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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

    There are numerous sites on the web that discuss constant speed propeller operation.
    Your variable pitch prop is a constant speed prop except that the governor is You!

    Most discussions assume a direct drive prop so you will need to divivd engine speed by 2.43 to get the Rotax Propeller RPMs .
    Rotax propellers turn slower than Lycoming and Continental engines.
    5800rpm = 2386 Prop speed... 5000rpm = 2057 Prop speed.

    - - -
    This assumes the limit stops have been properly set.
    You will Take Off with Wide Open Throttle(WOT) and the prop set to Fully Fine (Minimum Pitch).
    Assuming you now wish to throttle back to cruise power of 5000rpm @ 26"map.
    Reduce the throttle to 5000 rpm.
    You may be seeing 24" of MAP.
    Increase the Pitch until the MAP comes up to 26"
    You do not have constant speed governor so the RPMs will drop a little (~4800rpm?)
    Increases the throttle to return to 5000rpm.
    The MAP will increase due to the throttle opening.
    Adjust the Pitch to correct the MAP. This also effects the RPM.
    Continue to alternate the Throttle and Pitch settings to get to the target.

    With experience, you will learn to overcorrect one setting so that when you adjust the other they both converge.
    Ex. You may find that if you initially reduce the throttle to 5150rpm when you increase the pitch to set 26"MAP the RPMs drop right to 5000 in just one try.

    Important! When reducing throttle to land be sure to reset the Prop to the "Fine" Position.
    You do not want the prop set to "Course" in the event of a Go-Around.

    You can fly around all day in the "Fine" position.
    It is like driving in low gear. You won't get anywhere fast but it won't do any harm.

    If you do not understand how MAP is controlling the power let us know.
    It will be a long answer. ;)

    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. Great info there.

    Yeah - I understand that the gauge is displaying the difference in pressure between ambient and that which is behind the throttle plate. But I am not sure how that correlates, and what it actually means - if you know what I mean.

    I feel I should know this, but it was just simply not in the material that was delivered in my training.
      
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  • Re: Need advice - in flight adjustable prop

    by » 6 years ago

    Hi John,
    if I may recommend to have a look at SI912-016-R1 you'll find that it pretty much covers your question. It is heard that this SI was released after endless discussions on the envelope one needs to stay in when fiddling about with revs and MAP. Perhaps you manage to somehow overlook the exclusions that are numerously included and identify the real important ones to stick to the. Example, I tend to keep revs up in the range of 5.000-5.200 and cruise at approx. 200 km/h by manually setting the pitch. A friend of mine cruises at the same speed but pitches the (identical) prop so his engine only revs at, sit down, 4.300. He has no probs with temps, pressures or anything else and saves like 20% gas. The chart in the SI mentioned seems to cover that as OK.
    If I find a way to upload the chart, I will do that. However, it's in the SI anyway.
    Regards
    Peter
      
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  • Re: Need advice - in flight adjustable prop

    by » 6 years ago

    I believe that would be the Service Letter SL912-016 Pertaining to Manifold Pressures.
    Not Service Instruction SI912-016 Pertaining to Fluids.

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

      
    Report

  • Re: Need advice - in flight adjustable prop

    by » 6 years ago

    Absolutely right, Bill. Thanks for puttin that right.
      
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