Gentlemen,
I think you are near the right track.
As I mentioned before once you are less than 97% TPS you are in the Stoichiometric or leaner "lean of peak" region. I am not Rotax, but any Engineer worthy of working for Rotax will have set up the system to be as lean as possible..without damaging the engine..with a good safety margin for bad fuel etc.
Therefore I believe fuel flow will be a suitable (from a general Pilots point of view) approximation for power.
For my Aircraft I did tests of rpm/MAP fuel flow at different airspeeds and density altitudes and referenced these back to the Rotax supplied performance data to create a table with a few ranges of "nice settings" they were 100%, 95, 90, 85 80 etc. I think I wasted my time doing this it was not really necessary. there are only about three settings that matter (see below)
Ross as you say, one factor here is "what you want". For some people burning and extra 5l/hr to get another 5 kts is well worth it, for others it is not worth the fuel.This depends very much on airframe, a nice slippery carbon fibre aircraft MAY speed up significantly with a few extra HP, a draggy "ragwing" designed for 65-80 Hp originally, may be already almost maxed out at 22 in MAP and the extra 5-8 L/hr may only add 3 kt !
I have Also settled on a cruise which is 15-16 L/hr at 5000 rpm.
So the three settings I use are
1) Full power (5800 rpm full MAP)no more than 5 min
2) Sustained power climb 5500 Full MAP or whatever I want
3) Cruise 5000 and whatever MAP I want depending on how much money i want to spend on fuel.
As regards fuel flow accuracy.
Again I am not Rotax. Having said that I have set up engines where the ECU uses a "referenced fuel injector" that is one where we as the engine manufacturer have "mapped the injector for voltage and fuel pressure" then the ECU is very very accurate at producing a fuel flow number from the pulse width. I would expect Rotax to be this professional as well.
To check this I have measured fuel usage (from a dipstick not the most accurate way) vs the Garmin G3x which takes fuel flow from the ECU can bus. I have done this over a number of 5+ hour flights and the two always agree within 2-3%. I have assumed, possibly inaccurately, that Rotax is correct and my wooden stick is off by 3%.
As regards Rotax instructions I do not find them unclear, In fact I find them simpler than most GA aircraft.No shock cooling, no mixture control and manual leaning 50 degrees lean of peak !. No letting temps stabilise before shut down.
Rotax states...
1) You can have 5800 RPM and full MAP for five minutes.
2) After that you can have 5500 rpm and any MAP you want/feel like for ever.
Additionally as an ex engine developer I would suggest that the most "efficient" RPM is 5000 (vs 4800 for the ULS I believe)
Where we seem to have a disagreement is that I see no reason or usefulness in any restrictions beyond that. You both feel that Rotax should instruct the Pilots as Geoffery says "on the way it has been done for 60 years"
I have an Ipad in the cockpit, GPS, and all the modern stuff, I am comfortable with moving on from 1956, I understand your reluctance, it is quite normal. Perhaps Rotax will produce something for Pilots whom want to do it the old way, but I would not stress yourself over it, you will not damage the engine so long as you follow the simple recommendations in the Manual.
I am very new to Rotax engines too, and we are all new to the IS engine. However my LAME whom has been working with Rotax/Lycoming/Continental engines for 30+ years said "it will not break like a Lycoming or Continental will"
