I've been told that Rotax 4-stokes "prefer" to run at 5,000 to 5,200 RPM in cruise.

I find that in gusty conditions I'm more comfortable flying a little slower at 4,500 RPM.

Am I doing any damage to the engine by cruising for long periods at 4,500?

  • Re: What is “best” cruise RPM?

    by » one month ago

    None whatsoever. Fly the airplane at whatever rpm you want to and follow the Rotax Operators Manual when it comes to rpm.  You’ll find there’s no minimum cruise rpm stated in that manual. If you search this website you’ll find several threads where folks discuss cruise rpm. 

  • Re: What is “best” cruise RPM?

    by » one month ago

    I guess you'll find the Rotax chart interesting which shows the frame one is free to use the engine in (RPM and MAP). The 912 provides a great scope of freedom. Stay within the operational frame and choose as you like. Adapt to current conditions and your personal preferences. That's all.

    If you choose to fly somewhat slower with reduced revs in gusty conditions you will not harm the engine.

  • Re: What is “best” cruise RPM?

    by » one month ago

    Flying at or below your aircraft's "maneuvering speed" is highly recommended in turbulent conditions, so kudos to you for doing so!  Provided that you're not operating an in-flight adjustable or constant-speed propeller, flying at 4500 rpm as you've described should be perfectly fine. 

    Whenever I've got a question like this, I like to see "official data" backing up the recommendations provided, so here are my go-to "official" sources for engine operation data:

    The first is the "Operator's Manual for Rotax Engine Type 912 Series (Edition 4, Rev 0)" that can be downloaded from this site.  There is a table on page 5-3 that shows performance data.  The statement that immediately precedes the table says "Run the engine in accordance with the following table."  The table itself shows power settings for take-off power, max continuous power, and then shows 75%, 65%, and 55% power settings.  Each of the table entries includes rpm and manifold pressure to achieve that power setting.  That would seem to indicate that cruising at any of those rpm ranges should be perfectly acceptable.  Your 4500 rpm setting is roughly mid-way between the table values for 65% and 55% power, so approximately 60%.

    The second publication I've found helpful is Rotax Service Letter (SL-912-016:  "Essential Information Regarding Engine Behavior, Performance and Manifold Pressure Data for Rotax Engine Type 912 and 914 (Series)" – likewise available on this site) that provides a good bit more information about those operational guidelines.  Among other things, paragraph 3.1.2 states:  "Take off RPM at WOT (wide open throttle) should not be below 5200 rpm to avoid over loading the engine."  In paragraph 3.1.3, it also states that "Fixed-pitch propellers should be set so that the takeoff rpm is above 5200 rpm (WOT).

    Note:  If I had an in-flight adjustable or constant-speed propeller, I would consider the rpm and manifold pressure settings in the Operator's Manual table (referenced above) to be what I would term "soft limitations" - meaning I would make every attempt to operate the engine at or below the manifold pressures listed in that table for those rpms (and interpolate the data for other rpm settings).  If I was .1 or .2 off on MAP, it would not cause me to lose a lot of sleep.  But knowing that low rpm and high manifold pressures are the ideal conditions for setting the engine up for detonation or pre-ignition, I would not be comfortable operating at higher-than-recommended MAP for a given rpm.  But that's not "official" guidance - it's just what I've adopted after many years operating normally-aspirated, turbo-normalized, and turbocharged aircraft engines in high-performance aircraft.

    Thank you said by: RotaxOwner Admin

  • Re: What is “best” cruise RPM?

    by » 6 weeks ago

    Thanks for the reference to this SL. A question remains, however: What is meant by "Take off RPM"? Of course, RPM at the start of the takeoff roll (essentially static) will be lower than at liftoff or Vy (in my case, 59 kt). The AFM for my plane specifies 5,100 rpm +/- 50 rpm, static - which is what I see. Yet it pulls only ~5,400 rpm @ 60 kt and ~5,600 rpm @ 70 kt. Complicating getting a good picture of what's going on is that, in spite of the Rotax requirement for a MP gauge on adjustable (3-position) prop installations, the airframe manufacturer did not install one.

    IMO, the "climb" prop position is set too coarse. Thoughts?

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