912iS Prop Pitch Considerations

Wow, very interesting. I have never seen this discussed before.

I have not seen this discussed either, which is why I thought it would be a good topic. And I should mention my engine is the iS Sport. The Sport engine develops torque and HP earlier (at lower RPM) than the non-sport engine. That being said, there is very little information on how Rotax intends you to set up the prop to utilize that benefit. I would be inclined to think that this earlier torque/HP would allow you to run a bit more prop pitch, thus leaving more RPM range for cruise.  But in practice running more prop loads the engine such that you experience the issues discussed in my initial post. 

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The Sport engine develops torque and HP earlier (at lower RPM) than the non-sport engine.

The ONLY value that is important is POWER. (It is ALWAYS about Power!!!) 
Max Torque = Max Stress!!!
The Max Torque point is the worst point to choose if longevity is anywhere on your list of desirables.

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Recently I adjusted the propeller back 1 degree to 22 degrees.  Now I get 5150 RPM at takeoff and 5650 WOT at 4000’.  The switchover from ECO to POWER mode happens at about 5340 RPM at 25.5 MAP.  The plane feels lighter and climb is  a bit improved. I can also access the 5200-5300 rpm range while staying in ECO mode, which makes finding the right power setting easier.  The only downside is that at most altitudes moving into POWER mode will push the engine over 5500 RPM in level flight, so it can really only be used for takeoff and climb. So the plane is a bit faster now in ECO mode, about 111 kts true, but you can’t cruise in power mode, which yielded 118 kts with the factory prop setting.

You are going the right drection, don't backtrack.

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...my plane is now a bit slower overall, but faster in ECO mode, which is the way I would normally fly it.   It also has a better climb and access to the RPM range that feels the most comfortable ...

Flattening the pitch more will increase the ECO range speed.

Ideally you should remain in ECO mode at all times below 5500rpm.
The is no need to Cruise in Power mode, unless you are involved in Air Racing.

The only time you ever want to get into Power mode is when you absolutle need power at all costs ... Climbing, and especially when you need to clear an obstical in a hurry. 

Bill,

I agree with your thinking.  At this point I’m at 22 degrees, which is the lowest pitch angle that DUC allows (in their documentation) when used with the 912iS/914 engines.  To be precise, the stated range is 22-32 with a recommended starting point of 24. However, they do allow 20-28 degrees for the standard 912 with the same 2.43 gearbox.  I’m going to fly it for a while with different load profiles and see how it works under all conditions. However, the thought of the engine staying in ECO right up to 5500 RPM is intriguing. Thanks for the help.

Good discussion,

      I suspect that the IS was designed for a Variable Pitch Prop, and with a fixed pitch it is always a compromise, like having a car with only one gear. Surprisingly even with a variable prop I occasionally (once a year) find that at certain weights, heights, about 10,000 ft and temps (OAT of -15) I cannot find the setting I want between speed, rpm and eco mode. I have been doing some 500+ nm flights and eco mode definitely a good idea !