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Propeller gearbox friction torque

in 912 / 914 Technical Questions 6 years ago
912ULS, propeller gearbox friction torque. Allowable range is 264-532 (max) in-lbs. Video says that if it is less than 264, gearbox needs rebuild. What are the consequences for values higher than 532?

  • Re: Propeller gearbox friction torque

    by » 6 years ago

    You will be putting higher torque stress on the crankshaft than specified.
    Not just in the event of a prop strike but also during normal flight at specific RPMs and air loads when the engine and the prop become harmonically coupled and try to fight each other.

    It is convenient to think that when the Tach says the engine is at 5000 it is at 5000rpm continuously.
    In reality, the engine speeds up during the power stroke and slows down during the other three.
    You feel this as vibration and most of it is absorbed by the engine mounts.
    The props inertia want to keep the prop going faster than the engine during those times.
    And the prop has a LOT of inertia.
    Within limits, the clutch allows some slack between the prop and the crankshaft to occur.

    If you are getting high torque values, the clutch is getting bound up.
    Are you using leaded fuel? (100LL?)

    Bill Hertzel
    Rotax 912is
    North Ridgeville, OH, USA
    Clicking the "Thank You" is Always Appreciated by Everyone.

    Thank you said by: Franklin Covington, RotaxOwner Admin
      
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  • Re: Propeller gearbox friction torque

    by » 6 years ago

    Bill,
    Your explanation is correct and I agree with it.
    There is no doubt that a properly adjusted clutch minimizes undesirable stresses on the crankshaft.
    However, we must remember that the clutch is an optional, so I think the crankshaft must be sized to withstand those forces correct?

    It seems to me that the worst case, not for the crankshat but for the dogs is when the clutch is lose and that is why the repair is a must.
    Thank you said by: Franklin Covington
      
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