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Why does the air flow faster over the top of the aerofoil than underneath?

  1. Aug 31, 2012 #1
    Because it is curved? -------------> why because it is curved will it flow faster over it?!
    What other reasons!?

    pressure is lower ontop, therefore faster??

    why is the pressure on top lower... is it because of the shape of the aerofoil?

    I just find it hard to put this into a sentence that someone could understand when reading my coursework..

    Thank You :)
     
  2. jcsd
  3. Aug 31, 2012 #2

    HallsofIvy

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    Because it is curved, the air has farther to go over the top. If the air flowed at the same rate of the top as the bottom, it would be less dense. There would be a partial vacuum which then "sucks" the air in and causes it to go faster.
     
  4. Aug 31, 2012 #3
    Apologies, but i still do not understand...
    because it is curved the air has a larger distance to travel but why is it faster than the underside...
    is it because the underside airflow gets obstructed as it will be at a slight angle to create lift??
     
  5. Aug 31, 2012 #4

    phinds

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    d=rt
     
  6. Aug 31, 2012 #5
    :( dont understand once again
     
  7. Aug 31, 2012 #6

    Andrew Mason

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    Your question is a good one. You are probably trying to understand the theory of an airplane wing based on the Bernoulli's law. You should not feel bad that you don't understand how the Bernoulli theory would result in lift. It doesn't.

    An airplane wing creates lift because it generates turbulent flow above the wing. The physics is very complicated. This NASA page tries to explain it. This NASA page explains why the Bernoulli theory is wrong.

    AM
     
  8. Aug 31, 2012 #7

    phinds

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    you asked

    I find it hard to believe that you don't understand that, so I think it's more likely that it is just awkward wording. To answer EXACTLY what you asked, it is as I said, distance = rate x time.


    Let's say you have to drive your car 60 miles in one hour and a friend has to drive his car to the same end point but has to go a different way that takes 120 miles but he has to get there the same time you do. Do you think he can drive the same 60 mph that you do?
     
  9. Aug 31, 2012 #8

    rcgldr

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    "Faster" depends on the frame of reference. Even a flat (non-curved) board moving forward at an angle of attack will generate lift. Link to an example article with a simple explanation of lift, and diagrams showing the movement of air affected by a wing:

    http://www.avweb.com/news/airman/183261-1.html

    Image of a pre-shuttle lifting body prototype (next to a F104 jet) that shows that the "longer distance" isn't always on top:

    m2f2.jpg
     
    Last edited: Aug 31, 2012
  10. Aug 31, 2012 #9
    My understanding is that that's a standard fallacy - the air does go faster over the top but not because it "has farther to go". An air parcel split at the leading edge does not in fact come back together at the trailing edge.

    I was under the impression that turbulent flow is important for supersonic aircraft, but that at subsonic speeds Bernoulli alone was a pretty good description.

    Perhaps some aviation expert can chip in here...
     
  11. Aug 31, 2012 #10
    d=rt ???
    What on earth does that mean.......is it some sort of assistance?
    Do you mean s = vt ?
     
  12. Aug 31, 2012 #11

    phinds

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    Read post #7
     
  13. Aug 31, 2012 #12
    What worries me is that the OP appears to be trying to answer a multiple choice question which does not offer the correct answer. What sort of institution promotes that?

    Where did this question arise?
    The answer is (quite difficult) is that the circulation adds to the flow velocity above the wing and opposes (or subtracts from) the flow velocity below it.
     
  14. Sep 1, 2012 #13

    Andrew Mason

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    I don't know that an aviation expert would have any better idea. I am certainly not an expert on fluid dynamics. It is perhaps the most difficult area of physics (hence Feynman's comment that turbulent flow was "the most important unsolved problem of classical physics").

    According to this NASA explanation, lift requires a vortex - spinning air.

    This is what I gather from the explanations: At the boundary between the layer of air that is very close to the wing and not moving relative to the wing (which will occur naturally if the surface is able to hold a small layer of air) and the air above the wing that is moving relative to the wing, vortices occur. A vortex causes air to move upward and then downward but the downward part is little farther along the wing. Since the wing is tapered, this allows the air in the vortex to descend farther than its initial position, so the net result is that the air above the wing surface is driven downward. Since the wing is also inclined a little to the direction of airflow, the bottom surface of the wing also drives air downward. The total effect is that in the regions above and below the wing, air is driven downward. The downward forces on the air created above and below the wing create lift (the downward force on the air by the wing must necessarily be accompanied by an upward force on the wing by the air). But without the vortices created by the top surface, the wing does not create enough lift. This is what occurs when a wing surface is coated with ice - the wing cannot hold enough air near its surface and does not create enough turbulence above the wing.

    AM
     
    Last edited: Sep 1, 2012
  15. Sep 1, 2012 #14
    Good afternoon Andrew.

    Can I respectfully point out that the question was about airspeed not lift?

    The lift indeed arises from the difference in airspeed but that is a different question.

    Incidentally I thought oxvillian provided quite a useful summary.
     
  16. Sep 1, 2012 #15

    Andrew Mason

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    Yes. But he is asking about the concept that air flows faster over the top than over the bottom (to create lift). It doesn't.

    The air does not flow faster over the top than over the bottom. The airplane moves through the air and carries a thin layer of air with it. That thin layer of air does not move relative to the wing. It is the turbulent flow created above that layer that causes lift. That air is not flowing faster over the wing than the air flow below the wing. That is the premise of the question, and it is incorrect. That was my point.

    AM
     
  17. Sep 1, 2012 #16
    Read post 7.... If conventional nomenclature had been used in the first place there may have been less confusion and your explanation would have received the credit it warranted?
     
  18. Sep 1, 2012 #17
    I don't know why we have this seemingly perennial problem of wing theory.

    The air over the top does indeed go faster than the air underneath.

    This has nothing whatsoever to do with the distance it travels or the pressure difference over and under the wing.

    Any half decent fluids textbook has experimental pictures to prove this and when I get time I will post some.

    Bernoulli's theorem is not violated but pressure is a poor variable to compare because there is a huge variation of pressure from front to back both above and below the wing. There is no simple representative figure for the pressure above and the pressure below.

    Here is a sketch to tie in Andrew's NASA explanation with my own. Yes indeed the boundary layer and vortex plays a vital part. I have not said otherwise.

    I have not drawn a position for the vortex in relation to the wing because it varies with shape, speed and angle of attack.
    However it does show how the vortex generates faster motion on one side than the other when placed in a fluid stream.

    This is the only simple part of the theory and it is qualitative. To achieve useful numerical results one has to do some quite complicated maths probably by numerical modelling.
     

    Attached Files:

    Last edited: Sep 1, 2012
  19. Sep 1, 2012 #18
    Perhaps you have this perennial 'problem' because students crop up perennially .
    Not all students have access to half decent Fluids textbooks and maybe they do not read all the previous posts from the experts.
     
  20. Sep 1, 2012 #19

    Andrew Mason

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    As I understand it, the vortices that enable lift are above the wing. That means that the air above the wing is moving at many different speeds relative to the wing and there is a layer next to the wing that is not moving at all. So I don't see how you can say that the air above the wind is flowing faster relative to the wing than the air below.

    AM
     
  21. Sep 2, 2012 #20
    Well I promised some visual proof, but rather than posting old photographs I did some googling.


    I noted some respected organisations who should know better still promoting the "It goes faster because it has further to travel over the top" myth so since you like NASA I am attaching some of their work.

    Link (1) The length of the streakline is proportional to the local velocity, but I could not find many animations using streaklines. This was the best I could do. You need to use the navigation buttons at the bottom of the page to step therough all the animations

    Link (2) Is a shorter and different presentation

    Link (3) in is an older video from the Federal Aviation Administration. (FAA)

    Link (4) Is a page from NASA teaching material. You should pay particular attention to fig 29 (which is in the document and also my ref 5) which clearly states and shows how the velocity is greater above the arifoil than below.

    Edit
    Neither I nor the FAA nor NASA can claim originality for this material, which was originally developed independently by Lanchester , Prandtl and Jukowski around 1900.
     

    Attached Files:

    Last edited: Sep 2, 2012
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