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Hi all,

WOT on my 912uls is 5700 rpm in level flight but my take off roll only gets 4800 rpm and climb out is 5000 rpm. Prop is 68” 3 blade CGS wood. Is there anything that I should look at on the engine that may be causing the poor take off rpm’s. I took a degree of pitch out but then rpm’s would go past 5800 and cruise speed suffered. Engine is 2016 with 177 hrs. No air box.

Thanks 

  • Re: Take off rpm

    by » 6 months ago


    Your first set of rpm's report is fairly normal for most with a ground adjustable prop. With a 5600 - 5700 WOT rpm in level flight I would have left it alone. You have to decide  if you want a well balanced prop rpm for climb and cruise or want a better climb prop with a slight loss of cruise speed.


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


    Thank you said by: Ross Derksen

  • Re: Take off rpm

    by » 6 months ago


    Yes but at 5700 WOT should I not also be achieving the 5200 take off rpm’s that Rotax suggests. At 4800 take off rpm’s am I not overloading the engine and asking for trouble. And yes I did put the one degree of pitch back into the prop after going past 5800 rpm’s.

    Thanks


  • Re: Take off rpm

    by » 6 months ago


    Hi Ross

    Just to be clear the Rotax call out is to do 5200 on climb.  Take off RPM might indeed be lower as you have not really built up enough speed to unload the prop.  The 5200 number is peak torque.  RPM is very much dependent on your aircraft type (forward speed) and prop settings.  Static RPM is not really a good marker other than your starting point on the runway.  The test is to be climbing at 5200.  In some cases you may have to lower the nose a bit to get that.  As pointed out the prop has to govern the engine speed to 5500, perhaps 5600 in my opinion, with a fixed pitch.  

    Don't get too fixated on some of the numbers just hit the big markers.  5200 on normal climb, 5500 continuous WOT in level flight and don't exceed 5800.  Naturally aircraft and propeller type can shift these numbers as well as ambient conditions. 

    Cheers


    Thank you said by: Ross Derksen

  • Re: Take off rpm

    by » 6 months ago


    That's normal. Depending on the cruise speed the prop is optimized for, the blade tip angle of incidence when your in the ground or in take off run is very high, possible above the stall angle for that airfoil, and so it's drag is massively increased.
    In my experience the MIN to MAX rpm gap can be a lot closer using composite propellers with a high blade twist.

    In my case FK9 mk4, with 912ULS, and prop set to around 110kt cruise:
    A 68 wooden prop does 5000 ground 5700 max cruise.
    A 68 warpdrive does 4800 ground 5500 max cruise
    A 68 sterna swift does 5000 ground and 5500 max cruise.

    This sterna swift I bought in Oshkosh this year and I'm using it for a month now.
    The performance gain in both take-off and cruise is quite massive.
    With the warpdrive I could cruise at max 115kt, and now I can do 125kt.


    Thank you said by: Rotax Wizard, Ross Derksen

  • Re: Take off rpm

    by » 6 months ago


    When I did a 14 different prop study years ago Warp Drive suffered the most. More on climb.


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


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