Rotax 912 "Burping the Oil Tank" question

SSDriver · Post by **SSDriver** » Wed Jul 13, 2011 10:10 pm

I'm a low time student. I'm a car guy though and run a 950hp Drag car that uses a oil sump system so the concept is not a foriegn one to me.

The Sport Plane I fly has the Rotax 912. Below is part of the POH during Pre-Flight..

"Check oil level and replenish as required. Prior to oil check, turn the propeller by hand several times to pump oil from the engine into the oil
tank, or let the engine idle for 1 minute.

This process is finished when air is returning back to the oil tank and can be noticed by a murmur from the open oil tank.

NOTE: Oil level should be between max. and min. mark of the oil level gauge but must never be below min. mark. Before longer periods of operation ensure that oil level is at least up to midposition.
Difference between max.- and min.- mark = 0,8 l"

Ok, makes sense.. remove the cap, check the dipstick.. remove dipstick. Turn the Prop by hand (CFI says 15 times/quarter turns that is). Sure enough it will normally start to burp. Check the dipstick again.. still reading a little low.. Spin it some more.. still burps. And I think if I continued to spin it 100 times it would still burp!.

My question is.. What is best rule of thumb to use on when you can feel comfortable that it's good to go in terms of burping. Obviously I want my oil level at mid range on the stick but I dont want to over/under fill the oil level either.

My drag car is easy... remove the belt, stick the air ratchet on the pulley and spin it over! LOL

As far as the POH saying "...or let the engine idle for 1 minute"?!?! I can't get that to sound like a good idea in my head if you were to assume that the plane might not have been flown for 2-3+ days before hand.

Thanks for the education!

3Dreaming · Post by **3Dreaming** » Thu Jul 14, 2011 9:05 am

First off if you have oil on the flats of the dipstick you are good to go. The reason to burp is unlike your drag car ther is no scavenger pump. It uses cylinder pressure bleed by to push the oil back into the tank. The slower you turn the prop the more bleed by you have, let the compression do the work. The burp that you hear is the end of the oil coming from the crankcase, and now the cylinder pressure is just pushing air into the oil tank. When you hear the burp you are good to check the oil. Tom

SSDriver · Post by **SSDriver** » Thu Jul 14, 2011 11:08 am

Tom thanks for the response. So even if on the low side of the flat you should be within acceptable range? Fair enough. But is there a point where the 'burp' should pretty much be over with or because of the nature of the rotax will it just constantly burp?

drseti · Post by **drseti** » Thu Jul 14, 2011 1:23 pm

SSDriver wrote:is there a point where the 'burp' should pretty much be over with or because of the nature of the rotax will it just constantly burp?

Once you get a burp, if you keep pulling the prop through, she will keep burping. After a couple of good gurgles, check the dipstick -- further burping will not raise oil tank level significantly.

ibgarrett · Post by **ibgarrett** » Thu Jul 14, 2011 1:36 pm

I know our club mechanic (who's a Rotax A&P) pitches a huge fit if the oil isn't touching the top of the flat spot on the dipstick. I certainly understood that anywhere on the flat spot was o.k. - but at his insistence I make sure the oil is topped off anytime I go out.

3Dreaming · Post by **3Dreaming** » Thu Jul 14, 2011 2:52 pm

SSDriver wrote:Tom thanks for the response. So even if on the low side of the flat you should be within acceptable range? Fair enough. But is there a point where the 'burp' should pretty much be over with or because of the nature of the rotax will it just constantly burp?

You might get a little more oil, but the amount would be negligible. The sound you hear when it gurgles is air being pushed from the crankcase to the oil tank because there is no more oil in the crankcase to push.

FastEddieB · Post by **FastEddieB** » Tue Aug 23, 2011 10:57 am

Late to this thread, but...

...I find it MUCH more convenient to check the oil level after each flight.

I'd say it takes about 2/3 less prop pulling to get the burp if the engine has just been shut down.

Of course, that would only apply if you were the only one flying the plane. On a rental or multi-pilot plane I would always verify the oil level before the flight.

Jim Stewart · Post by **Jim Stewart** » Tue Aug 23, 2011 11:35 am

If I'm flying several times a week, I'll just pull the prop through 2 turns and take a look at the oil. It's always on the flat mark, but lower than if I gurgled the engine. I can't imagine a situation where gurgling the engine would cause the level to go down. I still do the gurgle thing at least once a week or if the plane has sat for a long time.

Pulling the prop through a couple turns gives me the nice feeling that there's no hydrostatic lock and lets me feel the engine compression.

drseti · Post by **drseti** » Tue Aug 23, 2011 11:58 am

Jim Stewart wrote:Pulling the prop through a couple turns gives me the nice feeling that there's no hydrostatic lock and lets me feel the engine compression.

True, Jim. Let's run some numbers. The Rotax 912 series is a four-cycle engine. So, to make sure all compressions are good, and all valves operating properly (IOW, to exercise everything in the engine), you need to run each piston through four strokes (up, down, up, down). Thus, pull through two full rotations of the crankshaft. The standard Rotax 912ULS uses a 2.43:1 gearbox ratio. That means the engine turns 2.43 times for every one revolution of the prop. So, to get two turns of the crank, you need only (2 X/2.43) = 0.82 turns of the prop. With a three-bladed prop, that means you need to pull the engine through (0.82 x 3) = 2.47 blades. So, to check all compressions, exercise all valves, etc., you should be pulling the engine through about 3 blades (at a minimum; more wouldn't hurt). That would indeed give me a nice feeling.

deltafox · Post by **deltafox** » Tue Aug 23, 2011 3:52 pm

I've been burping her on the start of each day, even if the oil is on the flat. Summertime temps take about 20 full rotations, winter doubles that. Am I wasting my time (and effort)?

drseti · Post by **drseti** » Tue Aug 23, 2011 3:55 pm

deltafox wrote:Am I wasting my time (and effort)?

No; think of it as your daily cardiovascular workout!

designrs · Post by **designrs** » Wed Aug 24, 2011 6:17 pm

So in reviewing the post at the top of the thread... POH...
"or let the engine idle for 1 minute"

Wouldn't it be easier to just run the motor for 1 minute....
especially for those who keep their planes outside and have to
hand prop like crazy on colder days??

Safe and acceptable for motor and pilot?

drseti · Post by **drseti** » Wed Aug 24, 2011 8:08 pm

designrs wrote:Wouldn't it be easier to just run the motor for 1 minute <snip> Safe and acceptable for motor and pilot?

That would certainly work for burping the engine, but I still recommend turning the prop through by hand, about six blades worth, just to feel the compression in each cylinder and help to spot problems before they become problems.

designrs · Post by **designrs** » Thu Aug 25, 2011 3:05 pm

OK great. So the plane is cold. Maybe even a cold day.
Run the motor for 1 minute.
Shut off the master & remove the key.

Check the oil. Hand prop 6 times, instead of 30++.
Check the oil again.

Anyone see any problem with this?

drseti · Post by **drseti** » Thu Aug 25, 2011 3:09 pm

designrs wrote:Anyone see any problem with this?

Not so long as you leave the oil tank uncapped, so you can hear the gurgle.

Now, remember, when somebody sees you on the ramp pulling the prop through, and asks you what you're doing, the correct answer is:

"winding up the rubber band."