There seems to be a wives tale going around about not cruising below 5000 RPM which I beleive to be FALSE I cruise my Rotax 912-S at 4200 RPM all the time, it reduces noise and fuel usage is much lower, and the engine seems very happy with almost 300 hours of this. I have read the Rotax manuals looking for limitations, and there are limitations on cruise RPM. There are RPM limitations listed, such as not idling below 1800 RPM, and 5500 maximum continuous RPM, so obviously thy did not just forget this issue, but there is NOTHING saying not to cruise below 5000 RPM. So it seems some guys have just " Invented ", or heard from someone else thier own limitation and pass it on as fact. Ever seen the AirCam twin engine Plane ??? That plane has two 912-S on them and they cruise them at very low RPM using about 3 1/2 gallons per hour per side as normal cruise procedure, Lockwood sells that plane...
My guess is that someone very correctly said not to have a high throttle setting and then pitch the prop to below 5000 RPM cruise, that would be lugging the engine, which would be very bad. Some simple minds probably just understood " Dont cruise below 5000 RPM " and spread it around, which is why there I think there is so much misunderstanding about this among the Rotax community. You should NEVER prop your engine to the point where it is lugging at low RPM and high throttle, This is very bad... If you have the prop pitch set correctly, and you pull the power back to a fuel saving, less stressful less than 5000 RPM cruise, I do not see a problem.
Rotax centers do not always get it right either, you may get a different stroy from every guy you talk to, guys that work at Rotax Centers could very well be subject to the same type of misconceptions as everyone else. Until I hear it DIRECTLY from the Rotax engineers, and they have had 20 years to put this limitation into the manual if it were important, I will continue to cruise below 5000 RPM and a low throttle setting.
The above is just what I have found out based on hearing the same garbage and researching it as much as possible over the years. So the above is just my OPINION. Just as guys that try to add thier own limitations to the Rotax manual are merely stating opinion, which I beleive to be with good intentions, but misguided.
Mike