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Homework Help: Power efficiency problem

  1. Jan 14, 2006 #1
    A 3.25 * 103-kg aircraft takes 12.5 min to achieve its cruising altitude of 10.0 km and cruising speed of 850 km/h. If the plane's engines deliver, on the average, 1500 hp of power during this time, what is the efficiency of the aircraft engines?

    I initially believed the power out would be: (Kinetic energy + Potential energy)/Time

    And efficiency would be 1500hp/power out. But after my calculations, I got 14.6% efficiency, when it should be 48.7% efficiency.

    What did I do wrong?
  2. jcsd
  3. Jan 14, 2006 #2


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    Gold Member

    I calculated 48.8% efficiency.

    Did you....

    1) Change minutes to seconds?
    2) Change km to m?
    3) Change km/h to m/s?
    4) Convert J to hp? ( 735.498 75 watt = 1hp/s)

    You have to do each of those in this problem.
  4. Jan 14, 2006 #3
    Check your units. Did you convert the power in hp to watts?

    1 hp = 746 watts.
  5. Jan 14, 2006 #4
    Looks like we typed at the same time.
  6. Jan 14, 2006 #5
    I don't know what I did wrong.
    I'm pretty sure I converted everything.
    It must have been a calculation error, because I just got the right answer.
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