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Does the equal transit theory work in viscous materials? It would seem here that if one were flying through something like gelatin the particles would come together at the tail end of the wing.

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It's the other way around. Look up Kutta Condition.

If the fluid were inviscid it would be able to flow around the trailing edge of the wing and join its cousins on the upper surface of the wing. Then there would be no bound vortex, and no lift.

Since it is not, the fluid on the upper surface feels a pressure gradient pulling it downward and to the rear. The net result is a bound vortex resulting in downwash at the rear of the wing. (There also is upwash at the front of the wing.) The idea that two neighboring molecules separated at the leading edge must rejoin at the trailing is a popular but wrong idea. In fact if it were true there would be no lift because lift depends on that vortex leading to the downwash and the upwash, and the associated momentum transfer.

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  • $\begingroup$ You say there wouldn't be any lift if two molecules separated at the leading edge had to rejoin- but you can calculate a lift from this by calculating a pressure difference- do you mean there wouldn't be enough lift? $\endgroup$
    – Bob
    Dec 18, 2015 at 0:06
  • $\begingroup$ @Bob: Here's a very good and easily understood explanation. Essentially the flow is a summation of linear and circular motion (a vortex) resulting in the downwash. If the molecules rejoin there is no vortex. No vortex = no downwash, and no downwash = no lift. $\endgroup$ Dec 18, 2015 at 0:40

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