"Oversquared" Powersettings

No The correct rpms put less load on the engine.

This is a Rotax operate it as Rotax reccomends

I think even 5000 rpm is too low for a 915. You'll get better speed while putting less strain on your engine cruising at 5450 rpm. Lower RPMs aren't going to make much of a difference in terms of a "quiet cockpit," and who doesn't wear ear protection these days anyway?

Hm I find 4500 RPM to be significantly less noisy  than 5500, and a more comfortable on the ears and overall fatigue after two hours than running at 5000. I do wear a Bose A20.

So apart from these subjective impressions of what is loud, do you have any objective, engineering based thoughts on potential damage risk? Why do you say 5000 RPM is "too low"? That seems kind of arbitrary when Rotax themselves specify this as their 75% power setting?? 

The application is in a fixed wing aircraft by the way, not a gyro or anything.

The mathematics and peak loads on crankshaft, conrod and the bearings.....literally fills a text book.

The factors are numerous related to geometry lubrication gas pressure in the cylinder at each phase and under various loads.....also another text book.

Professional engine design companies employ qualified engineers to do these calculations and then design the engine and operating conditions accordingly.

I was privileged to be involved with a small number of engine developments, the calculations for the full design before the CAD start took several months. The Dyno testing several months or years more after the build was complete.

Trying to give a simple one sentence answer as to why these settings from the manufacturer are best used and also why more RPM is often less strain on the engine is just not possible.

In the end you are the person whom controls the engine in flight...you can do as you wish. However if the engine goes bang as the over strained Conrod exits the engine then all quiet on you one day at the most inconvenient time you also have to deal with that too.

Hi Mike!

While I don't doubt that you & Evan have figured out that 5450 is better than 5000, what's interesting is that the Rotax 915 operator manual gives the impression that the engine *should* produce more power at the same fuel flow at 5000rpm vs 5500.

Consider lines 284, 834 & 844 in the Rotax 915 operator manual (pages 115 & 127):

Number rpm p_amb T_amb Throttle alt power FuelFlow p_Plenum T_Plenum - RPM bar C % ft kW kg/h bar K

284 5500 0,724 -3 94,1 8999 56,08564836 15,46 0,8869 274,65

834 5000 0,724 -3 97,4 8999 70,4409981 19,24 1,0936 274,85

844 5000 0,724 -3 94,1 8999 54,23631209 14,65 0,8955 274,65

(at 9000', standard atmosphere) if you extrapolate the two 5000 RPM numbers so that they produce the same fuel flow (15.46) as the 5500 number, (which is reasonable since the power chart on page 77 is a straight line at this point) it looks like 5000 @ 94.6% power should produce 57.08 KW, or a full KW more (or 1 1/3 hp) at the same 15.46 kg/h fuel flow that at 5500 RPMs produces 56.08 KW,

That seems consistent throughout this chart - that 5000 RPM is more efficient than 5500 RPM.

Why that is not the case in practice (at least for Slings) is intriguing...