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Prop pitch and the Rotax 912 ULS minimum takeoff rpm

Brett H

Active Member
Rotax states “Take off RPM at WOT (wide open throttle) should not be below 5200 rpm to avoid over loading the engine.”

Having set my prop pitch to the 71.4 degrees specified in the Van’s KIA, my WOT takeoff rpm tends to be in the low end of 5,100 - 5,150 rpm range. Flattening the prop pitch to 72.0 degrees has resulted in little change of the WOT takeoff rpm.

What pitch have people set their two blade propellers to on their 912 ULS engines to meet this requirement?

Thanks.

Brett
Columbus, IN N4BH
 
Brett,
I had the same issue. I have a 912 ULS. I used the Sensenich prop tool.
I set the prop for 5250 on the ground.
This gives me 5340 rpms in the climb and 1k per min climb rate with passenger in the winter (800 in the summer with passenger).
Level flight she will overspend by about 50 rpms (I don't remember exactly)so there's is some extra throttle there.
Something I do is measure from blade tips in on both blades where the tool goes and record that measurement. I then put a strip of tape chord wise so that I can put the measurement tool in the exact location everytime. Just a little movement inboard or outboard will change your reading. Could easily be off by .6. I would try adjusting a little more flatter ( get a noticeable rpm change) and then go from there. I adjusted my 2 blade about 6 times before I was happy....
Good luck!
 
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The recommended pitch value is one that has been found to be a good compromise for most owners. It will result in a climb RPM that is just slightly below the 5200 recommended minimum.
There has been lots of discussion about this here in the forum that you can find with a bit of searching. There has also been discussions with engineering from Rotax and the Rotax tech reps for north America, and they have stated they are fine with 50 rpm below the recommended because of the coolant temps that the RV 12 runs at.
 
Thanks for the responses. Having reviewed the archives, I was unclear about the Rotax statement about WOT takeoff rpm needing to be above 5,200 rpm.

Referencing the discussion with Rotax about the installation of the 912 ULS in the legacy RV-12 was very helpful. Based on this, I will stop attempting to flatten my propeller pitch to achieve at least 5,200 rpm on takeoff.

Also, Scott I appreciate you taking the time to answer questions now that you are retired.

Brett

Columbus, IN
N4BH
 
they have stated they are fine with 50 rpm below the recommended because of the coolant temps that the RV 12 runs at.

Does that imply that coolant temp/CHT limits should be reduced from the Rotax values?

The rpm limit is presumably based on a particular corner of the envelope, probably sea level with CHT and coolant at temperature limits and fuel that only meets the minimum specifications. That doesn't necessarily come from steady state flight where the coolant temperatures are predictable.

One of the previous discussions came after someone was held on the ground in hot weather, took off with coolant and/or CHT approaching the limit and had an almost immediate engine failure.
 
One of the previous discussions came after someone was held on the ground in hot weather, took off with coolant and/or CHT approaching the limit and had an almost immediate engine failure.

I don’t know which particular incident you are referencing…. The RV-12, with tightly-cowled engine, shouldn’t be subjected to long ground ops. Very little air is introduced into the small cowl openings. Long taxi downwind reduces cooling air even further. Fuel manifold hoses are run top-side on this engine, and if using auto fuel, a higher chance of vapor lock can occur. Perhaps the incident you mention was just that and not necessarily high engine temp in and of itself….
 
Does that imply that coolant temp/CHT limits should be reduced from the Rotax values?

No

I am only aware of 2 engine failures in the 14 year operational history of the RV-12.

On the first one a clear cause was determined to have been improper assembly by the builder that caused a serious induction leak which made the engine run very lean.

On the second (the one I think you are referencing), no absolute cause was ever determined because for unknown reasons the owner partially disassembled the engine before shipping it to a Rotax service center to have a diagnosis inspection completed. It arrived at the service center without all of the components.
I spoke personally with both of the U.S. Rotax technical reps. that were involved in evaluating the issue. If I remember correctly (it has been a long time so I may not be....), they had an opportunity to look at the EMS data download for the engine and look at the partial engine at the service center, and they were both adamant that the damage that occurred was not because the engine was at less than 5200 RPM in a WOT climb.
We specifically asked then and in meetings with Rotax engineering at later times whether we should be worried about being 50-60 RPM short of the 5200 RPM minimum recommendation, and in all instances we were told no.

With 14 years of history and almost 800 RV-12's now flying, and a large # of them likely using the recommended base line propeller pitch angle value, I am not aware of any other issues.
If anyone knows of another, please let me know.
 
Rotax states “Take off RPM at WOT (wide open throttle) should not be below 5200 rpm to avoid over loading the engine.”

Having set my prop pitch to the 71.4 degrees specified in the Van’s KIA, my WOT takeoff rpm tends to be in the low end of 5,100 - 5,150 rpm range. Flattening the prop pitch to 72.0 degrees has resulted in little change of the WOT takeoff rpm.

What pitch have people set their two blade propellers to on their 912 ULS engines to meet this requirement?

Thanks.

Brett
Columbus, IN N4BH

I ended up at 72.4 pitch after many attempts to get to 5150 rpm on climb.
Started at 71.4 and was at 5000 rpm.
Maybe that helps you.
 
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