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Incorrect Lift Theory

Old 6th Sep 2006, 13:28
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Incorrect Lift Theory

Ugghh, these NASA people! First, they tell us Pluto is no longer a planet, and now this!! What should we now teach student pilots on the theory of lift? It appears that the "Long Path" or "Equal Transit" theory is incorrect??
Check out this page from the NASA website and also conduct the experiment given below ... interesting stuff. If you've been teaching the "Equal Transit Theory" or the "Venturi Effect" theory for explaining "the force of lift" to your students, will you now change after reading this?
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Old 6th Sep 2006, 17:04
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Question

Very interesting point. I notice the website doesn't give any particular answers saying that the Equal transit method is appealing because it is mostly correct. As an graduate of aerospace engineering I can see where they are coming from but as an instructor I ask the question: which explanation does the CAA expect the student to use? Obviously many thiings are simplified, such as the four forces (no mention of resultant force!), but we have to think of this practically. Everything is much more complicated that the simple explanations and science fact is derived from a method of deduction and constants based on repetition through experimentation. All liquids flow downhill? not quite.. glass and lead are solids etc..

Good point well made tho!

Oveur
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Old 6th Sep 2006, 19:23
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This is not a new idea, it has been known for years that the simplistic " Bernoulli" type explanation of lift used at ppl level and even cpl level theory is flawed. It can't explain why a flat plate produces lift.

Try reading Barnard and Philpotts Aircraft Flight book, this readily explains why this theory is flawed; furthermore it explains some alternative theories on why a wing produces lift.

Captain Oveur, as an aero graduate then you should have encountered Kutta Joukowski (incorrect spelling I know) wing lift thoerems.
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Old 6th Sep 2006, 19:40
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Ha ha Touche!

Sorry long day at the office... You're absolutley correct. Was trying to make the point about CAA aerodynamics and theory and the real world! Not unlike CAA paperwork and the real world.

Kutta-Joukowski subscribed to the principles of conservation of momentum and newtons 3rd law. As such downwash from their rotating cylinder resulted in an opposing force pushing it skyward. This still doesn't explain flat plate sufficently. A good explanation is here: http://hyperphysics.phy-astr.gsu.edu...ids/kutta.html.

To be honest i'm digging deep here so i'll have a think and see what I can come up with!

Oveur

Last edited by Captain Oveur; 6th Sep 2006 at 19:54.
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Old 6th Sep 2006, 21:51
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Why a wing produces lift? there several theories, however I am probably not the best qualified to comment which is most valid. Anyway whichever explanation you believe there will always be someone who can provide a counter argument and this is something that could be discussed forever.


Mingal, the theory the ppl technical exam recognises for wing lift has always been incorrect since ppl training began. The way I approach the subject is to explain this is what you are required to learn for the exam, but please be aware that it isn't strictly correct, but I would avoid explaining any other theory due to the complexity involved and it would just generally confuse the student. I prefer teach to accurate facts rather than just teaching people to pass an exam, but in this instance it is unavoidable.
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Old 6th Sep 2006, 23:38
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I was beating out a similar tune in this thread.

http://www.pprune.org/forums/showthread.php?t=237796

Last e-mail I got from the FAA was to the effect that they're re-writing the book.
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Old 7th Sep 2006, 10:52
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As far as exams are concerned, questions should not be asked about theories only facts.
Who is to say that a students "answer" is not a valid theory?
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Old 7th Sep 2006, 12:53
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Theory of lift

Seems there is another member of the aviation community who disagrees with this. The person behind this claim is a Swedish guy by the name of Martin Ingelman Sundberg.

His thoughts are as far as i understand them that the "bernoulli" principle is correct in the fact that the principleis correctly defined, but does not apply to the theory of lift.

He theorise that the air over the top of the aerofoil is not the part that creates the lift, but the underside. Find it hard to believe myself, but thought i would throw a fox in amongst the chickens...

MD
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Old 8th Sep 2006, 12:41
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Originally Posted by portsharbourflyer
This is not a new idea, it has been known for years that the simplistic " Bernoulli" type explanation of lift used at ppl level and even cpl level theory is flawed. It can't explain why a flat plate produces lift.
A flat plate produces lift in the same way as an aerofoil does, it just produces a lot less becuase the flow over the plate is virtually completely non laminar when a small angle of attack is presented. Bernoulli's Theorem is not flawed at all and does explain why a flat plate produces lift but his theorum was written in regard to streamline flow, a point missed by many authors and comentators.

There has been no change in the way CAA (or knowledgeable instructors) teach or examine basic aerodynamics and the NASA site is correct.
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Old 8th Sep 2006, 13:12
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Originally Posted by theresalwaysone
Bernoulli's Theorem is not flawed at all and does explain why a flat plate produces lift but his theorum was written in regard to streamline flow, a point missed by many authors and comentators.
Exactly why aircraft don't just have 'flat plates' for wings!
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Old 8th Sep 2006, 13:21
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Originally Posted by portsharbourflyer
Why a wing produces lift? there several theories, however I am probably not the best qualified to comment which is most valid. Anyway whichever explanation you believe there will always be someone who can provide a counter argument and this is something that could be discussed forever.


Mingal, the theory the ppl technical exam recognises for wing lift has always been incorrect since ppl training began. The way I approach the subject is to explain this is what you are required to learn for the exam, but please be aware that it isn't strictly correct, but I would avoid explaining any other theory due to the complexity involved and it would just generally confuse the student. I prefer teach to accurate facts rather than just teaching people to pass an exam, but in this instance it is unavoidable.
With respect, what a bizzare statment! You say in one line you only teach what is required for the exam and then on the next line you state you prefer to teach accurate facts rather than just teach people to pass an exam, I would say that is a gross contradiction wouldnt you!?


How is the technical exam incorrect, please explain what theory it supports that makes you say QUOTE the theory the ppl technical exam recognises for wing lift has always been incorrect since ppl training began. I cannot see any questions in the exam papers that supports this statement.
(Just a small point but PPL training has been going a lot longer that the technical exam!)


Teaching basic aerodynamics dosnt requires COMPLEXITY just a spoon under a tap,a garden hose or the example of a door slaming shut with a draught. Teaching top surface pressure velocity relationship teaches students to respect clean wings in icing conditions and thats when lack of understanding of aerodynamic theory can bite very hard! Do you teach that?
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Old 8th Sep 2006, 13:36
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I think he means he would much prefer to teach the accurate facts, but due to the complexities involved and conflicting reading material - he can't!
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Old 9th Sep 2006, 20:19
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Mazzy,

Thanks for that, my point exactly, I don't think I would have many students left if I started lecturing Kutta thoery or Navier stokes equations.

I should correct myself, Bernouillis principles are not flawed, it is the explanation used in ppl/ATPL texts to explain how these lead to wing lift that are flawed.

Theresalwaysone,

I would suggest that you read one the trevor thom technical book or any similar ppl level technical text ( even your cpl/atpl technical notes), then read Barnard and Philpotts Aircraft Flight (it is a non mathematical text), you will soon see what I am referring too.

Last edited by portsharbourflyer; 9th Sep 2006 at 20:32.
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Old 9th Sep 2006, 22:06
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Personally I think that Glauert's 1926 book "Aerofoil and Airscrew theory" is far superior, but a little less accessible.


Seriously, I really don't see a problem. I've learned so many different explanations of how an aeroplane flies, that I long ago ceased to believe that I know how an aeroplane flies - I lost track of that somewhere in the middle of my doctorate (in aerospace engineering).

What I do know, which is much more important, is how to predict whether it will or not. Which brings us back to the grossly simplistic, and largely innacurate model which has been used for training pilots for years.

The fact is, it's good enough. It does a fair job of explaining about the stall, the relationship between speed, lift and g, and how the controls work - it does a good enough job for any private and the majority of professional pilots, without loading them down with theory that is frankly of no practical use to them whatsoever.

But, seriously, if anybody really wants to see the theory - get hold of a copy of Glauert - it is beautifully written, and in 1926 had already shot down the theory we all learned when we did our PPLs.

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Old 9th Sep 2006, 23:50
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Lets get back to basics - stick your hand out of the car window on the motorway and angle it upwards into wind - your hand wants to go up.
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Old 11th Sep 2006, 07:45
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If the aerodynamcis of lift can't be properly explained, then how do engineers design wings for aircraft - especially functionally different wings (e.g. fighter or big transport)?
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Old 11th Sep 2006, 08:32
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Originally Posted by GroundBound
If the aerodynamcis of lift can't be properly explained, then how do engineers design wings for aircraft - especially functionally different wings (e.g. fighter or big transport)?
By building a wing, sticking it in a wind tunnel, testing it, and changing it until it works at it's best
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Old 11th Sep 2006, 09:44
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As I said, it's not important that you can explain lift clearly - it's important that you can predict it accurately.

Through a combination of computer modelling and wind-tunnelling, fortunately we're pretty good at that nowadays.

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Old 12th Sep 2006, 16:30
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None of you have any idea what you're talking about,,, it's money that keeps you flying. More money, bigger plane- no money, no plane.
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Old 12th Sep 2006, 16:48
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I've done a bit of fluid dynamics too, and was always troubled by the number of approximations and fudges needed to make the theory work precisely

But, in the case of the NASA 'experiment', surely they are missing the obvious? The symmetrical wing section does have a long path (and a short one) as soon as an incidence is applied. The static point moves swiftly down around the leading edge, whereas the theoretical separation point is fixed at the sharp trailing edge. (In the case of +ve lift, and vice versa). Which is why Bernoulli still works.

Edited. it to is.

Last edited by boguing; 13th Sep 2006 at 21:20.
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