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I noticed recently as the weather has gotten cooler that my fuel flow seems to have increased during takeoff. I assume it's because of the weather, but maybe not? I just know that my EMS displays a red value for fuel flow during the first few minutes of my takeoff and displays 7.1gph or a tick or two higher. Is this something I should get addressed, or is it fine if it's only for a few minutes per flight? Once I reach 2,500 or 3,000' I think the fuel flow comes back enough that it's in the yellow, then finally into the green when I reach TOC and pull back to eco mode.

If 7.1gph is fine, what actually would be too high even just for a short time? Because if I know that value, I can adjust my EMS warning system so that it doesn't give me a red indication until it reaches that value.

Thank you!

  • Re: 912iS fuel flow - 7.1gph+ at SL T/O

    by » 3 months ago


    Out of curiosity what is your WOT rpm in level flight?

    There are a few things that absolutely affect fuel usage in both the 912ULS and the 912iS from plane to plane and how pilots use their plane. These variables need to be considered.

    One factor to consider that plays a big part in fuel use is prop pitch. When prop pitch starts getting too course fuel use goes up along with stress on the engine. Another factor is what rpm do you use for cruise rpm? When I was doing a study with two Flight Design CT aircraft (same prop rpm at WOT), one with the 912ULS and the other with a 912iS engine the 912iS got better fuel economy when under 5300 rpm in level flight. As that rpm went up past 5300+ then the 912iS engine fuel economy became worse. This was repeatable for many flights. The one place the 912iS engine did much better was at high altitudes because the 912iS could meter fuel better than the 912ULS. All my research projects are in real time and in flight and not just bench test.

     


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


  • Re: 912iS fuel flow - 7.1gph+ at SL T/O

    by » 3 months ago


    First, the carb version and the injected are completely different.  One should not mix the 2 for comparison.  Within the injected engine there is a fuel map that is based on density and fuel flow will adjust.  Cooler weather has more dense air so it is reasonable to expect more fuel will be used.  The measure of that for you would be the EGT, if they are normal then the ratio is fine and don't worry about fuel flow.  On the iS it also has to be understood that at WOT there is considerable increases in fuel flow as this is take off power.  ECO mode only starts when you back out to less than 97% power settings.  This is explained in your operators manaual and other documents on the iS models.  

    Cheers


  • Re: 912iS fuel flow - 7.1gph+ at SL T/O

    by » 3 months ago


    Roger Lee wrote:

    Out of curiosity what is your WOT rpm in level flight?

    There are a few things that absolutely affect fuel usage in both the 912ULS and the 912iS from plane to plane and how pilots use their plane. These variables need to be considered.

    One factor to consider that plays a big part in fuel use is prop pitch. When prop pitch starts getting too course fuel use goes up along with stress on the engine. Another factor is what rpm do you use for cruise rpm? When I was doing a study with two Flight Design CT aircraft (same prop rpm at WOT), one with the 912ULS and the other with a 912iS engine the 912iS got better fuel economy when under 5300 rpm in level flight. As that rpm went up past 5300+ then the 912iS engine fuel economy became worse. This was repeatable for many flights. The one place the 912iS engine did much better was at high altitudes because the 912iS could meter fuel better than the 912ULS. All my research projects are in real time and in flight and not just bench test.

    WOT rpm in level flight depends on altitude, as you know. But during my climb, I'm turning 5400 rpm +/- 50 rpm. Down at 4,500' cruise, I'm in eco mode cruise at 5,400-5,450rpm. Up at 10,500', I'm in eco mode cruise at 5,350rpm. But it's specifically the area of WOT between SL and 2,500' MSL that I'm asking about because that's the only place where I'm getting a red indication (7.1GPH) on the fuel flow when it's colder out.


  • Re: 912iS fuel flow - 7.1gph+ at SL T/O

    by » 3 months ago


    Rotax Wizard wrote:

    First, the carb version and the injected are completely different.  One should not mix the 2 for comparison.  Within the injected engine there is a fuel map that is based on density and fuel flow will adjust.  Cooler weather has more dense air so it is reasonable to expect more fuel will be used.  The measure of that for you would be the EGT, if they are normal then the ratio is fine and don't worry about fuel flow.  On the iS it also has to be understood that at WOT there is considerable increases in fuel flow as this is take off power.  ECO mode only starts when you back out to less than 97% power settings.  This is explained in your operators manaual and other documents on the iS models.  

    Cheers

    Thanks, Rotax Wizard. Yes, the EGTs are nominal and consistent. So you would say not to worry about a red 7.1GPH indication between SL and 2,500' MSL when at WOT when it's cooler out, right? What GPH fuel flow is too high and actually dangerous? Because if I know that value, I can go into my EMS settings and change the red range so it doesn't go red until that point. 


  • Re: 912iS fuel flow - 7.1gph+ at SL T/O

    by » 3 months ago


    Hi again

    Fuel flow on the information supplied to the instrument is a calculation and not actual fuel flow if your running an instrument that is reading the data stream from the ECU.  (it is possible to run fuel flow instruments but you need to calculate the delta between fuel supplied and fuel returned to tank) The Rotax ECU fuel flow is "calculated" and not measured, it is done by counting the injector pulses.  The reading of your EGT is what will tell you if it is out of balance.  As you say the temperatures are normal so then don't worry.  The only worry would be a lean burn and you would see high EGT.   

    Just a note, the system is smart enough that should you fail an injector you will get a warning lamp.  The ECU will then double pulse the remaining injector to that cylinder to make up the lack of fuel so it will run as normal until you land and can address the problem.  Again you will not see a change in the EGT since the ECU will look after the fuel ratio to keep it running.  

    Cheers


    Thank you said by: Jonathan

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