Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What causes low pressure on a wing?

  1. Feb 10, 2019 at 5:41 AM #1
    Even today there are lot of misconception about how and WHY static pressure reduce above wing.

    1)Faster air velocity cause low static pressure

    One involves holding a piece of paper horizontally so that it droops downward and then blowing over the top of it. As the demonstrator blows over the paper, the paper rises. It is then asserted that this is because "faster moving air has lower pressure"

    One problem with this explanation can be seen by blowing along the bottom of the paper: were the deflection due simply to faster moving air one would expect the paper to deflect downward, but the paper deflects upward regardless of whether the faster moving air is on the top or the bottom.Another problem is that when the air leaves the demonstrator's mouth it has the same pressure as the surrounding air; the air does not have lower pressure just because it is moving; in the demonstration, the static pressure of the air leaving the demonstrator's mouth is equal to the pressure of the surrounding air.A third problem is that it is false to make a connection between the flow on the two sides of the paper using Bernoulli’s equation since the air above and below are different flow fields and Bernoulli's principle only applies within a flow field.

    2) Curved airflow above wing cause low static pressure

    Why curved airflow cause low in pressure?
    Because of centrifugal force make air molecules far appart so pressure drop or?

    3) Why air follow curved wing surface insted goes in straight line?
    Is reason for that visocsity which "stick" air to the surface or high ambinet pressure above wing push low pressure in to upper wing surface?
    Last edited: Feb 10, 2019 at 7:03 AM
  2. jcsd
  3. Feb 10, 2019 at 11:11 AM #2

    This is a link to a video which explains the reason due to which the fluid follows the curvature of the surface.
    In case of wings, the fluid follows the curvature and the inclination of the wings.So we can apply newtons second law(in the vertical direction) to the stream of fluid to find the vertical force acting on it and third law can be used to find the reaction acting on the wings.This reaction is the lift force.
    I think there is a different explanation using Bernoulli's theorem.
  4. Feb 10, 2019 at 12:54 PM #3
    This test with cup of tea is not coanda effect it is more about surface tension.... (McLean in "Understanding Aerodynamics" states that the water deflection "actually demonstrates molecular attraction and surface tension.")

    here is 50min talking only on lift misconception,even scientis can not arggue what cause lift.
    this men write book only on lift misconception

    one more just about what cause lift
    Last edited: Feb 10, 2019 at 2:57 PM
  5. Feb 10, 2019 at 6:46 PM #4
    I dont know about the cup experiment.But I learnt in a forum that the sufficient condition for lift force in a wing is that the air stream should be redirected downwards(by any mechanism).Even the aerofoil cross section is not necessarily needed.The aerofoil cross section just increases the efficiency.Just an inclined plane can produce lift.(here we are not considering the case where beyond a certain inclination stalling occurs)
    This is true even in case of turbines (can be analysed by using the velocity triangles).
    Last edited: Feb 10, 2019 at 8:04 PM
  6. Feb 10, 2019 at 8:04 PM #5


    User Avatar
    Homework Helper

    The simplest explanation I've read is that a wing, even a flat plane at an effective angle of attack, sweeps out a volume of air as it travels through the air, leaving what would otherwise be a void behind the peak upper surface of the wing if the air didn't accelerate into the the volume swept out of what would otherwise be a void. If the flow doesn't detach, then it will tend to accelerate mostly downwards (lift) and somewhat forwards (drag). If the flow detaches, then what would otherwise be a void is filled with vortices, greatly reducing lift and increasing drag.
  7. Feb 10, 2019 at 10:07 PM #6


    User Avatar
    Gold Member

    Consider lifting body such as the XM-1 or the space shuttle fuselage ignoring wing surfaces. α angle of attack of the moving body generates lift as it travels through the fluid. In fixed wing aircraft increasing alpha also generates lift from wing surfaces while also inducing drag.

    Rotating wing aircraft such as helicopters generate lift in similar fashion by varying rotor angle relative to the plane of rotation. The rotating "disk" operates as a "wing" at sufficient angular velocity relative to fluid flow.

    Perhaps @boneh3ad can comment on fluid pressures and relevant equations that answer the original post.

    Example: hold your hand flat in the wind generated by a moving car. At some modest forward speed rotate your hand at an angle (α) to the flow and feel your hand lift normal to the vehicle motion. I attached a picture of the M-1 lifter from the air museum mounted at high α; and video of an XM-2-F1 flight and an M-2 drop at Edwards.

    Last edited: Feb 11, 2019 at 7:24 PM
  8. Feb 11, 2019 at 1:25 AM #7

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2018 Award

  9. Feb 11, 2019 at 2:00 AM #8


    User Avatar
    Science Advisor
    Gold Member

  10. Feb 11, 2019 at 7:51 AM #9


    User Avatar
    Science Advisor

  11. Feb 11, 2019 at 5:14 PM #10
    We turned away from topic,main question is way curved stremline cause low pressure?
  12. Feb 11, 2019 at 6:00 PM #11


    User Avatar
    Gold Member

    Did not intend my post to cause thread to diverge. This excellent article addresses your post:
    While this article addresses streamlines and provides much useful analysis: https://www.physicsforums.com/insights/demystifying-the-often-misunderstood-bernoullis-equation/

    Those of us interested in aerodynamics may have been trying to provide actual (aircraft) examples in place of teacups and paper :cool:.
    Last edited: Feb 11, 2019 at 6:32 PM
  13. Feb 12, 2019 at 9:49 AM #12

    https://user.uni-frankfurt.de/~weltner/Misinterpretations of Bernoullis Law 2011 internet.pdf
  14. Feb 12, 2019 at 11:39 AM #13


    User Avatar
    Science Advisor
    Gold Member

    I don't know what point you are trying to make with this link, but it isn't strictly correct. There is really no way to make a claim that curved streamlines cause a pressure gradient rather than the other way around. You could just as easily argue that the pressure gradient acts as the centripetal force that bends the streamlines and you would still be correct. It's a chicken and egg situation. In reality, they are coupled phenomena and it would be essentially impossible to say that one causes the other.

    Additionally, the Coandă effect applies to the behavior of a jet when it interacts with an object. It doesn't strictly apply to an infinite free stream flowing over a surface. That having been said, Coandă generally isn't taught in typical fluid mechanics courses because it's really just a subset of a larger phenomenon, which is that of the boundary layer that "sticks" to a surface as a fluid flows around it. In other words, the behavior is qualitatively similar in a free stream to that in a jet. The streamline curvature that results from viscosity can certainly still be used to describe the flow around a wing, but it would be improper to call this the Coandă effect, in my opinion. There are limits to this, however. Specifically, a sufficiently adverse pressure gradient can cause the boundary layer to separate and a large recirculating bubble to form at the surface, redirecting the free stream away from the surface.

    Finally, the idea that energy conservation is not appropriate for the derivation of Bernoulli's equation is pure nonsense. Bernoulli's equation is equally a statement about conservation of energy as it is a statement about conservation of momentum (i.e. force balances). Neither interpretation or derivation contradicts the other. I take issue with the idea the article suggests that this is a source of confusion in the use of the Bernoulli equation. A well-taught unit on Bernoulli's equation and its origins should include all of the caveats on its use, which can be deduced directly from conservation of energy or momentum.

    I should also mention that at no point in either of the two articles I wrote for this site did I claim that Bernoulli's equation could explain why low pressure forms on the upper surface and high pressure on the lower surface of a wing. In fact, I specifically mentioned that Bernoulli is not an explanation for how these flow fields form, and is only a tool that can be applied to a flow field that is already known to find lift.

    Really, the biggest problem with how Bernoulli's equation is typically taught is not the way it is derived, but instead the fact that many teachers/instructors themselves do not know how it works or its limitations. In fact, I've never seen a university course make any of the various erroneous claims. That seems to be a problem that lives primarily in the realm of primary school science teachers and YouTube.
  15. Feb 12, 2019 at 3:06 PM #14
    Here is nice old school video where test curved stremline pressure change and also talk where bernoulli can not be applied.

  16. Feb 12, 2019 at 6:34 PM #15


    User Avatar
    Science Advisor
    Gold Member

    Nothing in that video contradicts anything I've said.
  17. Feb 13, 2019 at 9:37 AM #16
    I didnot say that..

    What is your explanation what cause low pressure on top of wing, faster velocity ,curved streamline or maybe faster velocity is consequnece of low pressure ?
  18. Feb 13, 2019 at 11:12 AM #17


    User Avatar
    Science Advisor
    Gold Member

    All of the above. There is no way to meaningfully decouple those phenomena. The fundamental truths here are that conservation of mass and momentum must hold. Conservation of momentum is a direct result of Newton's second law, and implies that a change in velocity is effected by some corresponding force. In fluid mechanics, that force is usually provided by the pressure gradient. So, the general, not-very-helpful answer is that the flow velocity and pressure gradients arise together in a way that satisfy both of the above conservation laws. (Note: for compressible flows, conservation of energy is also important as it cannot be decoupled from the other two conservation laws as it can in an incompressible flow.)

    Generally, the distinction of which effect caused the other is not important. One is necessary and sufficient for the other. The only example I can think of that might have a clear-cut "answer" is that of a flow through a pipe with a constriction. Conservation of mass implies that the velocity must increase through the area reduction no matter what, and then you could separately apply conservation of momentum to argue that the pressure must be decreasing in that region as well to provide a force for that acceleration. Still, this is not a very useful or insightful exercise.
  19. Feb 13, 2019 at 11:58 AM #18

    Air blowing across the bottom of the wing pushes the wing up and itself down. Straightforward.

    Air blowing across the top of the wing keeps going for awhile, even after the shape of the wing pulls away underneath it, thus pulling the wing up, and itself down. Call it "centrifugal" for your own visualization purposes, if you like.
  20. Feb 13, 2019 at 1:55 PM #19
    Do you agree with me that air jet from hair dryer,blower ,blowing with mouth have identical pressure as ambient pressure ?
    So the air does not have lower pressure just because it is moving.

    If air velocity will lower pressure with speed than altimeter in plane will allways show increase in altitude when plane speed increase even plane do not climb.Because static ports on fuselage allways "feel" airflow(800km/h) which aircraft flys...

    Classic example of missaplied bernouli theorem..
  21. Feb 13, 2019 at 8:45 PM #20


    User Avatar
    Science Advisor
    Gold Member

    I do agree with that. A jet of that source will be at constant pressure throughout and that pressure will be atmospheric.

    Altimeters, however, are not an example of misapplied Bernoulli. They apply Bernoulli just fine and have for years.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?