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Homework Help: Force of a propellor in fluid

  1. Mar 17, 2012 #1
    1. The problem statement, all variables and given/known data

    Dear physics forum users, I have a bit of a question about using a propellor to provide resistance. I am looking at designs for fall arrest devices and one idea I've looked at is using a propellor connected to a reel so that when the reel is unwound the propellor spins and is resisted by the fluid. The only thing is I'm not really sure on how to go about calculating the force exerted on the propellors. I know that we have a reel that is is spinning at roughly 60 RPM and the reel is connected to directly to the propeller so will spin at the same speed. However. what I need to do is work out how big my paddles on the propeller would need to be to provide a force equal to 981 N. Would it be best to use the force to work out the torque on the connecting shaft, and then work out the torque of the propeller? Im not really sure how to work out the second bit. lets assume the propellor has three paddles facing perpendicular to the fluid to give maximum surface area. How much force would each paddle exert on the fluid, if we had a fairly viscous fluid such as a motor oil with 0.3 Pa.s

    2. Relevant equations

    Not really sure what to use, obviously we have the basics of:


    3. The attempt at a solution

    Havn't got this far yet...
  2. jcsd
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