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912 ULS engine wear at take off WOT RPM

in 912 / 914 Technical Questions 14 years ago
I'm new to the 912 ULS having just purchased a SportStar. I'm having discussion with other owners as to RPM setting for take off and climb out. Concern is that with too low take off RPM the engine may be taxed. Right now the RPM only hits 5,100 and I'm about to set the pitch. Take off is brisk at 5,100. Any thoughts on engine wear in setting take off RPM to 5k or there abouts?

  • Re: 912 ULS engine wear at take off WOT RPM

    by » 14 years ago

    Hi Raymond,

    5100 take off rpm is not out of spec by any means and this setting gives you good climb, but you are loosing on the cruise and the fuel economy side. I would bet at 5100 rpm take off rpm that you get around 5700+ WOT flat and level? If we all had a constant speed prop we could make good use out of our full rpm range. Since we only have a ground adjustable prop then we need to balance for climb, cruise and fuel economy. A good place to be at WOT flat and level at your normal cruise altitude is 5500-5600 WOT rpm. Don't set the WOT setting below 5500 as it starts to cause excessive load on the engine and the lower you go below that the worse the load is. The take off rpm will then be around 4950-5000 depending on your take off style. (i.e. steep verses shallow take off, flaps verses no flaps) Rotax has in their documentation they would like to see 5200 on take off, but that equates to 5800 rpm WOT and isn't a balance for the ground adjustable props, great for the constant speed props.
    Set your prop for WOT and the other rpm ranges will follow that. Set the idle rpm around 1750-1800 because the 912ULS has a higher compression than the 912UL. Low idle rpms means greater vibration to the gearbox and engine. The stick controls the speed on approach not the prop rpm.


    Here is another plus to 5500 WOT rpm. If you break a throttle cable or it comes loose the carb should go wide open on the offending side because of the carb springs. If while flying the engine all of a sudden feels like it will shake itself to death you can check to see if one did go wide open by going full throttle and if it smooths out then you know what happened. Of course it it doesn't smooth out time to kill the engine to save it. But if it did smooth out you can fly at 5500 rpm as long as you want without any issues as this is the constant speed rpm for your engine. Then shut down on approach. If your engine gets lets say 5700 rpm WOT then that only has a 5 min. run time limit.

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

    Thank you said by: YEN NIEN YU
      
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  • Re: 912 ULS engine wear at take off WOT RPM

    by » 14 years ago

    Roger,

    Thanks for the useful information. The engine will go 5,700+ WOT on cruise. I will stay with the current pitch as I agree that setting at 5,500 or greater for take off will give too high rpm WOT at cruise. I am a llittle confused how this steeper pitch would compromise economy.

    I am just getting used to the SportStar and it seems to cruise great at 5,000. The manual says economy cruise is 75%. Would that be .75 X 5,500 = 4,125?

    Thanks also for the insight on the throttle linkage/cable breaking. I'm learning something new everyday and my ears are wide open if anyone has words of wisdom to give.
      
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  • Re: 912 ULS engine wear at take off WOT RPM

    by » 14 years ago

    Hi Raymond,

    If it were my plane I would increase the prop pitch about 1.25 - 1.5 degrees to bring my WOT rpm setting to 5500-5550 and even up to 5600. The Rotax 912 produces its best torque at 5000 rpm provided the engine is set for 5800 rpm. After 5000 rpm the more throttle you give the lower the torque, but the higher the HP goes. Since we don't have the luxury of a constant speed prop then that balance between climb, cruise and fuel economy comes into play.
    Here is an example. If you have a prop pitch set up to only see 5200 WOT flat and level then you loose torque and climb and you'll have to give it more throttle which uses more fuel to get any speed or HP. If it is set at 5200 RPM WOT then your fuel use will be around 6+ gph. The engine is way overloaded and can be damaging. If you set the prop to get 5500 WOT then it can better achieve a more desired torque and the engine is less loaded. The speed will increase and you can throttle back and cruise at 5100 rpm and only use about 4.8 gph.
    If you set the prop to get 5700-5800 rpm WOT then you get more HP and good torque, but you loose cruise speed and fuel economy trying to cruise at enough rpm to keep your loss of cruise speed up. Too much WOT rpm and too little WOT will use more fuel, and cause a loss of climb or cruise performance depending on which way you propped it.
    So here we are back to balancing climb, cruise and fuel use. Fuel use just kind of follows the rpm setting. I always set up a prop for someone to get 5500-5600 WOT at their normal or average cruise altitude. Some people live at sea level and fly at 1000' MSL while others live at 6000' elevation and fly at 10K MSL.

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

    Thank you said by: YEN NIEN YU
      
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  • Re: 912 ULS engine wear at take off WOT RPM

    by » 14 years ago

    Roger,

    Thanks again for the great explanation. I'm beginning to understand. I was concentrating on take off as that is what my friends were focused on. I will take a real close look at the cruise rpm and adjust the prop pitch to acheive 5,600 at cruise.
      
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  • Re: 912 ULS engine wear at take off WOT RPM

    by » 14 years ago

    5550-5600 WOT flat and level. Do it for about a minute and it will tell you everything you need to know.If you want to add 100-125 rpm then add .75 degree more pitch in the prop, 150 rpm about 1 degree.

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

    Thank you said by: YEN NIEN YU
      
    Report
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