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Simple explanation - wings in an aeroplane

  1. Sep 20, 2010 #1
    on an aeroplane, why does air fly FASTER over the top than the bottom.

    I get everything else; i.e: higher pressure moves to lover pressure.

    What I don't get is how the air flies faster over the curved part of the wing.

    Please explain simply

    thanks
     
  2. jcsd
  3. Sep 20, 2010 #2

    mathman

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    The air on the upper surface has a longer path than the air on the lower surface, so it has to go faster to cover the same distance from front to back.
     
  4. Sep 20, 2010 #3

    cjl

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    A common (and wrong) explanation.

    This explanation is based on the "equal transit time" assumption, which states that air traveling over the top of the wing and the bottom both take the same amount of time to reach the back of the wing. In reality, this isn't the case (in fact, the air traveling over the top tends to arrive before the air traveling past the bottom). The actual reason is because the wing shape causes a net circulation of air around the airfoil. This circulation effectively causes the air along the upper surface to accelerate and the air along the bottom side to slow down.

    Of course, this doesn't really answer the question, since all it does is change the question from "what causes the air to move faster" to "what causes the circulation". The answer to this is related to the shape of the airfoil itself. Any object traveling through a viscous fluid with a sharp trailing edge will tend to have smooth flow past that trailing edge (with a streamline in effect contacting the object at that point). An airfoil without any net circulation would not have smooth flow past the sharp trailing edge - instead, the flow would separate somewhere on the top surface, and flow from the bottom of the airfoil would need to go around the trailing edge. Viscous effects tend to cause this to not happen (since this would require the flow to have extremely high velocity gradients around the trailing edge), in effect creating a circulation around the airfoil such that there is smooth flow past the trailing edge.

    (For more information, look up the "Kutta Condition" - this page has some good diagrams and is a good starting point)
     
  5. Sep 20, 2010 #4

    russ_watters

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    Mathman did not say they reach the back at the same time, just pointed out correctly that the path is longer.
     
  6. Sep 20, 2010 #5

    Danger

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    And, to further mess up a popular misconception, the wing deflecting air downward contributes to lift far more than does the "Bernoulli effect".
     
  7. Sep 20, 2010 #6

    cjl

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    Yes, but that statement in and of itself tells you nothing. It only implies faster air velocity if you make an assumption about the transit time (which was clearly implied in that post).
     
  8. Sep 20, 2010 #7

    rcgldr

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    It turns out that it's more efficient if wings reduce pressure above the wing more than they increase pressure below, and the greater differential in pressure versus ambient (surrounding air) above a wing results in greater speed.

    Many diagrams for flow near a wing show mostly the horizontal component relative to a wing. If the frame of reference was to the surrounding air, there is an aft component of flow above a wing and a forwards component of flow below a wing. Since all wings produce drag, there's a net forwards flow induced by a wing (the drag force corresponds to a net forwards acceleration of air) so the forwards component of flow below a wing is greater than the aft component above.

    The total flow relative to the air is mostly downwards (corresponding to lift) and somewhat forwards (corresponding to drag).

    Not on all wings, the air foil on this lifting body has the longer path on the bottom:

    m2f2.jpg
     
  9. Sep 20, 2010 #8

    russ_watters

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    No, it really does: the longer path makes the air move faster over the top precisely because it is longer. The air has a certain horizontal component of velocity to which is added a vertical component of velocity by the curvature (camber) of the airfoil. Hence: higher speed.

    At the same time, the air over the top reaches the back later due to the higher drag associated with that higher speed.
     
  10. Sep 20, 2010 #9

    russ_watters

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    With a positive angle of attack, I highly doubt that that is true. It isn't true of an inverted cambered airfoil, for example. The high angle of attack moves the stagnation point downward on the front of the airfoil, causing the air over the top to have a longer path.

    [edit] Note, the profile is somewhat deceptive: it looks perfectly flat on top but in fact starting 2/3 of the way back, by the tail, the top surface angles down to meet the bottom surface. So the top surface is quite a bit more curved than it looks. The craft is also much wider at the back than at the front, so the bulbus front in profile is actually a lot less of the full airfoil than it looks to be. http://www.google.com/url?sa=t&source=web&cd=13&ved=0CBkQFjACOAo&url=http%3A%2F%2F65.165.5.234%2Fcenters%2Fdryden%2Fpdf%2F87738main_H-479.pdf&ei=FguYTNKzMcX_lgfH-_wo&usg=AFQjCNHAa2PHYMddzRzp3DivzQB8yzMB_g [Broken]
     
    Last edited by a moderator: May 4, 2017
  11. Sep 20, 2010 #10

    cjl

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    As I said above, the air going over the top of the airfoil actually reaches the back before the air going over the bottom on most airfoils, not later as you said. This is due to the circulation imposed by the kutta condition. In general, the skin friction drag is negligible compared to inertial effects on the flow (basically, the reynolds number is extremely high), so the flow acceleration due to circulation is much more significant than the increased skin drag on the upper surface caused by the higher local flow speed.
     
  12. Sep 20, 2010 #11

    russ_watters

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    I stand corrected - I forgot about the effect of the circulation.

    Though I was wrong about the particulars, the effect of the longer path - or perhaps, larger obstruction - above the wing is still a proper explanation:
    http://en.wikipedia.org/wiki/Lift_(force)#.22Popular.22_explanation_based_on_equal_transit-time

    In essence, it isn't so much the path the partcles on the upper surface itself (as I said) but rather that the bigger upper surface tries to squish together the air above the wing, making it have to accelerate to avoid being compressed.
     
    Last edited: Sep 20, 2010
  13. Sep 20, 2010 #12

    cjl

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    I just discovered a really neat video that shows the flow around a wing:

     
    Last edited by a moderator: Sep 25, 2014
  14. Sep 20, 2010 #13

    rcgldr

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    At sufficient speed, such as seen in the photo, the visual angle of attack of the leading upper surface is zero. At higher speeds, the m2f2 glided a bit nose down. The pdf file mentions a negative angle of attack used at higher speeds, but doesn't mention how the "chord line" for the m2f2 was defined which would determine the described angle of attack.

    The aft part of the m2f2 is tapered, and both the top and bottom surfaces angle towards each other, but they don't actually meet, the aft surface is vertical and a bit over 1 foot tall. This increases the drag, but being a re-entry vehicle, a high lift to drag ratio isn't one of the goals.

    Although drag would be high, an airfoil could be a simple wedge with the narrow end at the front and the wide end at the rear. Given sufficient air speed, this wedge could fly with the top surface horizontal or pitched downwards a bit. This would be an example where the longer path was below a wing.

    However this is going somewhat astray from the OP, which was asking about wings on aircraft, and the examples I mentioned are unusual air foils, and not the efficient air foils used on aircraft.

    Regarding the longer path, the air above a typical wing travels backwards, while the air below travels forwards, as demonstrated in the video above (which is a symmetrical airfoil at various angles of attack). So the distance per unit time of air flow above and below a wing is significantly greater than the distance of the surfaces above and below a wing, even when taking into account that the flow separates below the leading edge of a wing.

    There's a signifcant component of reduction of pressure near the peak of the cambered upper surface of a wing due to acceleration perpendicular to the direction of flow near the surface of a wing. This causes the air further above to accelerate downwards towards that low pressure zone, with a dynamic situation where the air flows downwards as the wing moves forwards, with a receding upper surface, with the flow eventually going downwards past the aft end of a wing. That low pressure zone also causes air in front of the wing to accelerate backwards, and aft of the lowest pressure zone, the air is decelerating (accelerating forwards).
     
    Last edited: Sep 21, 2010
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