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Need advice - in flight adjustable prop

in 912 / 914 Technical Questions 6 years ago

  • Re: Need advice - in flight adjustable prop

    by » 6 years ago

    Yes but as I mentioned I have an ACTUAL constant-speed prop. The way to set 5500 rpm is to set the target rpm to 5500. There is no coarse/fine adjustment because the constant-speed governor electrically does that as needed, automatically. I was asking if full-throttle maximum-speed cruise is best done at 5500 rpm or at something a bit lower, like 5300 rpm. I can do wide-open throttle at literally any RPM setting I want, but I just wanted to know what was best.

    Likewise I wanted to know if I should lower the RPM when reducing the throttle a bit (again, for sustained cruise) or if 5500 is the best cruising RPM regardless of throttle setting.
      
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  • Re: Need advice - in flight adjustable prop

    by » 6 years ago

    Yep! Thanks Bill... forgot the MAP bit.

    I have never been at WOT yet - have had to back off LOL, and yes, take your point about MAP!

    In fact, if I were to go to WOT and set the pitch to hit 5500RPM, I would be screaming along and probably way above VNO!!

    Cheers
    John
      
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  • Re: Need advice - in flight adjustable prop

    by » 6 years ago

    Yes but as I mentioned I have an ACTUAL constant-speed prop. The way to set 5500 rpm is to set the target rpm to 5500. There is no coarse/fine adjustment because the constant-speed governor electrically does that as needed, automatically. I was asking if full-throttle maximum-speed cruise is best done at 5500 rpm or at something a bit lower, like 5300 rpm. I can do wide-open throttle at literally any RPM setting I want, but I just wanted to know what was best.

    Likewise I wanted to know if I should lower the RPM when reducing the throttle a bit (again, for sustained cruise) or if 5500 is the best cruising RPM regardless of throttle setting.


    With a Constant Speed Prop, you can NOT arbitrarily set any RPM you want and move the throttle to any setting you want.
    The Continental/Lycoming drivers have a rule of thumb to keep the MAP below the RPM divided by 100.
    Example: If they were to set the prop at 2400rpm the throttle could go no further open than 24 inches of MAP. This works because the Earth has an atmospheric pressure that produces a Max MAP of about 28 inches and the Lyc/Cont engines have a max rpm of about 2800 rpm. This is just a happy coincidence.

    This Rule of thumb is not going to work for a Rotax engine because of the Higher RPMs and lower Prop speed due to the gearbox. A working approximation might be to keep the MAP below the RPMs divided by 200.
    Example: 5600rpm/200 = 28" MAP, 5200/200 = 26" MAP, 5000/200 = 25" MAP (or LOWER)

    If you want to go WOT you will need to be at 5500 rpm or more, 5300rpm would be killing the engine!
    You can NOT go WOT at any RPM you want.
    At 5500 rpm and above you can set any MAP you wish without consideration.
    Below 5500rpm, you must limit the MAP.

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

      
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  • Re: Need advice - in flight adjustable prop

    by » 6 years ago

    Thanks Bill, but Rotax's service bulletin seemed to indicate there was no map pressure limit above 5200 rpm.
      
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  • Re: Need advice - in flight adjustable prop

    by » 6 years ago

    Hi Ed,

    I did see that too and thought it curious!! The graph shows that exact thing - if you are looking at the same data I am??
      
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