Oil Breather Tube Orientation

Sean

Air balance within the cowling should give you a neutral pressure.  The inlet cowl area vs the outlet will determine if you have low pressure at the outlet.  Where you mount the vent line exit is critical and I suspect you perhaps have the line in a low pressure zone which may be pulling oil vapor from the tank.  i have fixed many aircraft for this issue over the years and almost always, other than simple overfull systems, it is just too much low pressure at the vent outlet.  Cooling outlets work best if they have low pressure behind the cooler to draw the air through the fins.  If you mount a vent line there it could be a lot of the problem. 

Perhaps try moving it to another part of the exit air were there is less airflow. I still recommend the notch.

Cheers

Thanks RW,

The "notch" is such an imprecise recommendation.

My T piece variation on the (reducing negative pressure effect)  concept should  work, as well if not better IF the air pressure at top of firewall/under cowl is actually high, relative to where the oil breather exit is located.

😈

Rotax Wizard, I would be curious to know if you would have a recommendation for oil tanks that are embedded in the nacelle... like those Tecnam P2006ts?   I know for a fact they never came with notches, but I do something like that with all the other birds I work on because our temps get downright chilly (-30C is not unusual) so breathers freezing is a concern especially as that is key for oil return.

As for the OP if you would like a preview of the page and I am sure you have seen it, you can technically have low spots, but it recommends a drain bore to prevent "condensate"

Hi Sean,

Like Jeff says, I haven't noticed any condensation escaping either – at least not that I'm aware of. My oil vent hose runs downwards and ends near the air outlet on the engine cover, but still inside. This is certainly not an ideal location, but I believe that the air pressure conditions there play a minor role, unless there is very high negative pressure, which is unlikely. The exhaust and vent hose are not far apart, so there is no chance of any water condensation freezing.

Water evaporates/condenses/freezes, oil does not. If oil droplets and water are mixed and escape as vapor, the oil or oil droplets should be slightly foamy or slightly lighter in color. If something is escaping from the vent hose, this should be investigated first.

Why is anything coming out at all? Are the height levels of the crankcase and oil tank/vent hose correct, oil pressure/oil consumption too high, water consumption too high, how many revolutions/compressions do you need until the engine gurgles during the morning oil check?

If everything is OK, where is the excessive pressure in the crankcase coming from? Like RW says, measure the pressure. Blow-by is probably the biggest cause of the pressure, but could evaporating water also be a factor? So first investigate where the excessive pressure is coming from and whether water is playing a role.

It can't be your flying style, you're not doing aerobatics or hanging the plane so steeply on the propeller that gasoline and oil are running out of the tanks, are you? :-) Surely not.

Now to your thoughts on a pressure-neutral location where the vent hose should end. This is the same problem as with the carburetor vent: you won't find a completely stationary, pressure-neutral point inside the cowling. At best, you'll find a place where the constant pressure fluctuations cancel each other out, i.e., the average value of the fluctuations results in a pressure that may correspond to the external pressure. A stationary location would have to be constructed in which the pressure fluctuations are dampened to such an extent that the pressure is practically stationary.

Where overpressure and underpressure prevail on the aircraft, especially in the front fuselage, is only clear in theory. There are areas on the aircraft, especially in the front fuselage area, where you can experience surprises, which can change significantly with the angle of attack and airspeed. However, explaining this in more detail is beyond the scope of this article.

My opinion is that the pressure conditions at the end of the hose should not be taken too seriously.

To avoid contaminating the fuselage, let the end of the hose terminate in a suspended transparent plastic bottle. Then look for the actual cause and report back. Your thoughts are of interest.

Greetings

H iHKK,

Thank you for such a long & considered response.

My last 912 ULS (1999 model) was just short of 1000 hrs,  did not require oil top up between changes .

This new  912 ULS  (2022) 185 hrs does require topping up (about 100ml/50 hrs) which is well within Rotax consumption specifications.

So far oil removed at service has been just short of 3L and 3L added.

I have checked the oil tank height several times and believe it to also be within recommended specifications. I have previously asked this Forum if high/low tank position correlates with oil consumption - I do nor recall getting a relevant unswear.

In its 185 hrs of operation, my cooling system has not shown any consumption of coolant.

Water vapour within the crankcase is common to all engines - this is one of the reasons for advising a 100C + oil temperature at least once in an operating day, to drive off volatile fractions.

I can get an oil tank "gurgle" within two revolutions of the prop - subjective compression feel is even.

I did have a home made "catch-can" on the oil breather - didn't catch much oil. Has been removed, pending installation of a revised design.😈