Rotax Oil Check / Gurgle

[rhanson]

After seeing a YouTube session about engine oil checking, I am beginning to

wonder about the standard procedure.

 

The author suggested that you can hear the gurgle sound almost immediately

after prop rotation, if this procedure is performed following engine shut-down.

 

Sure enough, this works, and full oil quantity shows on the dipstick.

However, while reviewing the "Oil Servicing" procedure, I noticed the

reference to "purge the oil from the engine":

 

"Before adding oil to the engine system purge the oil from the engine by

rotating the propeller by hand",  ( in my case, no need to add any oil )

 

Now I am concerned about the resulting "lack" of oil in the engine for 

a day or a week before the next flight.

 

Is there a benefit of leaving the oil in the engine while not running ?

 

And is there a downside to starting an engine after a period of time,

after the post-flight purge of the oil ?

 

Thanks

 

RH

 

[Anticept]

I think you are mixing different procedures.

 

First, Rotax uses a dry sump lubrication system. That means that oil doesn't stay inside of a sump on the bottom of the engine, rather it is moved to an external reservoir. They do this by using the piston ring blowby, which pressurizes the crankcase and pushes the oil out. You also do this when you rotate the prop by hand. Note: rotating that prop like a hamster on steroids won't make it go faster, just rotate the prop slowly, and give the air time to push past the rings.

 

Now, what this means is, if you rotate the prop by hand after shutting down, a properly working system should not require a lot of prop turns before you get the gurgle. This is more of a troubleshooting and service thing, and not really a normal procedure after you've flown unless you suspect a problem. I had a very brief look at the 912 uls operator's manual, and I don't see it stating that this is standard procedure after a shut down anyways.

 

Now, what IS important, is you do this before you CHECK or SERVICE the oil. This means checking before you fly, when you add oil, or when changing it. You need to make sure excess oil is all in the reservoir. If you have oil remaining in the crankcase, then it means it won't show up on the dipstick. When checking or servicing, it means you're going to add too much oil to the lubrication system. When the extra oil is pushed out of the crankcase, it will overfill the reservoir and it's just going to be blown out of the breather tube and all over your airplane's belly. In extreme cases, significantly overfilling might also make it more difficult for air to separate from the oil, which is BAD (foamy oil does NOT lubricate!). When changing the oil, doing this purging pushes the excess oil into the reservoir to drain. If you don't purge first, you'll overfill the system.

 

Note: there's going to be a little bit of oil that you can't get as some will stick to the crankcase and slowly drain to the bottom. It's also possible for a little oil to backfeed from the reservoir into the case. This isn't a big deal as long as it's not EMPTYING the reservoir :-). That's why your preflight check might take a few more rotations to get the burp.

 

Leaving oil in the case after shutdown is inconsequential.

[Roger Lee]

There is no lack of oil if the engine sits. This is a dry sump engine and no oil is stored in the engine. There is residue that sits in the bottom of the case and varies engine to engine, That is the little extra oil that gets pushed back to the tank with crankcase pressure. The amount can vary. It may only be an ounce or two, but I have see better than a qt. If you hear the gurgle right away then very little oil is sitting in the bottom of the case. If you crank and crank then you have more.

It isn't anything to lose sleep over.

[FlyingMonkey]

It's not really different from a traditional wet sump system.  In that case the oil just pools in the sump at the bottom of the case.  In our case the oil pools in an external tank.  Either way, the only oil "in" the engine proper and lubricating critical components is residual oil from the last time it was run and you had oil pressure sloshing things.

 

As for burping, I check the oil before flying.  If the level on the dipstick is in the normal range (on the flat of the stick) I don't bother burping.  I just pull one blade through a compression stroke to make sure there is no hydro-lock, and then go fly.  If the level is not on the flat I burp and re-check.  I have never lost more than a few ounces of oil between oil changes, and have never had to add any between changes.

[Tom Baker]

I would like to add that there is always a positive supply of oil moving from the oil tank to the engine through the oil pump when turning the propeller in the normal direction of rotation. This is the oil that the engine needs for start up. The oil sitting in the bottom of the crankcase does nothing to provide engine lubrication while sitting there. It must go back to the tank and through the pump to be of any use.

[rhanson]

Thanks Gents,

 

For all of the good advice.

 

The oil reservoir is where we determine correct quantity,

no matter when.  If I fill it up post-flight, with 2 turns of the prop,

then I am ready for the next start, instead of 10+ turns

before each flight.

 

RH

[FastEddieB]

Thanks Gents,

 

For all of the good advice.

 

The oil reservoir is where we determine correct quantity,

no matter when.  If I fill it up post-flight, with 2 turns of the prop,

then I am ready for the next start, instead of 10+ turns

before each flight.

 

RH

That's how I do mine, since I'm the only pilot and the plane is kept in a hangar.

 

Mine takes about 10 blades when warm, vs maybe 30 when cold.

[gbigs]

Thanks Gents,

 

For all of the good advice.

 

The oil reservoir is where we determine correct quantity,

no matter when.  If I fill it up post-flight, with 2 turns of the prop,

then I am ready for the next start, instead of 10+ turns

before each flight.

 

RH

 

Some start to get a little lazy and do not burp the engine before first start of the day.  This is a poor practice.  Not only are you unable to check the oil level properly but I am told burping makes sure there is no air in the system. 

 

It does make a difference if the outside air temp is below say 50f...it will take more prop rotations to hear the burp.  If warm outside, say more near 70f the burp can happen as soon as three or four turns.

 

The best way to add oil is just after burping not after a flight.    The level should be no lower than the flat part of stick.  When I top it off I put it near the top of the flat spot to make sure.

[FlyingMonkey]

That's how I do mine, since I'm the only pilot and the plane is kept in a hangar.

Mine takes about 10 blades when warm, vs maybe 30 when cold.

How do you verify oil level before flight?

[Roger Lee]

An oil level check can and does lie to owners all the time. It is not exact and should be more a guide or a close proximity. 

The burping comes from yesteryear and the old filters and Kitfox mounting oil tanks way to high. You could get hydra lock years ago. I suppose the possibility always exist, but has a lower risk of that now days with newer filter design and more MFG and owners mounting oil tanks at the proper height. You shouldn't have to add oil between changes. If you are either you didn't properly fill it at the oil change or you have a problem. If you use an American brand oil and only use 3 qts. then you are under filling by 8 oz. AeroShell is in liters. 3 liters is 8 oz. more than 3 qts.  

If you put in 3 liters and keep adding you will shortly overflow.

 

If you ever get hydrostatic lock the picture below will demo what you need to expect.

 

 

Burping hasn't anything to do with removing air from the system just before each flight. I does however help fill a new empty oil filter.

[FastEddieB]

How do you verify oil level before flight?

I don't.

 

If it was OK after the flight, and there's no puddle of oil under the plane and no one surreptitiously snuck into my hangar and drained the oil, it should be fine.

 

I do turn the engine over as I check the prop blades, but don't think it's necessary otherwise.

[Anticept]

Hydrolock is famous in the radial world!

 

 

I don't.

 

If it was OK after the flight, and there's no puddle of oil under the plane and no one surreptitiously snuck into my hangar and drained the oil, it should be fine.

 

I do turn the engine over as I check the prop blades, but don't think it's necessary otherwise.

 

Haven't you heard of gremlins???

 

Anyways, you also assume that it hasn't run down somewhere else . Chances are tiny, but wouldn't you hate to be a poster child on the "accidents are a series of linked events" spiel?

 

Also, you don't need to spin it like crazy. Maybe your sky arrow is unique in mounting, but I personally put the prop just past TDC and let the air go by, then move on to the next rotation. I can get the burp even on a cold day with only a few blades of rotation (less than 10).

[Runtoeat]

Tom made a statement that caught my eye.  Does the burping also "prime" the engine with oil so it is lubricated when one starts up?  I know that an oil film is left for lubrication but I'm wondering if the plane sits for a while and the oil drains out, does burping run oil into the engine crank and cam galleys so that it is available for the start up?

[FastEddieB]

My understanding is that the worst thing you can do for an engine is to run it, or even just rotate it, with no oil pressure.

 

And that the quickest way to get oil pressure in an engine is to start it.

 

"Pulling it through", to pre-lubricate it or to "loosen up" the oil is unnecessary, and possibly counterproductive.

[FlyingMonkey]

Since burping moves oil *away* from the engine and into the sump tank, I don't think pulling the engine through is doing anything for the engine.  Plus I don't think you can pull it through fast enough to generate any oil pressure worthy of the name.

 

The ideal would be to have an electric aux oil pump that generated oil pressure in the engine before starting.  But then you'd probably burn a bit of oil on startup.  Engines seem to do okay without anything like that, even after sitting a long time.  Some wear associated with that, which is why frequently flown engines seem to last more hours than those that sit a lot.

[Ed Cesnalis]

I haven't burped Woodstock in years.

[FlyingMonkey]

Haven't you heard of gremlins???

 

Anyways, you also assume that it hasn't run down somewhere else . Chances are tiny, but wouldn't you hate to be a poster child on the "accidents are a series of linked events" spiel?

 

I have to say I disagree with Eddie on this.  If I'm the only pilot and the airplane sits in a hermetically-sealed vault with white floors to show any oil spills and the cowlings off to see any loss, I'm *still* checking the oil before the first flight of the day, at least.

 

Losing oil is just one of those "very low probability, very high consequence" events I want to avoid.  If the engine locks up I don't want to have to explain to the FAA & NTSB  why I didn't check the oil level before flying.  "I was going to check it after I landed" might raise some eyebrows.  

[FlyingMonkey]

I haven't burped Woodstock in years.

 

There's no need to burp, really.  I only do it if I check the oil and it's below the normal range, then I'll do it to get a more accurate reading.  I do pull one blade through to make sure there's no hydrolock.  No need for that really either, but it's super easy to do and much cheaper than a few bent rods.   

[Tom Baker]

Tom made a statement that caught my eye.  Does the burping also "prime" the engine with oil so it is lubricated when one starts up?  I know that an oil film is left for lubrication but I'm wondering if the plane sits for a while and the oil drains out, does burping run oil into the engine crank and cam galleys so that it is available for the start up?

 

 

 The way the system is designed the pick up in the oil tank should always be under the oil level, and the line to the pump full of oil. The pump is a positive flow pump with very close tolerances. When you rotate the engine the pump is going to move oil into the engine. I have no idea how far into the engine it is going to go. It certainly is not going to build oil pressure, but that oil is going to start filling the oil galleys in the engine. The whole point of my comment was that the positive flow of oil from the pump is what the engine needs on start up, not the oil that has been pushed out of the crankcase.

[Tom Baker]

For Lycoming and Continental engines the oil pump is moving oil to the bearing surfaces too, but this is out weighed by the oil that is being scraped of the cylinder walls. For these engines the cylinder wear is the bigger issue.