Time it takes air to travel over and under an airfoil

cool92092

cool92092

Well-Known Member
So I was reading Wikipedia this morning and came across an article with a list of common misconceptions. In that article it states
It is not true that air takes the same time to travel above and below an aircraft's wing.[96] This misconception, illustrated at right, is widespread among textbooks and non-technical reference books, and even appears in pilot training materials.
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Here's the linky:
http://en.wikipedia.org/wiki/Equal_...ar.22_explanation_based_on_equal_transit-time

Amazing how much stuff there is out there that you believe is fact until you are shown the light...
 
True. Even the FAA endorses this misconception in their basic pilot training documents. Good for you for digging deeper to find the truth. I believe at some point in time, many pilots begin to yearn for a deeper understanding of the physical laws of flying. That typically doesn't occur early on...as the beginner is just trying to learn to maintain altitude, track radials or master a Vmc demo. But after all those have been mastered...there's always something new to learn. It actually feels good to learn something new!
 
So let me get this right. If I understand that article correctly air still moves faster over the top of the wing which would create a low pressure system the only difference is that there is no requirment for the particles to meet at the back of the wing?
 
Jbroz04 said:
So let me get this right. If I understand that article correctly air still moves faster over the top of the wing which would create a low pressure system the only difference is that there is no requirment for the particles to meet at the back of the wing?
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In essence that is what I got from it except that it moves faster because of the natural compression of the stream moving upward and not because it has a longer distance to travel. The article then goes into explaining integrals with squiggly lines in them so I just had to stop reading.
 
Jbroz04 said:
So let me get this right. If I understand that article correctly air still moves faster over the top of the wing which would create a low pressure system the only difference is that there is no requirment for the particles to meet at the back of the wing?
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No, there is no "requirement." They move at different velocities and will reach the trailing edge according to that velocity.

two particles of air may strike the leading edge at the same time and separate, one above, one below - but no law of physics state they must meet up together again.
 
Jbroz04 said:
So let me get this right. If I understand that article correctly air still moves faster over the top of the wing which would create a low pressure system the only difference is that there is no requirment for the particles to meet at the back of the wing?
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That's NASA, and other aero engineers take on it :).
 
cool92092 said:
Amazing how much stuff there is out there that you believe is fact until you are shown the light...
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The fallacy is only a slight perversion of the very real Kutta Condition, which states that the circulation around an airfoil will increase until the the two airflows meet at the trailing edge at a stagnation point. Without this, the stagnation point would be on the top of the airfoil, although still towards the rear. The Kutta Condition requires that the velocity over the top of the airfoil increase until the stagnation point moves to the trailing edge of the airfoil.

While this isn't the same as the equal transit theory, the latter is not so wrong as to be stupid.
 
With a checkride coming up I wonder what the DPE would say as I try to explain why equal transit is wrong?:cool:
 
Jbroz04 said:
So let me get this right. If I understand that article correctly air still moves faster over the top of the wing which would create a low pressure system the only difference is that there is no requirment for the particles to meet at the back of the wing?
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Im kinda simple, and by far from an aerodynamics guru, but I understand correctly, if it were required for particles to re unite, how would a wing stall?
 
Cmill I never thought about that. Than again I would have to say that explaing aerodynamics is my weak point. I understand myself but I have a hard time expressing it to others. But I never thought about how airflow serperation related to equal transit theory. I guess that is why I fly the airplane and dont design it.
 
Jbroz04 said:
With a checkride coming up I wonder what the DPE would say as I try to explain why equal transit is wrong?:cool:
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I believe I would keep the topic simple. "The wing performs work on the airflow in such a way that a pressure differential exists to create the lifting force."

"How?"

"Air is accelerated over the top of the wing."

"How does it do that?"

"Either through the design of camber in the wing or introducing an angle of attack. The fluid mechanics involved are much more complex, however understanding those complex factors is not materially related to pilot training."

Just don't say that air bounces off the bottom of the wing and deflects back towards the ground...or some crazy way of relating this to Newton's 3rd law of motion!
 
So we're arguing the Particle A with Particle B rule? It's false, and here's why. If you believe that particle A is friends with particle B, and after they hit the wing the split. Well they can't be alone! So they make sure they meet up at the other end. Since the top of the wing has a larger surface to travel, the top one (A) speeds up to catch up with B right?

Ok, so how come when you splash water, the water that went away doesn't come back to meet up with it's friend. Look, there's EM fields everywhere, but nowhere does it say that in common nature do any two particles have to match up from one point, displaced to another.

I'll draw a simple diagram of Bernoulli's principle tomorrow and explain. It's late, I don't want to teach at this hour, and really, scanning in is too much work tonight, Cya tomorrow with more info.
 
Jbroz04 said:
So let me get this right. If I understand that article correctly air still moves faster over the top of the wing which would create a low pressure system the only difference is that there is no requirment for the particles to meet at the back of the wing?
Click to expand...

The truth is that the particle on top of the wing most likely reaches the trailing edge first!
 
"Either through the design of camber in the wing or introducing an angle of attack. The fluid mechanics involved are much more complex, however understanding those complex factors is not materially related to pilot training
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what if his reponse to this reply is that I am the examiner and I decide what is materially related to pilot training? I dont know that a examiner would say something to this point but I am just curious. I mean how in depth did your fed examiner go on your intial cfi on this topic?
 
Jbroz04 said:
what if his reponse to this reply is that I am the examiner and I decide what is materially related to pilot training? I dont know that a examiner would say something to this point but I am just curious. I mean how in depth did your fed examiner go on your intial cfi on this topic?
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Try it and let us know how it goes.
 
shdw said:
Try it and let us know how it goes.
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I'm still in training for today with a flight tonight, so I may not get to the way I teach it until tomorrow afternoon/evening.

However, yeah, you should know the in depth reasoning behind it, AND be able to dumb it down to a first time student. You're the expert, be one.

I will give my 10 minute lecture on pressures, velocities, and air densities (where it applies) and it's relationship to airfoils.
 
ProudPilot said:
However, yeah, you should know the in depth reasoning behind it, AND be able to dumb it down to a first time student. You're the expert, be one.
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The point was the concepts behind this are not simple, they are extremely complex. Also, knowing this will not give a pilot any greater intuition on how or why an aircraft reacts the way it does. It falls in the same box as IAS changing with altitude IMO. Delving into the realm of advanced flight theory will only confuse.

Now I love aerodynamics and teach it readily, however, I try to stick with things that I find useful: drag, lift, AOA, stall, and how the environment and aircraft configuration effect these.

Now if you feel you can dumb down the Kutta condition and show it's usefulness, by all means go for it. As tgray pointed out, that is what this is related to and I spent many hours researching this on another forum and all I can say is...:insane: I am however looking forward to seeing how you will dumb this down, and I am not trying to be sarcastic or mean with this.
 
Yea some of the stuff required for oral exams is alittle out there I think. I mean yes it does serve you to know why things react the way they do but sometimes it is just overkill. Don't get me wrong I read as much as I can because I just like to know but there is alot of stuff I am studying that I wouldn't even bother trying to teach a primary student because it would just overwhelm them. It's like what a instructor that flies the King Air once told me. If I can't fix it from the cockpit than I dont want to know about it!
 
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