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So, the Max Continuous Power is 95% of full rated takeoff power, and this is obtained at full throttle and 5500 rpm, right? At full throttle and 5500 rpm my G3X often indicates 97-98% power. Do I need to pull the throttle back so as not to exceed the 95% MCP limitation? One trusted source told me this is not necessary, as the 915 is incapable of exceeding 95% when it’s limited to 5500 rpm. Is the percent of power indication on the G3X just an estimation, or is it directly reported from the Rotax ECU?

What say you?

  • Re: Max Continuous Power

    by » 9 months ago


    Hi Mike

    The injected engines have channels that report to the ECU.  One channel is the throttle position sensor, TPS.  For ECO mode the engine has to be off of WOT, usually down to about 97/96% to allow it to have the ECU run the engine in eco mode to save fuel.  The ECU looks at WOT as take off power and that is going to be a rich condition.  If you have a adjustable, or constant speed propeller then 5800 is the max operating RPM for no more than 5 mins.  After that you have to pull back to 5500 RPM for maximum continuous power.  This is not necessarily 95% power, it relates to RPM and it should always be below the limit for ECO mode operation.  Prop loading is an issue especially with a fixed pitch.  With fixed pitch you must compromise and limit RPM to below 5500 at takeoff to compensate for prop unloading as you have more forward airspeed.  (unloading the propeller) the target with fixed pitch is not to exceed 5500 in level flight WOT with fixed pitch.  In that case you understand that the propeller is your governor to control overspeed of the engine... 

    So with that said.  The ECU looks at the load and you need to not ever exceed 5800 in any condition.  (on adjustable props takeoff power only for 5 mins)  The RPM for max continuous has to be 5500 or lower (lower is better fuel burn).  The TPS number value is reported on the CANBUS to the instrument and on most instruments they are displayed, this is helpful especially to know for starting position of the throttle and if you are into ECO mode of operation.  The percent shown is not an HP value, it is only the position of the TPS relative to the engine power and the opening of the throttle.  It is not a linear value.  This is seen when we have a 912iS at 100 HP, WOT is 100 %.  The type 915is WOT is 141 HP and 100%.  Type 916 is WOT 100% and 160 HP....so no it is not really an HP reading. 

    Cheers. 


  • Re: Max Continuous Power

    by » 9 months ago


    I'm not talking about ECO at all. I'm talking about MCP, which is clearly well above ECO and is commonly used in climb. I'm also not taking about TPS number, I'm talking about the percent power indication on my G3X. (Yes, G3X indication is percentage of POWER, not TPS. TPS is only shown when the engine is off or if I go into the FADEC menu.)

    I'll try to restate my question more clearly. I have a 915 and an Airmaster constant speed prop. When I have the prop set to "Climb" it is programmed to never exceed 5500 rpm. When I have full throttle and prop set to climb, my G3X indicates 97-98% power. 

    Question is, is the G3X percent of power indication actually correct, in which case I'm exceeding 95% power? Or, is that indication just an estimation, and it's no problem leaving full throttle as long as the RPM is 5500 or less?


  • Re: Max Continuous Power

    by » 9 months ago


    Mike...?  There is no output of the CANBUS that gives power.  Power is torque (T) X RPM/5252 =HP.  The CANBUS only gives out RPM and %TPS output.  Perhaps this is something within the G3X but I have reservations regarding this.  The power in the torque side is by boost pressure and that is not transmitted on the CANBUS to the instrument side, that is internal to the ECU only.  I am not sure if that can be looked at by the log files from the ECU reading with a BUDS reader.  I will check that however.  On the 915 the max HP drop to max continuous, 5800 too 5500, is only a few HP.  The limitation with the Airmaster is that you never have 100% power as this can only be attained by getting the full 5800 in climb as you would with a constant speed.  If you are in climb at 5500 you have a few HP short of the max output so it is not a bad HP for that.  

    Sorry if I am confusing the issue.  The engine has 141 HP at 5800, 100% power.  At 5500 it produces about 134 HP, that is about 95% power.  I have no idea where they get 97/98% ? You can run it all day long at 5500...but fuel burn is  higher than you really need.  Were we saw the best compromise with most propeller types was flying using peak torque, 5000 to 5200 RPM.  Usually gave plenty of speed and gave us good fuel range.  (Aquilla with 915 and constant speed MT propeller) It was fun to fly using fuel burn to get performance and save fuel.  You can fly nicely at 5 to 5.5 gallons per hour and still get great performance at 7500 flight elevation. 

    Cheers


  • Re: Max Continuous Power

    by » 9 months ago


    Seems you're unfamiliar with Airmaster (which is one of the most popular props used with the 915). In the Takeoff position it's governed to 5800 RPM, so it does get 100% power for takeoff. After a max of 5 minutes (per limitation), pilots switch to Climb position, where the RPM is limited to 5500. That's where my G3X is showing 97 - 98% power. Since you say there's no CANBUS output for power, I can only assume the G3X is estimating this.

    Your statement that "You can run it all day long at 5500" is the confirmation I was seeking. So, it sounds like full throttle and 5500 RPM is complying with the MCP limitation, and if the G3X shows greater than 95%, it's just an incorrect estimation.



  • Re: Max Continuous Power

    by » 9 months ago


    Hi Mike

    You are correct, I misunderstood you and thought that the Airmaster did not go to full 5800 (100%) in climb.  I was thinking it was only 5500.  So sorry and thanks for pointing that out.  I have not flown with an Airmaster myself, only fixed pitch and constant speed.  

    The specs call out 5800 as 141 hp. (105 kw)   5500 gives us 136 hp.  (99 kw) so 141 X .95 = 134 hp.  (105kw X .95 = 99.75kw)  I would not worry about the few HP differences.  So you are right in that the G3X must have an incorrect estimation.  We know that Rotax states you can run it all day long at 5500 without issue.  Of course fuel savings are much greater at 5000/5200.  You would still be on the good side of the power curve there and fuel burn will be significantly less.

    Cheers


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