Expert: Paul Soderman - 3/11/2009

Question
can you explain what tangential discontinuity means (in refernce to the "principle of equal transit times)

Answer
Jane
If you are talking aerodynamics, equal transit time was the idea that because air flowing over the top of a wing had to go farther to get to the trailing edge than air flowing under the wing, the upper surface air had to go faster to arrive at the trailing edge at the same time as the lower surface air. This would explain the higher velocities and lower pressures on the upper surface and would avoid the tangential flow discontinuity at the trailing edge.  But the idea is wrong.  There is no physical reason why air particles that separate at the leading edge have to meet at the trailing edge.  And there is experimental evidence showing that the upper surface flow travels farther than the lower surface flow. See:  http://user.uni-frankfurt.de/~weltner/Flight/PHYSIC4.htm

The above reference gives a much better explanation for surface velocities and lift by using the Coanda effect and Newton's law (centripedal force) to show that flow following a convex surface must have lower pressures than flow further above the surface. A pressure gradient is created by the geometry and flow field. The low pressures on the upper surface cause a velocity increase because of Bernouilli's law.  It is a good read.
Paul