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

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.

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

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.

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. 

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