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Direction of Pressure

  1. May 12, 2010 #1
    I know that pressure is a scalar quantity. But I am confused in cases when there are two areas of different pressures. I was trying to understand LIFT and was wondering why does the high pressure under the wings push up and not to every other direction like the pressure trying to crush a plastic vaccum bottle?

    Thanks is advance for replying
  2. jcsd
  3. May 12, 2010 #2
    Pressure pushes against any surface that you offer to it. So because the surface points down the force from the pressure points up.
  4. May 12, 2010 #3
    It is the DIFFERENCE in pressure (between upper and lower wing surface) that causes lift. There is lower pressure above the wing due to the curvature of the surface causing the air to move faster over the upper surface...Faster velocity of a fluid over a surface reduces the pressure...Bernoulli's Principle

  5. May 12, 2010 #4


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    Staff: Mentor

    It does. It pushes in every direction, including up.
  6. May 12, 2010 #5


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    Homework Helper

    The higher pressure does push in all directions. In the directions other than upwards against the wing, the higher pressure is causing the affected air to accelerate away from the high pressure zone in all directions. Since the wing (in level flight) prevents upwards acceleration of air, you end up with a net downwards acceleration of air.

    A similar effect happens above the wing, air accelerates towards the low pressure zone from all directions, except that it can't accelerate upwards through the wing, so again you end up with a net downwards acceleration of air.

    This is "hump theory" and it's not true. Here's an example of a M2-F2 glider with the hump on the bottom, it glided just fine.


    This is only true when the faster speed is due to internal forces and no external work performed, essentially a backwards way of stating the obvious, that as a gas or fluid accelerates from a higher pressure zone towards a lower pressure zone, then during this transition, as speed increases, pressure decreases. Clearly the exhaust from a jet engine is both very high pressure and very high velocity, because work was done on the affected air + fuel mixture. Getting back to the no work, internal forces only case, Bernoulli equation approximates the relationship between speed2 and pressure during a transition from higher pressure to lower pressure, ignoring issues like turbulent flow.
  7. May 13, 2010 #6
    Thanks everybody. I guess what I understood is that air in the high pressure zones tries to push air into the low pressure which eventually ends up as LIFT. am I right?
    I have come across few articles which have mentioned that the plane LIFT is not based on Bernoulli's principle, which makes sense. Because, there wont be inverted flight happening on this principle alone. This link explains the actual principle http://www.allstar.fiu.edu/aero/airflylvl3.htm
  8. May 13, 2010 #7


    Staff: Mentor

    Hi shivagss,

    Regarding vectors and scalars. You are correct that pressure is a scalar, but don't forget that area is a vector. The vector for an area points in a direction normal to the surface and the length is proportional to area. So the product of a pressure scalar and an area vector is a force vector.

    What you usually do is take a differential element of the surface area and that times the pressure gives you a differential force. You then sum up all of the differential forces around the entire surface to get the net force. As Creator mentioned, the difference in pressure is what causes lift because if the pressure is constant then all of the differential forces cancel out.
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