# Principle of equal transit times

1. Nov 27, 2004

### Munch_E

How does the principle of equal transit times works?

the Bernoulli principle explains lift by high speed = low pressure but these relays on the fact that two particles of air living the same point A will arrive at the same point B at the same time and this is explained by the "principle of equal transit times" my question is what is this priciple and how does it work?

2. Nov 27, 2004

### Clausius2

I think that the lift explanation provided by Bernoulli principle is a little bit primary and not realistic. The lift forces exerted on an airfoil comes generally from circulation considerations, more than that simplistic explanation.

About that principle you mentioned, think of the next experiment. Imagine a vertical front of fluid moving through the airfoil. All the particles have to reach simultaneusly the end of the airfoil because if not there would be a tangential discontinuity.

In fact this discontinuty exists, and it's a vortex behind the airfoil generated by the Kelvin Theorem. So that, we come up with the conclusion that such time principle is unreal and do not waste much time thinking on such a simplistic descriptions when you try to achieve an explanation about lift.

3. Nov 27, 2004

### Munch_E

first of all I thank you for your quick and extensive answer.

but I didn't quite understand what "tangential discontinuity" means.
(english in not my native tounge =\ )

4. Nov 27, 2004

### Clausius2

A tangential discontinuity is a discontinuity of some fluid maginitude such as velocity, density or pressure. I mean if the flow goes on x direction, a tangential discontinuity is a line such that flow magnitudes vary suddenly through "y" direction. The problem here is that just behind the airfoil the flow is no longer irrotational. Therefore, the Bernoulli principle is not applicable. A vortex is generated behind the airfoil, in part because of the principle you mentioned is not yielded, and because of the Kelvin's Theorem.

English is not my native language too.

5. Nov 27, 2004

### Munch_E

thank's
now I've got it.
and by the way your english is very good