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Prop Extensions: good, bad or ugly?

in 912 / 914 Technical Questions 8 years ago
In the Rotax 914 installation manual I find a spec for moment of inertia (6000 kgcm2) and a spec for maximum length of prop extension (120 mm). I'm wondering if when testing for moment of inertia, the prop extension is included, and if it would add much to the moment of inertia of the propeller alone. Is there a consensus view on the use of prop extensions? I'm being told that using a prop extension has a big effect on moment of inertia, but after doing some studying I am beginning to doubt that.

  • Re: Prop Extensions: good, bad or ugly?

    by » 8 years ago

    I would doubt it too. The mass of the extension is in the center and not out in the critical moment of arm.

    Maybe some others will chime in.

    Roger Lee
    LSRM-A & Rotax Instructor & Rotax IRC
    Tucson, AZ Ryan Airfield (KRYN)
    520-349-7056 Cell

      
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  • Re: Prop Extensions: good, bad or ugly?

    by » 8 years ago

    Let's put some real numbers to it.
    Assume a solid Aluminum Prop extension the same size as the 912 prop flange. 120mm (~5") in diameter.
    And also assume it extends the prop the same distance. 5"

    This solid block will weigh about 9 lbs. ~4Kg.

    It will have a moment of inertia of 72 kg•cm2 before adding any holes for the Bolts or boring out the center to lighten it.

    By the time you are done, it will eat up less than 50 of the 6000 kg•cm2 you are allowed.
    That still leaves 5950 kg•cm2 for everything else.

    A dime taped to each tip of a 60 inch prop is about the same.

    Small enough to generally ignore.

    What you might Not want to ignore, is the change in the Center of Gravity (Weight and Balance) caused by moving the prop (10-15lbs) 5 inches!

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

    Thank you said by: Ken Ryan
      
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  • Re: Prop Extensions: good, bad or ugly?

    by » 8 years ago

    Thanks!
      
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