Belly Oil

Hi Sean,

You do not want to run an oil vent tube into an airbox intake. That would end up being a huge mess. Is your oil tank vent out in the air stream under the belly or cut back approx. 2" inside the cowl?  I don't know of a specified oil catch can, but I have seen some make their own. It seems to be fairly rare. I don't have any clients that do that. You'd have to make sure it has easy air flow and no vacuum or pressure issues that may affect the tubing flow. You should be able to have the oil level at the top of the flat area on the dipstick without any issues. Everyone I know gets some oil vapor residue on the bottom of their plane. I don't know anyone with a clean belly. Yes some are better that others.

Tim B wrote:

Someone must have thought venting the oil tank into the induction system was a "Very Bad Idea", since it's been specifically prohibited for TC'd aircraft in the US since at least 1945. The prohibition was retained in various iterations of US airworthiness standards (and later EASA equivalents) until they were changed from prescriptive to "performance-based" standards in 2017. Unfortunately I can't find any official records of why they thought it was a bad idea, which isn't helpful for assesing whether it's actually risky or not in an experimental aircraft.

I might be stating the obvious, but have you tried running a lower oil level? Until 2003 (refer to SB 912-040), the "max" level on the dipstick was 20 mm lower than the current "max" level. I've found that in my aircraft, running the oil level at about halfway between the "min" and "max" marks makes a noticeable difference. (That's still 5 mm above where the "max" level was when my aircraft came out of the factory)

Hi Tim,

Veeeeery interesting comment.

It is not unheard of, for Government bodies to issue prohibitions/directives that are later found to have no scientific foundation - this may just be one of those.

Oil level - after 120+ hrs of operation, I am running my oil at about 1/2 way up the dipstick. This may have reduced the oil misting on my belly.

It is the above observation/response, that lead me to ask the question about oil tank position. My tank is within the Rotax recommended height range but on the high side. 

Hi Roger,

"You do not want to run an oil vent tube into an airbox intake. That would end up being a huge mess."

Hmmmm!

I think that, if I were to explore this idea, to specifically reduce "mess" I would position the breather:

                 1/2 way between the two carb intakes

                  on the up side or top of the airbox

The existing drain holes would deliver any oil residue, pooling in the bottom of the airbox,  to a vented sump can.

I would anticipate most of the breather vapour to be consumed in the combustion process.

I have no doubt of the process would work (in the short term) but wonder would ;

                 the breather vapour have an adverse effect on the carburettor/inlet valves?

                 the pressure (+/-) inside the airbox have a negative effect on oil breather function?

😈

Venting the hot combustion gases into the airbox will give reduced power output would be the main downside. It might also create a vacuum in the oil tank which may affect oil supply to the pump. Maybe explore the root cause of why it's blowing oil out the breather, I am not sure yours is normal.

(Second go)

Hi Murray,

"Venting the hot combustion gases into the airbox will give reduced power output would be the main downside"

I would expect that the amount of "combustion gases" entering the induction system, to form a very small component of the air:fuel mixture and the tiny amount of crankcase oil, adding to the combustible mix. The heat ? yes could have an impact, much like the heat of a summer day or  Rotax cone filters sucking hot cowling air.

This way of reducing contamination of the environment has been around for along time now.  I have never heard that there is a noticeable power reduction in ground based engines so configured. 

" It might also create a vacuum in the oil tank which may affect oil supply to the pump."

Given that the tank cap is not "airtight" I would expect that it would take a considerable +/- pressure, to make a significant difference.

"Maybe explore the root cause of why it's blowing oil out the breather, I am not sure yours is normal."

Oil consumption well within Rotax specifications - I log my single small oil addition/service interval.

Crankcase breathers have always ejected a mist of oil. I would go so far as to say that ALL operators of IC tractor engines, are irritated by the mist of oil that ends up on the belly of their aircraft. The exception would be aero engines fitted with catch cans (they do exist) or where the breather is routed back into the air inlet (don't know if this is done).

I would be more concerned about the potential for oil coking up the back of the inlet valves/stems and/or causing metering difficulties in the carburettors, than any of the above.

I would have thought that this solution, to breather excretions, would have already been explored by Rotax and  a "known" rather than speculative response given.

😈