With the Rotax and Jabiru engines, do they like to be run at high rpms, or can you fly with power back(ie. 50 percent) for some slow cruise flight?

Thanks Gary

The 912 seems to run smooth at all RPM's but it is a high RPM engine. I'm not certain of the redline but it's close to 6000 rpm. The Jab is a direct drive engine much like a traditional aircraft engine and runs much lower RPM's. My two stroke likes 6400rpm

I've been told by the Just Acft factory folks that the 912 is best run (ie. longest life) above ~4800 rpm and preferably >5000. It will certainly run smooth at 4k and 55mph or so.

fwiw - low time Rotax dude.

K

The guys on the Rans group I belong to run their engines at 5200 RPM in cruise, maybe a little less when just lolling around the local area. Rotax engines like to run at higher RPMs and that's how they are designed.

I don't know about the Jab.

Same with the 914 (turbo charged and intercooled) used in the Stemme S10-VT motorglider. The engine likes 5200+ rpm w/ redline (IIRC) of 5800. Normal cruise climb is 5200-5400.

bumper

EAA Sport Pilot magazine has a short series of articles written bya guy who built a Kitfox-4 and hung a 4-cylinder Jabiru 2200 on it. He said 75% power cruise was 2930 rpm, with a 3300 redline. The 6-cylinder Jabiru 3300 appears to be about the same engine but with a third bank of cylinders, so I assume the rpm numbers would be about the same.
Those jabiru's seem like a pretty nice engine- the machining looks awesome. A guy I know hung one on his Avid after several hundred hours behind a Rotax 582 and he loves it-- has about 3-400 hours on it now & no problems.

Eric

hotrod150 wrote:EAA Sport Pilot magazine has a short series of articles written bya guy who built a Kitfox-4 and hung a 4-cylinder Jabiru 2200 on it. He said 75% power cruise was 2930 rpm, with a 3300 redline. The 6-cylinder Jabiru 3300 appears to be about the same engine but with a third bank of cylinders, so I assume the rpm numbers would be about the same.
Those jabiru's seem like a pretty nice engine- the machining looks awesome. A guy I know hung one on his Avid after several hundred hours behind a Rotax 582 and he loves it-- has about 3-400 hours on it now & no problems.

Eric

The big problem is the Jabiru is a direct drive motor and the powerband is set to the high end, ie 1/3 of the HP is in the last 15% of the rpm range. On a fast plane using a small diameter prop it works great, on a bush plane or one with floats the prop tips go supersonic on larger diameter props.
I am a machinist and I have to admit they are a work of art though...

Ben.

With only 160 hours or so on the Jabiru 3300, installed in an experimental J series Jabiru aircraft, I have had no issues at all running at 1500 to 2500 RPM while just putttering around the sky looking at things. as was mentioned, the power curve is sitting in the lower ranges, but that is all I need at that point. While 2K may seem pretty high RPM, the redline of 3300 rpm and cruise settings of 2800 make the 1500-2500 seem sedate. m additionally the 3300 is hands down the smoothest running (and prettiest) aircraft engines I have seen. Additionally, I have not experienced any hesitation when throttle is applied after such a lazy run.

KB

The 912UL (80hp) runs 5500 max continuous and 5800 for short periods of time.
-It's a geared, relatively high compression and short stroke engine so it's made to run at high rpm.
-With the gearing, the prop only spins at 2500rpm with the engine at 5500. As long as the prop tips stay below the speed of sound, then the slower the prop rotates the less efficient it is.
-And, engines generally run more thermodynamically efficient at higher rpms.

You can fly it all day long at 5000+ rpm. Wind 'er up and let 'er run.

Stol wrote:.....The big problem is the Jabiru is a direct drive motor and the powerband is set to the high end, ie 1/3 of the HP is in the last 15% of the rpm range. On a fast plane using a small diameter prop it works great, on a bush plane or one with floats the prop tips go supersonic on larger diameter props........

The small prop is a disadvantage for STOL work, but maybe with a properly designed prop blade the disadvantage is minimized. It does help keep the prop outa the weeds though.
What I don't like about small,high rpm engines with reduction drives to slow down the prop speed & allow for larger props, you usually also get water-cooling & sometimes multiple carbs. The simpler the better is my philosophy.
The Jabiru 3300 is about 200 cubic inches, just like a Continental O-200. I wonder how a different cam ground would work out- ground for more low-end torque, and about 100 hp at 2700 rpm max (again, like the O-200). The you could run a bigger prop, and still have the advantages of a lighter, tricker, (slightly) more modern engine.

Eric