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I have a 912 ULS and an Ivoprop inflight adjustable prop. 

I’ve been adjusting my prop angle at WOT to 5,600 rpm while in the pattern in level flight.  For takeoff, I go to WOT and reduce by 200 rpm using throttle once I reach 700’ and keep on climbing.  For cruise, I again go to WOT and slowly change prop angle to bring rpm to 5100 rpm.  My manifold pressure (MAP) is about 25.   That looks like it’s within the parameters of Rotax Service Letter SL-912-016R1 .  But this letter also says that, “Continuous use of engine speed below 5200 rpm with WOT should be avoided”.   So does that mean I could be over stressing my engine?  Any recommendations?

Thanks. 

  • Re: Inflight adjustable prop questions

    by » 6 weeks ago


    Hi William,

    Great question. I will be very intersted in the responses from our Rotax experts.

    You probably already have the following but just in case, this is what I get from the Rotax Operators Manual:

    4800 RPM, 65% /26.5Hg”/13L/h

    5000 RPM, 75% / 27.2 Hg

    5200 RPM 27.5 Hg”/15L/h

    5500 RPM 28 Hg”/19L/h

    Expressed in a slightly diffrent way:

    Max Cruise 94 hp @ 5500 rpm/ 28 Hg /19Lh

    Econ Cruise 65 hp @ 4800 rpm/26.5 Hg /13L/h

    The suggestion is, that the above settings are within Rotax operating recommendations.


  • Re: Inflight adjustable prop questions

    by » 6 weeks ago


    I am a bit confused about your settings.

    WOT and 5500-5800 for less than 5 minutes in the TO and initial climb is within parameters and gets your best power for climb.

    However WOT in any other part of the pattern is too much, downwind should be cruise or less.

    Cruise is normally defined as 75% power so probably 4800-5200 rpm and MAP of 23-25 inches

    again "cruise" should not be WOT but about 65-75% power 4800-5200 rpm and about 23-25 MAP.

    You imply that WOT at 5600 rpm is a MAP of 25...this seems low unless you are operating at high density altitude. I often see 28+ MAP at WOT, so perhaps check your inlet for restrictions.

    Aircraft piston engines are most reliable when NOT operated at Full power continuously set a reasonable RPM for cruise and adjust MAP. At altitude of course you can use WOT as it will not result in a high MAP, so at say 10,000 feet I usually only get 22-23 MAP at WOT and operating continuously there is OK, but I need to be cautious when descending.

     


  • Re: Inflight adjustable prop questions

    by » 6 weeks ago


    There is a side note to the chart that says (if I remember correctly) that the limits shown only apply at WOT AND below 3500 ft (@ ISA?). Thus, to me, “normal flying”  should be no issue. 
    Given a certain cruise speed keeping revs up in the range of 5000-5200 rather than at something around 4500 (with an extensive pitch of the prop) is said to be better because of lower side forces on the pistons.

    I cannot double check but expert explanations on that matter sound plausible to me.


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