Editing Bernoulli's principle (section)

=== Airfoil lift ===
[[File:Equal transit-time NASA wrong1 en.svg|thumb|right|586px|An illustration of the incorrect equal transit-time explanation of airfoil lift]]

One of the most common erroneous explanations of aerodynamic lift asserts that the air must traverse the upper and lower surfaces of a wing in the same amount of time, implying that since the upper surface presents a longer path the air must be moving over the top of the wing faster than over the bottom. Bernoulli's principle is then cited to conclude that the pressure on top of the wing must be lower than on the bottom.<ref>{{cite book |author=Technical education research center|title=Physics That Works |publisher=Kendall Hunt |location= |year= 2006 | pages= |isbn= 0787291811 | quote = One of the most widely circulated, but incorrect, explanations can be labeled the “Longer Path” theory, or the “Equal Transit Time” theory. |oclc=61918633}}</ref><ref>{{cite journal|quote=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 over the top surface farther 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. |title=Bernoulli and Newton in Fluid Mechanics |first=Norman F. |last=Smith |journal=The Physics Teacher |date=November 1972 |volume=10 |issue=8 |page=451 |doi=10.1119/1.2352317 |bibcode=1972PhTea..10..451S |url=http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=PHTEAH000010000008000451000001&idtype=cvips&doi=10.1119/1.2352317&prog=normal|url-access=subscription }} {{dead link|date=January 2018|bot=InternetArchiveBot|fix-attempted=yes}}</ref>

Equal transit time applies to the flow around a body generating no lift, but there is no physical principle that requires equal transit time in cases of bodies generating lift. In fact, theory predicts – and experiments confirm – that the air traverses the top surface of a body experiencing lift in a ''shorter'' time than it traverses the bottom surface; the explanation based on equal transit time is false.<ref>{{cite journal|quote=...it is often asked why fluid particles should meet up again at the trailing edge. Or, in other words, why should two particles on either side of the wing take the same time to travel from S to T? There is no obvious explanation and real-life observations prove that this is wrong. |title=How do wings work? |first=Holger |last=Babinsky |journal=Physics Education |volume=38 |issue=6 |date=2003 |pages=497–503 |doi=10.1088/0031-9120/38/6/001 |bibcode=2003PhyEd..38..497B |s2cid=1657792 |url=https://www3.eng.cam.ac.uk/outreach/Project-resources/Wind-turbine/howwingswork.pdf }}</ref><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."<br>{{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><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. Experimental results and computational fluid dynamic calculations clearly show that a fluid element moving over the top surface of an airfoil leaves the trailing edge long before its companion element moving over the bottom surface arrives at the trailing edge.}}</ref> While the equal-time explanation is false, it is not the Bernoulli principle that is false, because this principle is well established; Bernoulli's equation is used correctly in common mathematical treatments of aerodynamic lift.<ref>{{cite web|quote=There is nothing wrong with the Bernoulli principle, or with the statement that the air goes faster over the top of the wing. But, as the above discussion suggests, our understanding is not complete with this explanation. The problem is that we are missing a vital piece when we apply Bernoulli’s principle. We can calculate the pressures around the wing if we know the speed of the air over and under the wing, but how do we determine the speed? |website=How Airplanes Fly: A Physical Description of Lift |first1=David |last1=Anderson |first2=Scott |last2=Eberhardt |url=http://www.allstar.fiu.edu/aero/airflylvl3.htm|title=How Airplanes Fly|access-date=26 January 2016|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>{{cite book|title=Introduction to Flight |first=John D. |last=Anderson |edition=8th |date=2016 |publisher=McGraw-Hill Education |chapter=Chapter 4. Basic Aerodynamics}}</ref>