We had RPM discussions before.
Low RPM is good! It keeps wear down – as long as temperature is within the specified range and as long as there’s no limitation on either engine or airframe.
It’s no problem to run a 912 on 4400 to max 4800. All our engines go far beyond TBO (closely monitored) and are typically operated in that range of RPM (the rest is landing training).
Thanks @europaxs . Actually, as you know, for a C/S prop Rotax suggests one can safely use a range of MP/RPM combinations:
I personally use 26/4800 for a compromise between noise and speed, and not interested in a few knots more with higher RPM, unless fighting a horrible headwind.
Maybe we should stop here, we’re highjacking the thread..
@aart – I think discussing possible cruise settings is okay in the context of the thread title ;-)
You’ve quoted the other “official Rotax document” for engine operation yourself (IIRC it’s from a Service Letter).
Not offending anyone, but I don’t understand why this is discussed to death when there’s excellent documentation and guidance from the engine manufacturer.
Just chiming in to say that I personally don’t mind thread hijacking – the main question has been answered after all and I find the discussion relevant.
UdoR wrote:
Low RPM is good! It keeps wear down – as long as temperature is within the specified range and as long as there’s no limitation on either engine or airframe.
Yes, within limits. But 7l per hour will be in the 2500 rpm range with a constant speed prop according to the Rotax manual. You are producing about 10 hp. We are not talking 55% power, but 10% 
55% power can be achieved from WOT at around 3800 rpm all the way to a more sensible throttle opening at around 4600 rpm. Rotax recommend 4300 rpm. There’s no recommendation from Rotax to run the engine lower than 55% in cruise.
With the Risen, cruising at 7 l/h probably can be done just fine. But, what have they done to keep the engine warm in that condition, at 200 km/h at FL100 ? The Alphatrainer can also “cruise” at 70 knots IAS, and use less than 7 l/h. It’s not something anyone does, because within minutes, the engine is all cold soaked.
I tend to split off threads when a new valuable topic is being discussed.
Surely much of these figures 7 l per hour are more down to aerodynamics of the airframe itself. The 912 (not i) on the Super Guépard tends to cruise at 180km/h at 4800rpm which is POH recommendation. At that we use use 13litres per hour.
The yellow line is at 160km per hour which is at or around 4000 RPM and we get around 11 to 12 litres per hour (difficult to be accurate with such a basic fuel system.
In circuits we are getting around 10 litres an hour. Basically everything is done at 110km per hour
Full power take off with Vr at 60km hour
Climb Vy 110km per hour 1000ft per min power to idle at the end of downwind leg to landing.
So 7 litres per hour cruise I would say is pretty damn good, perhaps it’s because of Swiss precision manufacturing.
But that quality does cost a lot more than the price one pays for a new SG.
200 km/h
Which would be commonly translated to 108Kts… not sure many a Risen pilot will be found loafing in the sky in this style. That would be slow, even to my standards (whatever that means) 😂
A 1 second look at that most famous Super Guépard vs the Risen explains it all 😉
