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Homework Help: Calculating mass of air from Speed and Density?

  1. Jan 11, 2010 #1
    For the hovering helicopter, it is assumed that all the air beneath the blades is pushed vertically downwards with the same speed of 4.0m/s. No other air is disturbed.

    The density of air is 1.2kg m^-3

    How can you calculate, for the air moved downwards by the rotating blades,

    i) the mass per second
    ii) the rate of change in momentum ??
  2. jcsd
  3. Jan 11, 2010 #2


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    You can't unless you know the area of the blades. Do you?
  4. Jan 11, 2010 #3
    The length of each blade of the helicopter is 0.70m. The area that the blades sweep out as they rotate is 1.5 m^2.
  5. Jan 11, 2010 #4
    could you not just calculate the area of the cylinder? which would be an area of 6m^3. then find out the mass of the air in the 6m^3. I believe that would be the mass per second.

    As for the change in momentum, I am unclear as to what the question is asking. Are you asking for the change in momentum of the helicopter? Or the change in momentum of the air?
  6. Jan 11, 2010 #5
    in case of any ambiguity, the 6m^3 arises from 1.5m^2(area of the blades), multiplied by the metres of air displaced in one second.(4m)
  7. Jan 14, 2010 #6
  8. Jan 14, 2010 #7
  9. Jan 14, 2010 #8
  10. Jan 14, 2010 #9
    the area = 85.9645m^2
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