912ULS High Oil Temperature

Roger -

"The only thing we saw from a 5600 - 5650 to 5800 at WOT was better climb,.............."

Better climb gives the pilot the option to lower the nose for less load and higher air speed which may assist in cooling, particularly in a  high ambient temperature.

An aircraft engine set up on WOT may compromise climb performance/terrain clearance, limiting the option of the pilot to lower the nose.

"... decreases in cruise speed...."

Of course but in an aircraft that probably struggles to achieve 100 knots this is hardly a big sacrifice.

"...and fuel economy."

In my limited experience, setting for Static, fuel econamy per hour improved/reduced . Of course the improvement may be lost by having a lower cruise speed/increased trip time. 

I am not suggesting that prop pitch is the only factor which should be considered - I support 100% your list of possible issue that need to be checked, including the doubtful performance of the liquid cooled oil cooler.

Hi Sean,

No argument from me I think we are just looking at it from different angles and perspectives.

I just received a new coolant radiator from the Ikarus supplier in the U.K. I figured its worth a try to change it out.

If I don't see any difference, I am going to try and figure out how to plumb an oil cooler system instead.

You could also remove the muffler, remove the baffle tube, you will find it has been red hot where the exhaust blows on it and it has distorted, this baffle creates back pressure in the muffler, increases the heat, the heat backs up into the cylinder heads, cooks up the oil in the heads, when you take rocker covers off you will see shades of brown in the alloy, the darkness is dictated by the heat/how much back pressure. Depending on how much back pressure relates to whether you feel a slight increase in power/fuel economy. If you are unfortunate enough to have a tight spot in swinging the propeller, it is likely to have been caused by excess heat in cylinder head travelling into crankcase distorting the cases.

The other issue I find a lot is the low pressure generator under the cowls in most cases is woefully inadequate which does not create enough draft thru the engine bay and or radiator or there a gaps around the outside of the radiator, AND/OR gaps around cowlings for air to leak, air will take least line of resistance ie I am not going thru that hot thing when I can go round the outside, much easier.

Rotax is not the only engine to have this issue I have found.

Onwards Terry

On the right in the attached picture is the new water radiator, the old is on the left. The new water radiator has one extra cooling channel compared to the old one. Not a huge difference, but a difference at least. Also - more surface area because of this fact as well.

I also fixed the exhaust leak - I changed a few exhaust springs and readjusted the exhaust pipe. I don't think the problem was that severe but its possible. I'll see where I am when I get the new radiator on and I am able to do a test flight.

As far as the radiator seal is concerned - it was totally airtight with no gaps. Originally the gasket was worn down and I sealed it temporarily with some weather stripping material and this helped the issue some.