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Homework Help: Torque and Angular acceleration

  1. Jul 18, 2010 #1
    Hi all, first time posting so lets see how it goes!

    My problems are with a piece of physics coursework I'm doing for my A-level courses. I chose to investigate the 'torque effect' on single propeller planes. (If you are unsure of what this is, its basically newtons third law, reasoning that when a plane makes a propeller spin, the propeller makes the plane spin).

    So i need a way to calculate torque so i can compare it to forces produced on the plane. The setup i used for the experiment is show in the picture:

    [PLAIN]http://img827.imageshack.us/img827/1451/physicsforum1.png [Broken]

    Basically, its a suspended mock up of a plane, with a propeller on the front. I'm using a tachometer to measure rpm and scales to measure the 'weight' induced on the wing, which i could convert into a moment.

    How can i find the torque of the propeller from this? I've spent ages trawling the internet. The variables i know are: rpm, length and mass of propeller. I also changed the distance alone the 'wings' of the mockup which i measured the force from, using the scales to measure the 'force' the torque effect applied on them.

    So what i understand, is:

    ω = 2*(pi)*f

    a_tangential = Angular acceleration*R

    τ = I*α

    But how do i link angular frequency with angular acceleration. How could i do this? Could i just use the ideas in the picture and trigonometry?

    Many thanks to all!!
    Last edited by a moderator: May 4, 2017
  2. jcsd
  3. Jul 18, 2010 #2


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    welcome to pf!

    hi egroeg93! welcome to pf! :smile:
    i think you're misunderstanding both the word "torque" and the way the propellor is connected to the rest of the plane

    "torque" can mean a couple (a pair of equal and opposite forces, not in-line), or it can mean the moment of a force

    the torque of a propellor is the first type, but your distance x force calculation is the second type

    each propellor exerts both a forward horizontal force on the wing, and a couple (a torque) on the propellor shaft

    the moments (torques) of the forward horizontal forces of the two propellors (assuming the plane is symmetrical!) cancel out, so those forces don't turn the plane

    the propellor shaft isn't connected to the wing or the body, it's connected to the engine … i don't know where that is or how it works, but i assume that either there's something in the engine that "absorbs the recoil", or that there is a torque, but its turning effect is very small since the moment of inertia of the plane is very large, and that that's catered for by a very slight adjustment to the rudder or to the plane's geometry

    anyway, i think you'll need an apparatus that incorporates a torsion balance rather than the usual linear one :smile:
  4. Jul 18, 2010 #3
    i was told i could calculate a torque force by my teacher :P

    Its modelled on a single propeller plane, so there is no cancelation of forces.

    This is a real scenario, and i'm just trying to experiment with it :)
  5. Jul 18, 2010 #4


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    If there's a single propeller, it's presumably in line with the centre of mass of the plane, so again there should be no moment (torque) about the centre of mass.

    I think either you're misunderstanding your teacher's answer, or (s)he's misunderstanding your question.
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