It is widely understood that the curved shape of a typical airfoil (such as a Clark Y) causes a pressure difference that creates lift. How do symmetrical airfoils do this when the top and lower surfaces are the same shape and length?
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asked Jun 21, 2017 at 16:54
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1$\begingroup$ Actually it is widely understood that the curved shape is not needed to create lift. Flat plate at slight angle to the flow does generate lift. Not as efficiently, but using the same principle. Also, the length is irrelevant, because the air flowing over the upper surface will cross the wing in significantly shorter time than the air flowing over the lower surface. $\endgroup$– Jan HudecCommented Jun 21, 2017 at 18:43
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1$\begingroup$ See also How do paper airplanes create lift if their wings are flat? and What is the performance of a flat plate wing? for explanation why even flat plate generates lift. $\endgroup$– Jan HudecCommented Jun 21, 2017 at 19:01
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2$\begingroup$ Even the Wikipedia article on Lift calls out the common misconception of equal transit times as completely unfounded in reality. $\endgroup$– Jan HudecCommented Jun 21, 2017 at 19:04
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By using a non-zero angle of attack. When the trailing edge is pointed downwards, and assuming the airstream leaves the trailing edge smoothly, the exiting airstream is deflected downwards. This causes lift via conservation of momentum. Increasing the angle of attack will increase your lift until such time as the airstream over the trailing edge becomes non-smooth. (When this happens, you're close to stalling.)
(Image from this page, which unfortunately appears to be down)
As you can see from the above graph, a symmetric airfoil at zero angle of attack generates no lift; see this site from NASA as well as the above Wikipedia page.
