RPM

Hey Sean,

I see that service letter SL-912-016 does suggest take-off RPM be set at 5200 minimum for the 912/914 series engines.  I have assumed this was not applicable to the injected 912, but I would like that confirmed.  The performance graph for the 912iS I posted earlier shows the takeoff phase starting at 5000 RPM. 

The 912iS engine makes its max torque earlier, at 5000 RPM, and in my experience you don’t get much for adding another 100-200 RPM for climb.  I could get about 5 HP more during climb by setting for 5500/climb.  However, there is another factor I have experienced. When I have tried settings up to 5300/climb, my exhaust temps during cruise go up almost to the limits, while water and oil temps run lower.  I attribute this to the engine being unloaded and thus sending more waste heat out the exhaust instead of into the cylinder heads from producing thrust.  In short, the engine did not seem loaded enough for the ECU programming.  As soon as I pitched the prop for <5100 RPM/climb, cruise EGTs went down 50-100 degrees.  This is something I would like to better understand. 

Hi Jeff,

My apologies my responses have all been for carburetted Rotax 9's - I have nil experience of the iS.

"my exhaust temps during cruise go up almost to the limits, while water and oil temps run lower. "

Speculation: In Cruise @ altitude, your ECU is sensing lower load/lower O2 and "leaning" the fuel mixture accordingly. This will see a rise in EGT but as the amount of fuel burning is less overall, the heat generated by the combustion process is reduced so your Oil & Coolant temperatures go down 

I'm  still a believer that with a GROUND ADJUSTABLE  prop that 5600 - 5650 is the BEST WELL BALLANCED WOT rpm in level flight at your average altitude for either the UL or iS series engine. Remember the manuals are also written for in flight adjustable props. Then yes higher rpms at take off is great since you can adjust the pitch after take off.  If you have need for a better climb prop then higher rpm at take off is good, but you will lose cruise speed, fuel economy and many times higher temps.

Bottom line is some in the world have the best prop setup with variable pitch props during flight, but if your's is only ground adjustable you need to make a decision on what you want. Either a balanced prop pitch for all flights or a good climb prop.

Hi Roger,

Me thinks your statements are a tad too black & white.

My Rotax 9 experience is limited to about 10 years on my last aircraft an ATEC Zephyr/912ULS, Fiti, 2 blade, ground adjustable, prop.

I set the STATIC to a smidgeon under 5200RPM / WOT. This gave the aircraft (full fuel 87L, 1 pax) a 100m ground role, on grass and a 1000+ft/min Climb Out (care taken to stay within the Rotax Max RPM/5 minutes). Cruise at 100 knots, 5200RPM. 12.5L/hr  Conservative fuel planning 14L/hr covered all weight, air speed eventualities. On rare occasions I would do a beach run at 500ft AMSL @ 120 knots, 5500 rpm.

I know the aircraft was capable of 125 knots but this would have required a STATIC of probably under 5000RPM/WOT. So I estimate I may have lost about 5 knots at an air speed I would not have used very often, if at all.

Fuel consumption was outstanding and I never added oil between services.

Engine temperatures were such, that a large temporary "cowl flap" was employed for winter conditions (under 25C)

Note: Air speeds are all as indicated ie subject to instrument error. Fuel consumption/sortie, calculated using graduated cylinder.