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Low max RPM

in 912 / 914 Technical Questions 13 years ago

  • Re: Low max RPM

    by » 13 years ago

    Hi Ricardo,

    At the point you are at with your prop pitch and allowing another 100+ rpm at WOT you will unload the engine a little more and yes you will get better fuel economy along with a little better climb and better cruise. When you work (load) the engine with additional pitch it makes the engine work harder and when it does that it takes more fuel to turn the prop under a loaded condition. When you unload the engine at the rpm you are at it takes less fuel to run at the same speed and rpm.

    You don't have to cruise at WOT, but when you throttle back let's say 5000 - 5100 rpm in cruise the engine can turn that rpm easier now and it will use less fuel. Try it you can always go back, but out of every single plane I have adjusted for an owner to get 5500+ rpm WOT over lower rpms not a single person has ever gone back.

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

      
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  • Re: Low max RPM

    by » 13 years ago

    Hi Roger, I will change the pitch to it work properly, but this is the point that is a bit confuse to me :

    "When you unload the engine at the rpm you are at it takes less fuel to run at the same speed and rpm."

    I understood your explanation, but the peak torque of the 912UL is at 4800 RPM, where is the max volumetric efficiency too, I believe. Then the point is : Is it more economic running with high load at 4800 RPM or running at lower load at higher RPM, when the prop load is exponentially higher ?

    Ricardo
      
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  • Re: Low max RPM

    by » 13 years ago

    Hi Ricardo,

    I do know the max torque for the UL is 4800 and the ULS is 5000, but we still set up to 5500 rpm to reap the balanced benefits. Unloading the engine a tad more will net you more over all benefits. If we were to get too far unloaded or too far loaded then you are right that it becomes a defeating problem, but at the rpm you are at right now it will help. Remember we are doing a balancing act with the engine prop setup and all its characteristics. I have a Dynon D-120 in my plane that shows fuel use and I know many others that have them too. Over the years I have flown many planes with fuel use instrumentation and seen the affects of the rpm prop pitch correlation. The lower the engine HP the less available HP/torque you have to turn a loaded engine and when we add more pitch then the engine has to strain more to turn the prop which translates into more throttle and fuel use trying to get where you want it.

    Like I said, give it a try and if you don't like it you can always go back. You would have noticed a big difference between 5200 and 5500 WOT and its flight characteristics, but may only notice and very small difference between 5400 and 5500 by the human body.

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

      
    Report

  • Re: Low max RPM

    by » 13 years ago

    Hi Guys,

    I almost forgot. Your prop setting has a direct bearing on the engine temps. Load the engine too heavy and the temps go up quick. Unload the engine and the temps will go down. So if your max WOT rpm is only 5200 (too low) then you have an over loaded engine that has higher temps than needs to be. In the summer and warmer climates this can cause some over heating issues during climb out. The engine is working harder trying to perform. More work more heat generated.

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

      
    Report

  • Re: Low max RPM

    by » 13 years ago

    Hello Roger, I changed the prop pitch, now the engine reach 5600 RPM and is working much better. The gain in the propulsion was great. Thank you.

    Ricardo Urio
      
    Report
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