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Why does the air above an aerofoil travel faster

  1. Mar 31, 2010 #1
    how does the air above an aerofoil(wing of an aircraft) move faster than the bottom surface just by increasing the angle up of the aerofoil.

    even if the upper surface and lower surface distance are same i.e not campered or symmetrical.
     
    Last edited: Mar 31, 2010
  2. jcsd
  3. Mar 31, 2010 #2

    russ_watters

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    This may get misinterpreted, but I'll just throw it out there: because the shape of the wing causes the air above the wing to move further than if the wing wasn't there due to the addition of vertical components to the direction of motion.
     
  4. Mar 31, 2010 #3
    I wasn't aware that wings did work in this case! I seem to remember my 8 grade science teacher lying to me--saying the bottom is flat, the top rounded... hmm
     
  5. Mar 31, 2010 #4
    source: "http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html" [Broken]
    "http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html" [Broken]

    but didn't find why?
     
    Last edited by a moderator: May 4, 2017
  6. Mar 31, 2010 #5

    Filip Larsen

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    Very simplified I would say, that any object in the air flow will "reduce" the area available to the air flow compared to the corresponding free up-stream area (picture the two areas as either end of a tubular control surface with no air flow across the sides and with one area up-stream and the other crossing the wing span-wise). For steady incompressible air flow the mass flow in and out of the two areas must be equal and since the area covering the wing is less than the area up-stream, the speed of the air must be larger that the free up-stream speed.

    So, you will find increased air speed around any object placed in an air flow, not just for wings. If you want to use the difference in speed of a laminar flow over an airfoil to explain lift (as opposed to explain it due to pressure difference), you could argue that since the speed over the upper surface is faster than air passing the lower surface, the combined airflow down-stream the airfoil must have been shifted downwards a bit since the "lower part" of the flow has been braked a bit compared to the upper part and we still got to have conservation of mass flow, and this downward change of momentum of the air mass is "balanced" by the lift force.
     
    Last edited: Mar 31, 2010
  7. Mar 31, 2010 #6

    DrDu

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    That's one of the most difficult questions in aerodynamics. Consider the aerofoil to be infinitely long, so that the problem becomes effectively two-dimensional. Also, let's first neglect friction. Then the symmetric air flow is only one of a family of solutions to the Euler equations. The other equations differ by the addition of circulation around the wing. Obviously, the addition of circulation will make the solution unsymmetrical with a higher speed on top than on the bottom (or the other way round). For all but one solution, the air will have to change speed abruptly around the rear edge of the wing which means infinite velocity and acceleration. When we take friction into account, this will lead to the appearance of turbulent eddies which will detach taking angular momentum with them until we are left with the stable solution for which the air flows from the upper and lower part of the airfoil join smoothly. The latter condition is called Kutta-Zhoukovski condition. That we only have to take into account this condition at the rear edge of the wing is due to the fact that the pressure decreases at the rear edge while it increases at the front edge. The increase of pressure stabilizes the linear flow.
     
  8. Mar 31, 2010 #7

    rcgldr

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    Faster relative to what frame of reference? If you only consider the "horizontal" component of flow relative to a wing, the flow is faster above, slower below, but relative to the air, the air is moved slightly backwards above the wing and a bit more forwards below the wing, so the "horizontal" component of air is faster below relative to the air.

    Relative to the air, most of the flow is downwards (corresponding to lift) and somewhat forwards (corresponding to drag).

    Wiki has a good short version of an explanation:

    a low pressure region is generated on the upper surface of the wing which draws the air above the wing downwards towards what would otherwise be a void after the wing had passed. On the underside of the wing a high pressure region forms accelerating the air there downwards out of the path of the oncoming wing. The pressure difference between these two regions produces an upwards force on the wing, called lift.

    http://en.wikipedia.org/wiki/Wing#Science_of_wings

    It turns out that somewhat ahead of the wing, some of the air is diverted upwards, because it's being affected by the low pressure area above and high pressure area below a wing before the wing actually arives. Air can also somewhat flow around high pressure stagnation zones as the wing moves through a volume of air, but has to fill in what would otherwise be a void as the wing's surfaces recede from the air, and the magnitude of the pressure reduction above a wing is usually greater the magnitude of pressure increase below.

    One of the better web pages about wings:

    http://www.avweb.com/news/airman/183261-1.html
     
  9. Apr 1, 2010 #8
    can it be simply said
    bcoz when the airfoil angle is increased up, the lower surface is exposed to flow of air and hit the surface directly and slows down but the flow of air above doesn't hit the upper surface. So flow smoothly along the surface and makes them travel faster than air below the wing.
     
  10. Apr 1, 2010 #9

    russ_watters

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    There is a piece I missed here: Even if the airfoil is geometrically symmetrical when viewed from a frame of reference parallel to the chord line, it is not geometrically symmetrical anymore when you give it an angle of attack. The stagnation point moves down when you give it a positive angle of attack, making a new effective chord line.
    No, it can't be said that way. That requires friction and would be an undesirable way to generate lift. And in fact, it is easier to make air flow smoothly along the bottom surface than the top because the higher pressure keeps the flow attached.
     
  11. Apr 2, 2010 #10
    sorry, but couldn't find anywhere why the air above the wing move faster in the first place

    there is a nice animation in "http://en.wikipedia.org/wiki/Lift_%28force%29#cite_ref-44"

    i could understand the downwash and the lift but not what makes the air above move faster
     
  12. Apr 2, 2010 #11

    Cleonis

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    An aircraft wing creates lift by deflecting air mass downwards.
    That is, an aircraft wing creates lift by accelerating a large amount of air mass downwards.

    Aircraft wings have their distinctive cross section (aerofoil shape) for efficiency. The aerofoil shape is not a necessity for lift. It's better, it's more efficient, but not a necessity.

    The crudest "wing" is a flat panel, at an angle to the direction of travel.
    Example: while it's unwise to stick your hand out the window of a car at speed, if you do so you will feel the lift if you make your hand flat and at an angle to the air rushing by.

    (Aircrafts that are designed for aerial acrobatics have pretty much flat panels for wings: that is why they can fly inverted almost just as well as the right side up. The purpose of the aerofoil shape is to induce a flow pattern where the air does not go into turbulence. The more turbulence, the less efficient.)

    Returning to the original question:
    Air mass is accelerated downward. That is, the trajectory of the air mass relative to the wing is curvilinear. The air mass under the wing can be thought of as moving along the inside of the curve, the air mass over the wing moving along the outside of the curve. What the aerofoil shape facilitates is that the airflow doesn't detach from the upper surface of the wing. Presumably the airflow doesn't detach because a sudden change of air flow direction is avoided.

    The air mass that moves over the wing travels faster than the air mass moving under the wing. It follows that some force accelerates the air mass that flows over the wing. Presumably a pressure difference is providing the force for that acceleration.


    This is a subject where different people can disagree about what they identify as cause and what as effect. What is the causal chain?
    I have seen people argue that the lift follows from the fact that the air mass over the wings moves faster.

    The causal chain that I see is:
    - Air mass is deflected downward
    - This downward deflection lowers air pressure above the wing, near the trailing edge.
    - The lower air pressure in the area above the wing, near the trailing edge, provides the force that accelerates the over-the-wing air flow.
     
    Last edited: Apr 2, 2010
  13. Apr 2, 2010 #12
    Not bad, at all Cleonis. The simple answer is that the air is accelerated into the lower pressure region over the top of the wing. So the question defers to "why is the pressure over the top of the wing lower?" As it turns out, the pressure is lowest about 5-15% back from the leading edge, and the average force normal to the airflow is about 1/3 back from the leading edge. Top picture.

    [PLAIN]http://www.cfse.ch/img/site/topic/Wings-Explanations.gif [Broken]
     
    Last edited by a moderator: May 4, 2017
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