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Lift on an airfoil

  1. May 27, 2006 #1
    Hi guys,

    I've heard that this topic has been discussed to death before but hopefully not here yet I apologise if it has been.

    I'm currently in high school we're just into bernoulli's equation and one of the applications is the airfoil. Our textbooks states that the fundamental source of lift is the difference in velocity of airflow over and under the wing hence the difference in pressures which will in return create lift.

    However it also states that as the airfoil flies through the air, for the velocity of air over the airfoil to be faster than the underside, the air that is separated at the leading edge must converge at the trailing edge. How true is this? If it is, can anyone offer an explanation beyond just the shape of the airfoil on why this will happen? I've done a bit of research on the internet and some sites states that this is not true, that wind tunnel tests have shown otherwise and that Newtonian physics is involved - the wing diverts air downwards and gains momentum.

    How is it possible for a wing to divert so much air downwards (it's not even close to vertically downwards I think?) to support the weight of say a B747.

    Which is the main principle of lift?
     
  2. jcsd
  3. May 27, 2006 #2

    Hurkyl

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    If you zoom in, I think this results in the following effect:

    The higher pressure on the bottom results in many air particles bouncing (downwards) off the bottom of the wing.
    The lower pressure on the top results in fewer air particles bouncing (upwards) off the bottom of the wing.

    So, this effect does result, overall, in downward-deflected air particles, which is what must happen for there to be lift.


    Why wouldn't it be possible?
     
  4. May 27, 2006 #3

    FredGarvin

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    The only issue I have with that statement is the use of the word "fundamental." The pressure differential experienced is definitely a part of the lift situation, but it is arguably not the main contributor in many situations. The standard example used to dispel this is the symmetrical airfoil with 0 angle of attack.

    This is known as equal transit time and it is a load of bunk. There is nothing in the boundary conditions that mandate that two molecules or control volumes meet at the trailing edge after one goes over the top and one underneath. The momentum idea is the direction you need to look as well as a situation in fluid mechanics called "circulation."

    I think this question stems from a lack of experience in knowing just how much air is moving across a wing during flight. You would be blown away (pardon the pun) to experience 1/10 of that kind of flow in person. When your personal feel for something seems impossible, you must always go back and look at the numbers. If your modelling of a situation is close, the numbers won't lie to you.

    There is a great video link by a professor at Cambridge (I think). He discusses quite nicely what we are really looking at. I don't think it is over your head at all. It is linked here somewhere. I'll see if I can find it.
     
  5. May 27, 2006 #4

    FredGarvin

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  6. May 27, 2006 #5

    LURCH

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    I went into this thing with a completely open mind, and I have to say that I am more convinced now than ever that the "old fashioned" explanation of how a wing generates lift is indeed the correct one.

    Certainly, the idea that the lift generated is a reaction force from deflecting air downward is not at all supported by the lecture in the Cambridge link. The photos or airflow clearly show air returning to its original path after the encounter with the airfoil. The person giving the lecture never says anything about a reaction force, or any net force downward on the air.

    Instead, he reasserts that lift is caused by a pressure differential between the upper and lower surfaces of the wing. He starts by saying that a curved airflow allways means that there is a pressure gradiant. By the end of the lecture he switches to saying that the curvature of the airflow causes the difference in pressure. At no transition point does he explain this apperent reversal in relation between cause and effect.

    Airflow over the top of a wing (I'm speeking of the standard, assymetrical wing used on most aircraft) is faster than that along the underside. This difference in wndspeed has been measured time and again. It is this more rapid airflow that creates the low pressure wich results in the curvature of the streamlines. And this same low pressure is what generates lift.

    Check out this link;

    http://www.av8n.com/how/htm/airfoils.html

    I think it contains a better explanation.
     
  7. May 29, 2006 #6
    Thanks guys very enlightening as well as informative.
     
  8. May 29, 2006 #7
    Hi LURCH and everyone..

    I've seen the video and I went into it with a open mind as well and I see where you're coming from. I've also just come across this piece of article which I thought was really logical as well, which addresses that bernoulli's principle is not the main explantion for lift.

    http://homepage.mac.com/richtherrn/physics/flight.htm

    Any thoughts on this? You might want to start reading from 1/4 down the page where It starts with "The Popular explanation of Lift". I think anything above that's not necessary for us here..

    Cheers
     
    Last edited: May 29, 2006
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