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Airplane propeller and plane's velocity relative to the ground.

  1. May 17, 2008 #1
    [SOLVED] Airplane propeller and plane's velocity relative to the ground.

    1. The problem statement, all variables and given/known data
    An airplane propeller rotates at 2200 rev/min while the airplane flies at a speed of 550 km/h relative to the ground. The plane's velocity is parallel to the propeller's axis of rotation.
    Part a) What is the speed of a point on the tip of the propeller, at radius 1.5 m, as seen by the pilot?

    Part b) What is the speed of a point on the tip of the propeller, at radius 1.5 m, as seen by an observer on the ground?

    2. Relevant equations
    v=w times r
    v=w times r times sin delta



    3. The attempt at a solution
    I got part a asking speed relative to pilot by converting 2200rev/min to m/s, but part b has me stuck...I don't know if I am missing an equation that I could possibly use but I tried to convert the speed relative to ground to m/s and add it to the 345.57m/s I got as the answer for part a.
     
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  3. May 17, 2008 #2

    Hootenanny

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    Notice that the linear velocity of the propeller tip is perpendicular to the velocity of the plane.
     
  4. May 17, 2008 #3
    Remembering that how would I relate 550km/hr to the speed seen by the pilot that is what has me confused. It should form a 90 degree angle and be the same as I calculated in a, but that doesn't work...I tried v= w times r times sin delta and it didn't work.
     
  5. May 17, 2008 #4

    Hootenanny

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    The speed is the magnitude of the resultant velocity. How do you normally find the magnitude of two perpendicular vectors?
     
  6. May 17, 2008 #5
    Oh...now I understand this. I would use pythagorean's theorem to get the resultant velocity. Thanks I appreciate your help.
     
  7. May 17, 2008 #6

    Hootenanny

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    Correct! No problem :smile:
     
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