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Airplanes and Birds-Debunking the Myth of Why They Fly

  1. Oct 12, 2009 #1
    My daughter came home from school on Friday and told me that her science teacher mismarked her test. The question was: EXplain the reason thzt birds can fly. she told em that the reason they can flywas Isaac's Third Law. I believed, aqs we weretaught by our elementary school teachers that it waz the Bernoulli Law. to my surprise I discovered she was correct! Any comments?
  2. jcsd
  3. Oct 12, 2009 #2
    I invite you to consult http://en.wikipedia.org/wiki/Lift_(force [Broken])

    It seems to be quite complete and explains why the bernoulli's principle (due to air particles splitting at the leading edge and joining again at the trailing edge) is not the reason for lift as most people think it is.

    P.S. It's Newton's Third Law or Sir Isaac Newton's Third Law.
    Last edited by a moderator: May 4, 2017
  4. Oct 12, 2009 #3

    D H

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    Both Bernoulli's Principle and Newton's Third Law can be used to explain lift if used correctly. Just because Bernoulli's Principle can be invoked incorrectly (e.g., equal transit-time) does not mean that this explanation is invalid. It just means that the invalid uses of Bernoulli's Principle are invalid.

    Newton's Third Law does not (directly) explain why an inclined plane makes a lousy lifting body. Newton's Third Law is a bit of a hand-wave: What makes the airflow turn downwards?

    Bottom line: Both explain lift superficially. A real explanation of lift requires a deeper model of fluid dynamics.
  5. Oct 12, 2009 #4


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    What a ridiculously ambiguous question.

    The question can be answered in myriad ways that have nothing to do with aerodynamics.

    Here is a perfectly valid answer:

    Birds can fly because they evolved their bodily hair into feathers, simultaneously developing hollow bones to reduce their weight. etc. etc.
  6. Oct 12, 2009 #5


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    The question also does not specify if it is in reference to just gliding or the act of producing thrust by flapping.
  7. Oct 12, 2009 #6
    Everyone know that things fly because of happy feelings. Surely you must have seen Peter Pan. : )

    I think that the question was too open ended to expect any specific answer. I imagine that the teacher was looking for a specific answer using a principle they just learned. He/she probably didn't really think it all the way through when writing the test.
  8. Oct 14, 2009 #7
    Both are correct, but some might say only half correct. Bernoulli explains the origin of the force, and Newton explains the result of the force.
  9. Oct 15, 2009 #8
    Seems like there was more to the question than is being stated.
  10. Oct 15, 2009 #9
    Maybe this is useful.
  11. Oct 15, 2009 #10


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    Would any classical force be very useful without Newton's 3rd? I don't think so...
  12. Oct 16, 2009 #11

    Chi Meson

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    I was going to say...

    What is the reason a person can climb ladder? Who is going to say N's 3rd is the complete answer? Yet it ultimately must be part of the overall explanation.
    Last edited: Oct 16, 2009
  13. Oct 16, 2009 #12
    Last time I checked, we aerodynamicists don't use Bernoullis equation to calculate lift -ever.
  14. Oct 16, 2009 #13
    That's a pretty bad link with poorly explained information. I don't think it helps :(
  15. Oct 16, 2009 #14
    I heard the Bernoulli Effect was negligable and that it's the Coanda Effect that mostly contributes to flight. And that's what evokes Newton's third law.

    And by saying that's what I heard, it takes any responsibility off of me if I'm wrong.
  16. Oct 16, 2009 #15
    The Bernoulli Equation and the Coanda Effect mean something very specific. Lift is the closed loop integral of the pressure forces over the wing. That's all you need to say to explain lift.
  17. Oct 16, 2009 #16


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    Of course not - but the concept is incorporated into lift calculations and measurements.
  18. Oct 16, 2009 #17

    Chi Meson

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    Bernoulli accounts for up to 6% of lift. As I recall, it was this that made the Wright Brother's propeller efficient enough to make the Kitty Hawk work; they built the airfoil design into the propeller blade which enhanced the thrust by a slim, though crucial, margin.

    But as a Physics teacher, I get so annoyed by the elementary texts, and middle school texts that go on and on about Bernoulli and how it makes planes fly. It's almost as bad as the misconceptions about "centrifugal" force.
  19. Oct 16, 2009 #18

    Chi Meson

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    That works really well for fourth-graders, Cyrus, but how are you gonna explain it to high-schoolers?:tongue2:
  20. Oct 16, 2009 #19
    I think the traditional method involves sticking your hand out the window and changing the concave of your hand and the angle the wind hits it to make it rise and fall. : )

    I learned this way when I was younger. Just explain why this is happening as he/she tries it and eventually it will make since to him/her.
  21. Oct 16, 2009 #20
    Why, conformal mapping of course!
  22. Oct 16, 2009 #21
    If I were to explain lift to you I would say that one could think of the bernoulli equation in regard to the upper surface because the air is accelerated, and we would expect via bernoulli to see a lower pressure. We are not guarenteed this will happen because the assumptions of bernoullis equations are not strictly valid near a wing. But it gives us a qualitative idea of what is going on.

    Futhermore, the air follows the curvature of the wing surface because of the coanda effect. This is again a phenomenon. It does not explain lift. It simply says that provided the curvature of the surface is not too large, the air will follow it.

    We do not use bernoulli or coanda to calculate lift on a surface in any way-shape or form. Period.

    There are advanced methods that do this. CFD will use a Reynolds Averaged Numerical Solver (RANS) solution. Way, way beyond the scope of this explanation. One can also use conformal mapping and complex variables, again, way beyond the scope. One could also use lifting line theory. The point, it should be clear, is that aerodynamicists don't - ever- use "Bernoulli Lift" and "Reaction Lift" when doing analysis. I have no idea where these made up terms came from in the link Andre provided - which is why reading it made me shudder, and curse at my monitor.
    Last edited: Oct 16, 2009
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