGraw-Hill|location=New York|year=2001|isbn=978-0-07-136377-8|quote=The first thing that is wrong is that the principle of equal transit times is not true for a wing with lift.|page=15}}</ref><ref>{{cite book|last=Anderson|first=John|title=Introduction to Flight|publisher=McGraw-Hill Higher Education|location=Boston|year=2005|isbn=978-0072825695|page=355|quote=It is then assumed that these two elements must meet up at the trailing edge, and because the running distance over the top surface of the airfoil is longer than that over the bottom surface, the element over the top surface must move faster. This is simply not true}}</ref><ref>{{cite web|url=https://www.telegraph.co.uk/science/science-news/9035708/Cambridge-scientist-debunks-flying-myth.html|title=Cambridge scientist debunks flying myth - Telegraph|access-date=10 June 2012|url-status=dead|archive-url=https://web.archive.org/web/20120630121849/http://www.telegraph.co.uk/science/science-news/9035708/Cambridge-scientist-debunks-flying-myth.html|archive-date=June 30, 2012}} ''Cambridge scientist debunks flying myth'' UK Telegraph 24 January 2012</ref><ref>{{cite AV media|url=http://web.mit.edu/hml/ncfmf.html|title=Flow Visualization|publisher=National Committee for Fluid Mechanics Films/Educational Development Center|access-date=January 21, 2009|url-status=live|archive-url=https://web.archive.org/web/20161021122939/http://web.mit.edu/hml/ncfmf.html|archive-date=October 21, 2016}} A visualization of the typical retarded flow over the lower surface of the wing and the accelerated flow over the upper surface starts at 5:29 in the video.</ref><ref>"...do you remember hearing that troubling business about the particles moving over the curved top surface having to go faster than the particles that went underneath, because they have a longer path to travel but must still get there at the same time? This is simply not true. It does not happen." 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> In fact, the air moving past the top of an airfoil generating lift moves ''much'' ''faster'' than equal transit time predicts.<ref>"The actual velocity over the top of an airfoil is much faster than that predicted by the "Longer Path" theory and particles moving over the top arrive at the trailing edge before particles moving under the airfoil." {{cite web|url=https://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html |date=Aug 16, 2000 |title=Incorrect Lift Theory #1 |author=Glenn Research Center|publisher=NASA |access-date=June 27, 2021| archive-url=https://web.archive.org/web/20140427084226/http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html|archive-date=April 27, 2014}}</ref>  The much higher flow speed over the upper surface can be clearly seen in [[#The wider flow around the airfoil|this animated flow visualization]].