Editing Lift (force) (section)

====Obstruction of the airflow====
[[File:Streamlines around a NACA 0012.svg|thumb|300px|Streamlines and streamtubes around an airfoil generating lift. The flow is two-dimensional and the airfoil has infinite span. Note the narrower streamtubes above and the wider streamtubes below.]]

Like the equal transit time explanation, the "obstruction" or "streamtube pinching" explanation argues that the flow over the upper surface is faster than the flow over the lower surface, but gives a different reason for the difference in speed.  It argues that the curved upper surface acts as more of an obstacle to the flow, forcing the streamlines to pinch closer together, making the streamtubes narrower.  When streamtubes become narrower, conservation of mass requires that flow speed must increase.<ref>"As stream tube A flows toward the airfoil, it senses the upper portion of the airfoil as an obstruction, and stream tube A must move out of the way of this obstruction. In so doing, stream tube A is squashed to a smaller cross-sectional area as it flows over the nose of the airfoil. In turn, because of mass continuity (ρ AV = constant), the velocity of the flow in the stream tube must increase in the region where the stream tube is being squashed." J. D. Anderson (2008), ''Introduction to Flight'' (6th edition), section 5.19</ref>  Reduced upper-surface pressure and upward lift follow from the higher speed by [[Bernoulli's principle]], just as in the equal transit time explanation.  Sometimes an analogy is made to a [[venturi tube|venturi nozzle]], claiming the upper surface of the wing acts like a venturi nozzle to constrict the flow.<ref>"The theory is based on the idea that the airfoil upper surface is shaped to act as a nozzle which accelerates the flow. Such a nozzle configuration is called a Venturi nozzle and it can be analyzed classically. Considering the conservation of mass, the mass flowing past any point in the nozzle is a constant; the mass flow rate of a Venturi nozzle is a constant... For a constant density, decreasing the area increases the velocity." ''Incorrect Theory #3'' Glenn Research Center NASA https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/venturi-theory/ {{Webarchive|url=https://web.archive.org/web/20230209112230/https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/venturi-theory/ |date=February 9, 2023 }}</ref>

One serious flaw in the obstruction explanation is that it does not explain how streamtube pinching comes about, or why it is greater over the upper surface than the lower surface.  For conventional wings that are flat on the bottom and curved on top this makes some intuitive sense, but it does not explain how flat plates, symmetric airfoils, sailboat sails, or conventional airfoils flying upside down can generate lift, and attempts to calculate lift based on the amount of constriction or obstruction do not predict experimental results.<ref>"The problem with the 'Venturi' theory is that it attempts to provide us with the velocity based on an incorrect assumption (the constriction of the flow produces the velocity field). We can calculate a velocity based on this assumption, and use Bernoulli's equation to compute the pressure, and perform the pressure-area calculation and the answer we get does not agree with the lift that we measure for a given airfoil." NASA Glenn Research Center {{cite web|url=https://www.grc.nasa.gov/WWW/K-12/airplane/wrong3.html |title=Incorrect lift theory #3|date=Aug 16, 2000 |access-date=27 June 2021 |archive-url=https://web.archive.org/web/20120717222459/http://www.grc.nasa.gov/WWW/k-12/airplane/wrong3.html|archive-date=July 17, 2012}}</ref><ref>"A concept...uses a symmetrical convergent-divergent channel, like a longitudinal section of a Venturi tube, as the starting point . . when such a device is put in a flow, the static pressure in the tube decreases. When the upper half of the tube is removed, a geometry resembling the airfoil is left, and suction is still maintained on top of it. Of course, this explanation is flawed too, because the geometry change affects the whole flowfield and there is no physics involved in the description." Jaakko Hoffren ''Quest for an Improved Explanation of Lift'' Section 4.3 American Institute of Aeronautics and Astronautics 2001 {{cite web|url=http://corsair.flugmodellbau.de/files/area2/LIFT.PDF|title=Archived copy|access-date=26 July 2012|url-status=dead|archive-url=https://web.archive.org/web/20131207102746/http://corsair.flugmodellbau.de/files/area2/LIFT.PDF|archive-date=December 7, 2013}}</ref><ref>"This answers the apparent mystery of how a symmetric airfoil can produce lift. ... This is also true of a flat plate at non-zero angle of attack." Charles N. Eastlake ''An Aerodynamicist's View of Lift, Bernoulli, and Newton'' {{cite web|url=http://www.df.uba.ar/users/sgil/physics_paper_doc/papers_phys/fluids/Bernoulli_Newton_lift.pdf|title=Archived copy|access-date=10 September 2009|url-status=dead|archive-url=https://web.archive.org/web/20090411055333/http://www.df.uba.ar/users/sgil/physics_paper_doc/papers_phys/fluids/Bernoulli_Newton_lift.pdf|archive-date=April 11, 2009}}</ref><ref>"This classic explanation is based on the difference of streaming velocities caused by the airfoil. There remains, however, a question: How does the airfoil cause the difference in streaming velocities? Some books don't give any answer, while others just stress the picture of the streamlines, saying the airfoil reduces the separations of the streamlines at the upper side. They do not say how the airfoil manages to do this. Thus this is not a sufficient answer." Klaus Weltner ''Bernoulli's Law and Aerodynamic Lifting Force'' The Physics Teacher February 1990 p. 84. [http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=PHTEAH000028000002000084000001&idtype=cvips&prog=normal] {{dead link|date=January 2018|bot=InternetArchiveBot|fix-attempted=yes}}</ref> Another flaw is  that conservation of mass is not a satisfying physical reason why the flow would speed up. Effectively explaining the acceleration of an object requires identifying the force that accelerates it.<ref>Doug McLean ''Understanding Aerodynamics'', section 7.3.1.5, Wiley, 2012</ref>