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Homework Help: Calculating helicopter lift

  1. Jul 23, 2011 #1
    1. The problem statement, all variables and given/known data

    I've been trying to find a simple equation for calculating the lift (or thrust) of a RC helicopter and i've found a few that are different.

    2. Relevant equations
    1) Lift = circular area swept by rotor blades [itex]\times[/itex] density of air [itex]\times[/itex] velocity of air exiting disk [itex]\times[/itex] velocity of air exiting disk

    2) Thrust = 2 [itex]\times[/itex] air density [itex]\times[/itex] length of rotor blades 2 [itex]\times[/itex] induced velocity

    3. The attempt at a solution
    I am unsure which is the correct equation. Also, can someone explain what 'induced velocity' is please?

    Last edited: Jul 23, 2011
  2. jcsd
  3. Jul 23, 2011 #2


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    Staff: Mentor

    The equations seem very similar. Each contains a term having units of length2 (we call that area), and density of air and velocity of air column. So I'd say you are in error in having velocity appearing twice in the first equation. This leaves us with two equations almost identical, except for a Pi and I'd surmise that you may have accidently left that out, too. :frown:

    So I'd conclude that induced velocity is the downdraught, the velocity of the air below the disk.

    Have you decided what units you would expect this 'thrust' to have? Do the units of the terms on the right hand side of the equations seems to amount to that?
    Last edited: Jul 23, 2011
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