Theory of equal transit time

[flyguy]

1. Do you believe it?

2. Do you teach it?

[Ralgha]

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1. Do you believe it?

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No, mostly because it's not true.

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2. Do you teach it?

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See above.

[BrettInLJ]

See this NASA link:

http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html

And a simulation program here:

http://www.grc.nasa.gov/WWW/K-12/freesoftware_page.htm

[flyguy]

I don't need any convincing that equal transit is incorrect. That I understand. But the problem I'm having is teaching exactly what is going on without at least introducing equal transit.

For one thing, equal transit is true in the sense that, taking airflow out of the picture, it will take the same amount of time for the top of the airfoil to pass through a plane of space as it will the bottom of the airfoil. And since the distance over the top is greater it would accelerate the air. That is a fact, and I do believe that explains at least some of the reason there is low pressure above the wing. Even in a symetrical airfoil, if it is at a positive angle of atack, the stagnation point is some place below the leading edge,so the air going over the top has to go up and over the leading edge, then over the upper camber, wheras the air taking the lower path just has to go straight to the trailing edge. Equal transit theoretically still holds water. But in reality, the weight of the atmosphere is also pushing down over the top of the wing creating a venturi, further accelerating the air, which is why the adjacent air molecules that seperate at the stagnation point never see each other again.

My point is, equal transit is, although not completely accurate, explaining that a low pressure is created above the wing by air moving faster. I think knowing that that is happening is more important than knowing exactly why that is happening. Equal transit it taught in nearly every one of our text books and even Rod Machado teaches it, at least at the private level. So if you've got an ambitions student who has studied hard out of every available resource, (written for private pilots that is) it might be hard to convince them that you know more than the author of every texbook they have.

Understanding exactly what is going on seems to me to require at least a small understanding of aerodynamics. And I'm wondering if teaching equal transit is a way to give the student the undertaning they need to be able to build on that and explain it more accurately. So to the more experienced CFIs out there, do you think equal transit is at least a good starting point? Or is it better to try to explain it accurately from the beginning?

[Ralgha]

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So to the more experienced CFIs out there, do you think equal transit is at least a good starting point? Or is it better to try to explain it accurately from the beginning?

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I think it's better to explain it accurately from the beginning. Don't even mention the words "equal transit".

"And the truth...shall set you free!"

[moxiepilot]

thanks for that link. gotta new site for students to go to to play & learn for a while.

[flyguy]

The problem with not even mentioning equal tranit is at some point they are going to hear it, and whoever it is that explains it to them is going to have countless resources to back them up. I think if you are going to teach lift accurately, you should explain why equal tranist is incorrect. Does this make sense? Can you tell I'm a little apprehensive about teaching my first student about it?

[Ralgha]

[ QUOTE ]
The problem with not even mentioning equal tranit is at some point they are going to hear it, and whoever it is that explains it to them is going to have countless resources to back them up. I think if you are going to teach lift accurately, you should explain why equal tranist is incorrect. Does this make sense? Can you tell I'm a little apprehensive about teaching my first student about it?

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If you're really that worried about it, then mention it, and show them why it's wrong. If you teach it correctly though, the student will recognize that the equal transit theory is wrong on their own.