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Homework Help: Bernoulli and small aircraft

  1. May 17, 2014 #1
    1. The problem statement, all variables and given/known data
    See attachment

    2. Relevant equations

    Maybe lift force = ClρAv2

    Cl= lift coefficient= 0.3
    ρ= air density
    V=air speed but don't know which one it represents..... Under or below wing or average?

    And the bernoulli equation seems to be needed as well.....

    3. The attempt at a solution

    To keep the plane just in the air the lift force must be equal to the gravitational force I guess. But I'm not sure how to continue from there....

    Attached Files:

  2. jcsd
  3. May 17, 2014 #2
    I think the diagram shows that explicitly.

    Yes, can you try and apply it here?
  4. May 17, 2014 #3
    But im stuck on how to continue.... Can you please help me a bit?
  5. May 17, 2014 #4
    Why don't you start by stating Bernoulli's equation and maybe try and substitute some values into it?
    Last edited: May 17, 2014
  6. May 17, 2014 #5

    But I don't know the pressure and I don't understand what to do with drag and lift force?
    And do I have to use the given engine power?

    Sorry this is completely new to me....
    Last edited: May 17, 2014
  7. May 17, 2014 #6


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    Forget that equation for lift force. I assume there are more parts to this problem, which you didn't show us. Some of the given information may be needed for those -- or may just be extra information.

    You have P1-P2=(1/2)ρ(v22-v12) .

    Do the subscripts, 1 & 2 refer to the bottom surface of the wing and the top surface of the wing respectively?

    How much lift force is needed?

    What part of the plane generates the lift?

    How are force, area, and pressure all related?
  8. May 18, 2014 #7


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    Going with the idealized assumptions about lift for this problem, you can assume the pressure below the wing is some ambient pressure P1 (for pressure below), and above the wing it's P2. The the difference in pressure (P1 - P2) times the wing area will equal the weight of the aircraft.
  9. May 18, 2014 #8
    But P x A= force isn't it? So you say (Pbelow-Pabove)xA=mass? Or mg? And I havent got any other info about P....

    I have substituted (Pbelow-Pabove) for mg/A and my result seems to be correct now. But im not sure why I can say that the difference of P is this....
    Last edited: May 18, 2014
  10. May 18, 2014 #9
    Maybe show your work explicitly but the difference in air pressure is what generates lift.
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