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Lift on an airplane

  1. Apr 18, 2015 #1
    I'm confused about how an airplane generates lift. If I'm correct the wing of the plane is bent so air can flow over the top of the wing faster than the bottom of the wing, the faster fluids somehow apply less pressure resulting in a net upward force. Why though does a bent wing allow air to flow faster over the top, and why do fast moving fluids apply a lesser pressure than slow ones?
     
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  3. Apr 18, 2015 #2

    phinds

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    There have been approximately 6,420 threads on this forum discussing it, so I suggest a forum search.
     
  4. Apr 18, 2015 #3
    The simplest way to describe it is that wing's angle of attack forces air downward, so (Newton's third law) - reaction of the wing is to be forced up.
    This also causes 'drag', so the aerofoil is contoured to minimise the element of drag.
    The details of it can get get surprisingly complex, but that's the basic idea.
    You COULD have a complete flat wing, but the induced drag would make it very inefficient
     
  5. Apr 18, 2015 #4

    russ_watters

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    Both of those are largely related to the Venturi effect (just a one-sided venturi):
    http://en.wikipedia.org/wiki/Venturi_effect
     
  6. Apr 18, 2015 #5

    russ_watters

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    While that's basically true it can be enough of a simplification that people lose sight of the fact that the top surface is a bigger contributor to the lift than the bottom surface. And in this case, the OP was asking about what's going on with the top surface anyway.
     
  7. Apr 18, 2015 #6

    boneh3ad

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    This is in no way similar to a so-called "one-sided" Venturi effect. The Venturi effect describes a system with finite boundaries. The flow over a wing has effectively infinite boundaries. The two situations are not the same.
     
    Last edited: May 11, 2015
  8. Apr 18, 2015 #7

    boneh3ad

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    This is at best misleading and at worst untrue. You can't decouple the top and bottom surfaces. Without one, the flow over the other does not develop the same way. Also, the pressure on a wing's surface is meaningless without the pressure on the other side of the wing since the net force only comes from factoring in both. If you only consider the top surface, you will get negative lift, so there's no way the upper surface can be the "bigger contributor".
     
  9. Apr 18, 2015 #8

    russ_watters

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    The air provides its own boundary in low-speed flow when it is essentially incompressible. The velocity increase/pressure drop happens because it is being "squeezed", just like in a venturi. This is similar to the principle behind an aerospike engine, where air pressure helps constrain the flow:
    http://en.wikipedia.org/wiki/Aerospike_engine
    I'm aware they can't be completely decoupled. Nevertheless, it is typical for the top surface to produce more of the lift and the OP was asking about the top surface, whereas the other answer implied it was about the bottom. The reality is that for most people what is going on with the bottom surface is intuitively obvious but what is going on with the top surface is not.
    That isn't true. Both the top and bottom surface have pressure profiles that are measured/expressed as gauge pressure because the default is atmospheric pressure. As a result, the pressure on the top surface is measured to be negative. For a simplified/idealized example, a flat-bottom airfoil with the bottom parallel to the airflow would essentially just have atmospheric pressure below it and all of the lift generated by the top surface. Here's a sample graph of a pressure profile (not a flat bottom but still showing more of the lift derived from the top surface):

    http://www.wfis.uni.lodz.pl/edu/Proposal/image093.gif [Broken]
     
    Last edited by a moderator: May 7, 2017
  10. Apr 18, 2015 #9

    boneh3ad

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    The fact that it is incompressible makes it less likely to "provide its own boundary," which does not accurately reflect what actually occurs. The velocity increase happens because he design of the trailing edge sets a certain separation point and the conservation laws then require the velocity to increase greatly over the upper surface to maintain physical equilibrium. For an explanation of why the Venturi approach is wrong, see this link:
    https://www.grc.nasa.gov/www/k-12/airplane/wrong3.html

    Also, that is not how an aerospike engine works. Aerospikes are, first and foremost, compressible flow devices and the Venturi effect is irrelevant to the phenomenon. The constraint by the outer atmosphere here (and importantly, variable constraint) is due to some of the unique features of compressible flows.

    So which is it? You can believe both things.

    The shape of the top certainly contributes to lift. You cannot measure lift an just the top surface, though. The top definitely plays a role, but it does so as a part of the whole shape.

    First, I know what gauge pressure is. In fact, I typically would give zero points if a student uses gauge pressure where absolute pressure is required because it's that important to keep straight.

    Second, plotted there is the pressure coefficient, the definition of which includes a numerator that is equivalent to gauge pressure, so in that, you are correct. That is where it ends. Just because the pressure coefficient is negative does not mean there is somehow negative pressure on the upper surface. The force on a given surface due to pressure is always dependent on the absolute pressure, not gauge pressure, and absolute pressure is always positive. The pressure on the upper surface is always downward. A more negative pressure coefficient just means there is less downward force on the top to counteract the larger pressure on the bottom.

    Third, a flat-bottomed airfoil would not necessarily have atmospheric pressure on its underside. This would generally only be true if the leading and train edges were sharp in a way such that the bottom approximated a flat plate, otherwise the curvature of the leading edge would accelerate the flow.

    Even if the bottom is atmospheric, the total lift force is still the integrated sum of the absolute pressure on the bottom minus that on the top. The force due to the pressure on the top is still downward. The importance of the upper surface contour is therefore effectively to make the force on the upper surface less downward for a given upward force on the bottom. This is what I mean by being unable to decouple the two.
     
    Last edited by a moderator: May 7, 2017
  11. Apr 19, 2015 #10

    A.T.

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    If you push a body with 20N from one side and 15N from the opposite side, which side "is a bigger contributor" to the net force on the body?
     
    Last edited: Apr 19, 2015
  12. Apr 19, 2015 #11

    A.T.

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    That is just a convention. You can express them relative to any reference pressure you want. But the forces on the two surfaces are unambiguous and a function of absolute pressure.
     
  13. May 4, 2015 #12

    A.T.

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    Also visualized here:

     
  14. May 5, 2015 #13

    rcgldr

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    From the video at about 1:10 into the video, pressure is reduced on the top "partially because it's shielded by the wing" as the wing travels forwards through the air. So the air fills in what would otherwise be a void by following the upper surface of a wing, or in the case of a stall, with turbulent flow consisting of vortices or mostly one very large vortice.

    A wing doesn't have to be curved. Flat wings and/or symmetrical wings are reasonably efficient on small model gliders, from the dime store type models to the larger ones like this one with nearly symmetrical airfoil.

    http://www.4p8.com/eric.brasseur/glider2.html [Broken]

    Aerobatic aircraft, both full scale and models, use symmetrical airfoils.
     
    Last edited by a moderator: May 7, 2017
  15. May 5, 2015 #14

    A.T.

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    True, and I don't think anyone implied otherwise.
     
  16. May 5, 2015 #15

    rcgldr

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    From the original post, "bent wings", which I thought he meant curved (as opposed to dihedral).
     
  17. May 6, 2015 #16
    You're all wrong. Lift is due to the bound vortex, in superposition with the airstream, induced be viscous drag. :P

    Am I making this up? Curved foils, or a planar foil at a positive angle of attack will not induce lift in an inviscid fluid. Invent any shape you wish. Immerse it in an inviscid stream. It will not produce a component of force perpendicular to the stream.
     
    Last edited: May 6, 2015
  18. May 6, 2015 #17

    A.T.

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  19. May 6, 2015 #18
    The lift can be explained either by using bernoulli's principle (which principle explains why in a fluid like air, in regions of higher velocity the pressure is lower) or using Newton's laws.

    According to wikipedia the explanations are equivalent https://en.wikipedia.org/wiki/Bernoulli's_principle#Misunderstandings_about_the_generation_of_lift.

    Bernoulli's principle can be derived from Newton's 2nd law. https://en.wikipedia.org/wiki/Bernoulli's_principle#Derivations_of_Bernoulli_equation

    However i prefer bernoulli's principle explanation because it is not so crystal clear how Newtons law aplly in fluid's particles that flow, we are used to apply Newton's laws in point particles or rigid bodies.

    As to why the velocity becomes higher in the upper surface of the wing it is because the air has to travel bigger distance in the same time.
     
  20. May 6, 2015 #19

    A.T.

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    Why?
     
  21. May 6, 2015 #20
    It is basically because of the equation of continuity and because we assume incompressible flow.
    In speed's well below the speed of sound the flow of air is incompressible.
     
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