Belly Oil

Now 200+ Hrs & still getting "belly oil"

As stated oil consumption within the 0.06L/hr stated by Rotax BUT a lot more than my last 912ULS, when at  20 years & 950hrs old,  was still not requiring oil to be "toped up" between 50hr changes.

I have installed a T piece in the end of the oil breather - One end to atmosphere (belly), the other to a tube that draws air from the top of the engine side of firewall. IF the tube exit was subject to negative air pressure (vacuum) this should remove that effect.

Recently did a 13hr (Hobbs) round trip. Engine ran very well, temperatures & pressures all good. Most time in Cruise at 5000 rpm, at altitudes between 4500-8500 ft.

Constant moderate turbulence, both ways.

Started with slightly overfilled oil from 200 hrs service (Yes I know overfilling will cause excess oil to be blown out but does not explain oil consumption below Max).

Rotax say oil from Min to Max on stick, is 0.5L .

At 6.5 hr return pre-flight check, discovered level to be approximately 3/4 full (on stick). Did not "top up"

Post return - lots of oil on belly. Stick showing oil level a tad over 1/2 way up. Near 0.25L used or 0.019L/hr. Well within the 0.06L/hr but annoying never the less.

I did not "top up" and a few days later, went flying again, for 1.5 hrs. Oil level now below 1/2 - starting to make me nervous!!!

Previous 912 ULS attached to fixed pitch prop. Cruise 5200 rpm for a whole of flight fuel consumption of about 14 L/hr. Prop pitch set to advantage Take-Off. Engine likly "unloaded" in Cruise at 5200 rpm

Current 912 ULS drives a CS @ 5000 rpm for 15 L/hr. Engine loaded at all times by CS automatic adjustment.

Leak-Down-Test - all good.

If this was a LyCon I would expect such oil consumption but not Rotax - whats going on?😈

Hi Sean

Higher engine load does increase crankcase pressure and thus a higher velocity in which oil is returned to the tank. It’s easy to imagine this would vaporize more oil. It would be an interesting test to run the engine under lighter loads at cruise and see what the difference is. I guess that would take setting the CS prop for a lighter cruise pitch.  A CS prop does get more out of the engine, and it seems like there is often some other consequence to that.