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I've seen the equal transit time theory being used to explain aerodynamic lift [basis is Bernoulli's principle] several times. However recently I've been told that there is NO physical compulsion for air above an aerofoil to have to travel across it at the same time as the air below the aerofoil.

There're quite a few questions on this site that deal with the fallacy but none [as far as I've seen] specifically mention WHAT PROBLEMS WOULD ONE ENCOUNTER BY ACCEPTING THE EQUAL TRANSIT THEORY.

I mean sure, plenty of sources bring up the concept of air parcels having been proved [by computer simulation] to travel FASTER above the aerofoil than below it, but I haven't come across any specific contradiction the equal transit theory would throw up.

Though I vaguely recall coming across some article that stated the equal transit time would violate Newton's laws of motion. If this is correct then WHICH law of motion would it violate and HOW would the law be violated?

If there's anything else [apart from the laws of motion] which contradict the equal transit theory, I'd love to hear it.

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As always, read John Denker's wonderful ebook.

To quickly answer the question, the equal-time argument is a wrong application of Bernoulli. Bernoulli is right. Bernoulli plus the Kutta condition (air can't flow up over the trailing edge) is what makes flight possible.

If one accepted the equal-time argument

  • it would not explain how airplanes can fly inverted, or use symmetrical airfoils, as aerobatic aircraft do.

  • it would result in zero circulation, resulting in zero lift. Read Denker about this.

Mike Dunlavey
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Equal transit time would violate Newton's laws of motion in the sense that the computer simulations that are based on Newton's laws of motion show that equal transit time is false. I don't think you can show that using a simple heuristic argument or a simple equation, but the computer simulations do count as a valid proof, albeit perhaps not one you find satisfying. (Provided, of course, you've been careful to show that the error bars are small enough to rule out the possibility of equal transit time.)

In fact, I don't think that equal transit time does violate any of Newton's laws of motion in general, because if you make the airplane wing perfectly up-down symmetric then presumably it would be true. So it isn't physically impossible in general, it just happens to not be true in the vast majority of cases.

tparker
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Equal transit time theory (ETTT) violates Newton's third law. ETTT claims that upward force is generated without an equal and opposite downward reaction. The typical diagram that accompanies equal transit shows airflow lines behind the wing that are undisturbed. Nice tranquil straight lines. The story has been told about the air speeding up over the top so that air molecule pairs get reunited at the trailing edge, leading to a mistatement of Bernoulli's principle and a claim this acceleration produces lift. Downwash is no part of the story. If the wing is lifted without massive downwash, that violates Newton's third.

JAMES LUCKETT
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