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Eureka! Oil siphoning Rotax 912ULS

 
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JonathanMilbank



Joined: 14 Apr 2012
Posts: 383
Location: Aberdeen area

PostPosted: Sat Nov 03, 2018 3:38 pm    Post subject: Eureka! Oil siphoning Rotax 912ULS Reply with quote

Since I wrote about my epiphany re oil siphoning, I've repeated the process of gurgling/burping after flight, last thing before putting the aircraft to bed and I'm happy to report that the system works. The oil level doesn't move down by the next time that I look at the dipstick. It can't, simply because the siphoning gets interrupted by introducing air into the return oil line. So no turning of the propeller required to check oil level before the next flight. Also far fewer propeller hand rotations required overall.

This is probably old news to you and I'm not looking for advice or help; just passing on my observation. My engine has around 300 hours and is going just fine with four similar compressions, apparently making as much horsepower as ever. Only thing is that it has usually needed quite a lot of rotations when gurgling/burping, to push oil back into the tank. But not always!

Somehow I came to realise that infrequently the oil level would stay unchanged near the top of the dipstick flat for days on end. Mostly my experience has been that the oil would sink away overnight down to about 1/4 inch below the flat, then never go lower no matter how many days had passed before the next flight.

It seems to me that the difference could be explained by siphoning of oil out of the tank after flight, which stops once the siphon is broken when the level in the tank gets below the end of a metal tube, perhaps? Or that the siphon can't be sustained after an air bubble from the crankcase finds its way up an oil hose? This might account for the oil generally not dropping below 1/4 inch below the flat, except when I choose to do the gurgling/burping after flight and then the level remains much higher!

If I perform the gurgling/burping about an hour after flight, it would be because I had decided to top up the oil level before going home. I can only imagine that the siphoning would be going on during that hour until I start turning the prop by hand, in which case it takes very few rotations to hear the gurgle/burp telling me that air has been pushed back up the hose. This air could likely prevent siphoning. Another factor might be that while the oil is still warm, any siphoning action is less likely to restart when the oil is less viscous thus allowing bubbles to travel more easily.

Maybe the routing of my oil pipes is a little different from yours and therefore you wouldn't experience this phenomenon the same as I do. I can't swear that it always behaves this way for me, but from now on I'm going to try to remember to do the gurgling/burping about an hour after flight until I've gained enough of a history.


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