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Centrifugal twisting moment of a propeller

  1. Sep 20, 2012 #1
    I am interested in calculating the centrifugal twisting moment (CTM) of a variable-pitch propeller blade. It's been a long time since I did anything with moments of inertia, so I'm looking for pointers to good info, or direct help if someone here can provide it.

    Assume there is a propeller with two blades. For simplicity, each blade is a rectangular solid with length greater than width and negligible thickness. The blades spin around the main axis of rotation at the center of the propeller. In addition, the blades twist (variable pitch) about an axis that is perpendicular to the main axis of rotation, extends radially away from the main axis, and passes through the center of gravity of the propeller blade.

    [ That was my attempt to describe the system. In plain english, it's a propeller with variable pitch blades. Hopefully my intention is clear even if my description is not. ]

    Ignore aerodynamic forces (the centrifugal twisting moment is much greater than the aerodynamic twisting moment).

    When the propeller is spinning, the CTM acts to reduce the blade pitch. In other words, when the propeller is spinning with no other applied forces, the blade pitch will go to zero degrees. For an actuator to hold a pitch angle in the blades, the actuator must be able to provide more force than CTM.

    If the moment of inertia, mass, center of gravity, and RPM are known -- how does one calculate the centrifugal twisting moment?

    http://www.scribd.com/doc/45224522/292/Propeller-Twisting-Moments [Broken]
     
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Sep 20, 2012 #2

    A.T.

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    Fig 10 in your link makes it pretty clear how to compute it, using vector math. For a simple approach put the points A, B into the centers of mass of the blades halves, separated by the pitch axis. In general you would have to integrate of the blade volume.
     
  4. Sep 20, 2012 #3
    It may be clear how to compute it for someone who is more familiar with these methods than I am. However, this is outside my normal field of work, so I could use some pointers.

    While I could do an integration over the entire blade, I thought it would be simpler to use a moment of inertia that describes the blade.

    Right now, I'm thinking along the lines of the following:
    (material density) * (rpm^2) * (Imajor - Iminor) * sin(alpha) * cos(alpha)

    alpha - angle between minor axis and plane of revolution


    I'll keep crunching to see if I can get the numbers to match the data I have. Any tips in the meantime would be appreciated.
     
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