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Rotax 912ULS Cruising RPM !!!!

in General Tech Discussion 8 years ago
Rotax 912ULS cruising RPM !!!!

Hi, I am here posing this question as the more I ask the more confusing it’s getting..

What Rpm are you operating your Rotax engines in cruise?? Some say 4900 other say 5000 others 4200-4500!!!

When I ask pilots that operate in the 4900 to 5000 they say the operating manual say’s its best for the engine etc… etc... while the other half says that at 4900/5000 you are pushing the engine etc..etc..

So I am getting mixed feelings about the rpm that I should operate (I normally cruise 4400-4300).. any suggestions from the pros !!

Ground adjustable prop set @ 5500 in S/L Flight WOT... MAP @ 4200 25'' @ 4900 27.5'' fitted on a BRM Okavango (like CH701 & ICP Savannah)

Tx

  • Re: Rotax 912ULS Cruising RPM !!!!

    by » 8 years ago

    Hi Michael,

    Your engine is rated to run continuously at 5500 rpm all day long. The majority run usually between 5100-5300 from what I have seen over the years and what is talked about on the forums. I've seen some that run the engine from 5400-5500 all the time. It was never designed to run down in the 4K's all the time. Those people cruising at 4200-4800 are wrong. The 4200-4800 cruise all day people should attend a Service level Rotax class and get some education. This should pull people up out of the 4k's. Many that I talk to on forums use 5100-5300 all the time. The engine runs leaner down in the mid 4k's too.
    Where did the 4k rpm people get that type of info because it isn't in print anywhere and it's talked about on all the forums not to cruise in the 4k's, but in the 5k's.

    Let the other guys do their thing and you can run somewhere around 5100-5300 and know you're doing better.
    Many pick these rpms to cruise at because there is a speed vs fuel crossover point where more rpm doesn't help speed that much, but just uses more fuel.

    Having the prop pitched to help your cause is a must too.

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

      
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  • Re: Rotax 912ULS Cruising RPM !!!!

    by » 8 years ago

    As a point of reference, I operate my 912ULS (RV-12 ELSA with 370 hours) usually @ 5,250 rpm in cruise. I will occasionally cruise @ 5,000 - 5,400 rpm depending on oil temperature/need-for-speed/etc. - I rarely, if ever, cruise below 5,000 rpm. I have set up my ground-adjustable Sensenich prop to yield @ 5,650 rpm WOT at 5,000' (my average cruising altitude) per Roger's and Van's recommendations. This gives me a nice balance of climb and cruise performance.

    A question: I gather that climbing at WOT (until oil temps require a throttle-back) is preferable due to the unrestricted main jet thus being fully open and supplying maximum fuel for cooling. Is it preferable to climb at WOT and Best-Rate-of-Climb to get up to cooler altitudes as fast as possible? As seems to be typical with many 912ULS engines, my engine is "oil heat challenged" in climbs during hot weather (> 90 degrees F ambient) operations and requires "step-climbing" in order to keep Oil Temps < 240 degrees F.
      
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  • Re: Rotax 912ULS Cruising RPM !!!!

    by » 8 years ago

    Hi David,

    Sounds like you're set up fairly well. The flight profile you described seems to be mostly what I see from owners. As far as the climb profile you may be limited like you mentioned to engine temps and with the time of year (summer vs winter). You can climb all you want, but if temps get too hot it's time to change something to bring them down. Each plane's cooling abilities can be different. Some never seem to get real hot and some get there fairly quick.
    Some high engine temps are correctable and some aren't.

    p.s.
    Just finished an 100 hr/annual condition on an RV12 with 185 hrs. He left this morning.

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

      
    Report

  • Re: Rotax 912ULS Cruising RPM !!!!

    by » 8 years ago

    For what it's worth I cruised a 914 at 5600 for 285 hours before I installed a Airmaster constant speed prop. I now cruise at 5400. At 10,000' I true 140kts in a Tecnam P2008.
      
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  • Re: Rotax 912ULS Cruising RPM !!!!

    by » 8 years ago

    Thanks for the info guy's

    Roger, do you mind me asking one last question?

    I will soon fit a DUC Flash prop my my BRM Okavango. Shall I set this as you suggested WOT 5500 in S/l? I do have a manifold pressure gauge do I take notice of this (prop is only aground adjustable). I have been told that the MAP readings will only have to be taken in consideration when fitted with a in flight variable pitch prop.

    On another note I saw YouTube clips and MAP readings was @ 24 !!! am I overloading the engine 26'' MAP @ 4500


    I had a Woodcomp Klassic 170 3 blade this was set @ 17.5 and I would have a MAP reading of 26 @ 4400...

    Tx
      
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