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How am i supposed to determine the speed of air?

  1. Apr 24, 2005 #1
    Please Help!!

    How am i supposed to determine the speed of air??
    I need it for Bernoulli's Equation, see i built a glider and i have to present on Wednesday, and i want to show that i did at least one calculation.
    As if things weren't bad enough already i have to figure out two velocities, one of the speed underneath the wing and the other of the velocity on top of the wing.
    how do i do it?? I was thinking that maybe the velocity of the air over the wing is the whole glider's velocity which would be really easy to calculate. and perhaps the velocity of the air underneath the wing would be the air's velocity at that time. (i can probably get it online at a weather website)
    Anyone have any suggestions?? R.S.V.P.!!!
    This is driving me nuts!
     
  2. jcsd
  3. Apr 24, 2005 #2

    Andrew Mason

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    Tell your teacher that Bernoulli's principle does not explain lift. Have a look at:
    http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html

    You might try calculating the lift that your glider creates. Have a look at the NASA site on glider flight:

    http://www.grc.nasa.gov/WWW/K-12/airplane/glider.html

    AM
     
  4. Apr 24, 2005 #3
    hmm.. the bernoulli principle doesnt explain lift... so what does?
     
  5. Apr 24, 2005 #4

    Clausius2

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    What does what?

    What does explain lift?----> Fluid Mechanics Theory.

    What does Bernoulli Equation say?----> Several things, but not everything.
     
  6. Apr 24, 2005 #5
    Bernoulli principle does explain the lift, the problem is that you cannot use Bernoulli equation to calculate the lift.
    Bernoulli equation deals with ideal nonviscous liquid. For that liquid no lift can be generated, neither air can provide a retarding force. However, if the viscousity is taken into account, the flow became complex.
    The rough explanation is that at relativelly low speeds vortecies are forming on the rear edge of a wing. Then, because the momentum should be conserved, an opposite vortex is formed along the wing. That attached vortex provide the neccessary difference of the air speeds above and below the wing. After that we can use the Bernoulli principle to calculate the lift.
    That theory was developed by Zhukovsky and Chaplygin in Russia before WWI.
     
  7. Apr 25, 2005 #6

    Clausius2

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    You're exposing yourself to replies of Arildno & Co. who has been discussing this topic very long long before. I am not going to reply to you in order to don't start it again.
     
  8. Apr 25, 2005 #7

    arildno

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    Am I a dangerous man? :uhh:
    Can't say I disagree terribly much with shyboy, since he notes the crucial importance of viscosity.
     
    Last edited: Apr 25, 2005
  9. Apr 25, 2005 #8
    I believe it is better to be exposed to critics than to be safe like this:
    why not Newton's laws ;)
     
  10. Apr 25, 2005 #9

    russ_watters

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    Easy guys - this looks like a question that needs a simple answer. MM - how about just using Bernoulli's equation to calculate the freestream velocity of the air(essentially, the airspeed of the glider) at the front of the glider by measuring the pressure with a pitot tube: http://www.du.edu/~jcalvert/tech/fluids/bernoul.htm
     
  11. Apr 25, 2005 #10
    well thanks guy for trying to help, but i dont really need it anymore. I have to present on Wednesday and i'm not really worried about it cus i'm pretty much done...i just gotta do the PowerPoint presentation.

    and by the way, you can find the NetForce using Bernoulli's Equation.
    First you take the change in pressure using Bernoulli's equation.
    Then, since Pressure is equal to Force over Area, you solve for force and get
    F=(pressure)(A) the area used is the wing's total area. you calculate Fnet and Voila, you're done!
    we did an example problem like this, and if the teacher was wrong you better not ride in another Boeing cus that's where he worked. hahaha
     
  12. Apr 26, 2005 #11

    Clausius2

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    You know, Arildno, you have a Toledan Knife just below your PC table. I saw it when you invited me to take dinner that day.... :biggrin:
     
  13. Apr 26, 2005 #12

    arildno

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    Toledan knife?
    Are you sure you don't mean the saracen sword my Viking ancestor got from Al-Mansur for helping to subjugate soft Spaniards of his time? :wink:
     
  14. Apr 26, 2005 #13

    Clausius2

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    :rofl: :rofl: Not really.

    Toledo is a medieval city of Spain. It is famous for its steel and swords:

    "All European armies knew the superior quality of Toledo's steel and many great warriors relied only on sabers of Toledan provenance."
    go here:
    http://www.tf.uni-kiel.de/matwis/amat/def_en/articles/steel_collector/swords_from_toledo.html

    The best steel? In Toledo. The best swords? In Toledo. :wink:

    Some time you come here I will show Toledo to you, if you want. :rolleyes:
     
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