1. Not finding help here? Sign up for a free 30min tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Propeller Problem

  1. Apr 17, 2006 #1
    An airplane propeller is 2.08 m in length (from tip to tip) and has a mass of 117 kg. When the airplane's engine is first started, it applies a constant torque of 1590Nm to the propeller, which starts from rest.

    Question I:
    What is the average power output of the engine during the first 5.00 rev?

    Question II:
    What is the instantaneous power output of the motor at the instant that the propeller has turned through 5.00 rev?


    I've already solved for the angular accelleration (after 5 revolutions) which is alpha, angular speed omega (after 5 revolutions), and work after 5 revolutions W, moment of inertia 42.18kg*m^2.

    For the last two parts I've tried to solve using P=torque+angular velocity .... that turned out to be wrong. Then I tried using P=Change in work/change in time but that failed. So now I'm a little at a loss. Is there any suggestions where to try next?:cool:

    I figured it out, my methods were correct...... my ending units were wrong! @_@
     
    Last edited: Apr 17, 2006
  2. jcsd
  3. Apr 18, 2006 #2

    Andrew Mason

    User Avatar
    Science Advisor
    Homework Helper

    Energy is torque x angle (force x distance).

    [tex]\tau\Delta\theta = \text{Work}[/tex]

    So [tex]P_{avg} = \Delta E/\Delta t = \tau\Delta\theta/\Delta t[/tex]

    All you have to do is figure out how long it takes to move the propeller 5 revolutions with that torque: Use [itex]\theta = \frac{1}{2}\alpha t^2[/itex] and [itex]\alpha = \tau/I[/itex] to find the time in terms of angle and torque (and I).

    To find instantaneous power, use:

    [tex]P = \tau\omega = \tau\alpha\Delta t[/tex]

    You have to assume that in the first 5 revolutions, the resistance to motion is only the moment of inertia of the propeller, not the propulsion of air by the propeller.

    AM
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: Propeller Problem
  1. Propeller Design (Replies: 1)

  2. Airplane propeller (Replies: 2)

  3. Propelling force (Replies: 2)

Loading...