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: Power question for aero class

  1. Apr 2, 2006 #1
    OK, I have a question regarding this homework that I have to complete about power and airspeed.

    I have a beech bonanza v-tailed aircraft and I need to calculate rate of climb in ft/min with a True airspeed of 110 knots. (110 knots = 185.7 ft/s)

    I am using R/C= (excess power)/weight My problem is that when I found the power required for level cruise at the same speed as above, I got a huge number because I did PR=Thrust required x Velocity (200.3 lbs x 185.7ft/s)

    This going to give me a negative number for my excess power. The power plant is capable of 345 hp at sea level.

    What am I doing wrong?
  2. jcsd
  3. Apr 3, 2006 #2


    User Avatar
    Science Advisor

    I'm calculating only 67.6 HP for cruise. Where did your huge number come from?
  4. Apr 3, 2006 #3
    I honestly don't know what I'm doing wrong. I used the equation I described above and multiplied by 60 in order to get ft lbs/min.

  5. Apr 3, 2006 #4


    User Avatar
    Science Advisor

    [tex]PR = TV[/tex]
    [tex]PR = (200.3 Lb_f)(187.5 \frac{ft}{sec})[/tex]
    [tex]PR = 37556.3 \frac{ft*Lb_f}{sec}[/tex]

    Use the relationship that [tex]1 HP = 550 \frac{ft Lbf}{sec}[/tex]

    That leaves [tex]PR = 68.3 HP[/tex]

    Why are you multiplying by 60?
  6. Apr 3, 2006 #5
    oh ok.....that makes total sense. I was multiplying by 60 to try and convert the units, but now I can see how to get the answer. I didn't use that relationship that you gave.

    Thank you!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook