Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Centripetal Acceleration calculation help

  1. Nov 1, 2003 #1
    a) calculate the centipetal acc at the tip of a 4.00 m long helicopter blade that rotates at 300 rpm.

    300 rpm = 31.42 rad/s = w

    Centrip Acc = rw^2

    Ac = (3.41 rad/s)^2 * 4 m

    Ac = 3948.87 m/s^2

    b)compare linear speed of the tip with the speed of sound which is 340 m/s

    v=rw

    v = 4m * (31.42 rad/sec) = 125.28 m/s

    So the blades are slower than the speed of sound.

    This does not sound right to me I figured the blades would be faster - but maybe just not at the tip???

    Thanks
    Nautica
     
  2. jcsd
  3. Nov 1, 2003 #2
    OK up to here, I think...
    But how do you arrive at this figure? - Seems much too large to me...
    IMO, your math is correct here. Not sure, but I think helicopters have been around since the 1920's or so, when no aircraft engineer was even close to handling supersonic speeds... Still those machines worked, didn't they? Another thing, I believe it's not possible to move supersonic with any propeller-driven aircraft because the blades are just not allowed to move supersonic - turbulence & friction would lead to destruction... Just compare a helicopter's sound to that of a whiplash (where the tip does reach supersonic) - it's very different, much smoother isn't it? ... Enough for now.
     
  4. Nov 2, 2003 #3
    Sorry, it was a typo, should have been 31.42

    Thanks
    Nautica
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook