Rotax 912 pulls through easy when cold

[BirdDog]

Hey all,

 

So I have never really noticed this until I started doing my own maintenance, and one step is to measure how much force it takes to pull the prop through, and noticed recently that if the bird has sat for a month or so, the prop pulls through really easy.

 

What I am saying is... when I turn up at the hangar and pull the prop through 5 or 6 times to burp the oil, she is really easy to pull through.  If I do the same thing after i go flying, she is a lot firmer.

 

Is this normal?  My LAME seems to thing it's OK.

 

She runs fine. Burns no oil, and is smooth.  She is a 912 ULS 2008 with about 350 hours on it.  All SBs up to date etc.

 

Any advice would be appreciated.

 

Cheers

 

J

 

 

[cscotthendry]

Have you done compression (leakdown) tests on the engine? That will tell you what you need to know. There is no other reason I know of for an engine to be "easy" to pull through when cold but not so much when warm.

 

 

[Thruster88]

I would say you have nothing to worry about. My lycoming has always been harder to turn over after flight than the next day when its cold. 

 

If your engine is cold in both case's it could be that the rings are not sealing as well after a month due oil draining away. The easy way to check piston engine health is static RPM (fixed pitch prop). All certified (VH) aircraft have this number in the POH. You can do this for any aircraft even if you have a non standard prop. Know the number, check it before flight if you have concerns 

 

 

[M61A1]

I agree with cscotthendry. A leakdown test will show up compression issues.

 

Generally the 912 feels tighter when hot.

 

 

[facthunter]

 If you had an indication of LOW blowby that would tell you if All's OK. I would always do a leakdown after a decent engine run. Not when it's sat for weeks You have to be familiar with what a good compression feels like or you are just guessing. When you are hand swinging props you get the feel . A "soft " one is very noticeable, usually  one particular cylinder will be much worse than the others. The Rotax being geared isn't quite as easy to pick. If in doubt do the proper test and back up your judgement with the figures. Nev

 

 

[Downunder]

I wouldn't worry about it. Lack of oil and cold piston giving poor bore sealing I think.

 

As above, a leakdown test will find any compression problems if you're worried.

 

 

[facthunter]

New 912's (after a few hours of bedding in) are very slow to return the oil and burp. the ring seal is exceptionally good . You have to turn them very slowly to allow some air to pass the rings or you are there for ages.. There's a difference between having good compressions and "stiffness" (friction.) A motor with low leakage will recover the compression force on the downstroke if it's turned continously as it  (compression) acts like a spring. You have to know what it should be like and at least compare each compression feel with others fairly slowly. They are not likely to be all bad at the same time. Nev

 

 

[BirdDog]

Hi lads,

 

Leekdowns were done 20 hours ago, and they are 78/80 across the board.

 

 

[facthunter]

Did it "feel" the same then, as it does now?   Nev

 

 

[Tucano]

Recently exposed to a 912 that had 1900 hours that I had been flying in with no issues. When it was ferryed back to base the pilot who is a L2 noted that it was tight to turn- real tight.

 

It was found it was in a batch that had pick up on the crank - this is listed in a Rotax notification - just have the serial number checked, this was one real unhappy crankcase and was condemned based on the internal damage.

 

Gary

 

 

[M61A1]

Hi lads,

 

Leekdowns were done 20 hours ago, and they are 78/80 across the board.

 

How long ago was the 20 hours? I have seen aircraft that have done 14 hours in 4 years. Sitting around can be worse on an engine than regular operation.

 

Do a leakdown check on it when hot, it's probably fine, but it should  reassure you if you are worried about it. 

 

 

[facthunter]

 I'm not sure that would be the way. Finding the cause of the stiffness might be required at this point. With one plug removed on each cylinder you should only have the valve spring tension to cause any drag and that's not much. If the motors not free it's got to be stripped to where the cause can be found.  Nev

 

 

[M61A1]

 I'm not sure that would be the way. Finding the cause of the stiffness might be required at this point. With one plug removed on each cylinder you should only have the valve spring tension to cause any drag and that's not much. If the motors not free it's got to be stripped to where the cause can be found.  Nev

 

Pretty sure the OP's issue was that after sitting several weeks that it felt as if it had lost compression when it was cold, firmer when hot, not that it was "stiff". Most 912s feel much firmer when hot. As Downunder mentioned the oil drains away after sitting and cold clearances can feel much different to when they're hot. I would be very concerned if it was "stiff" at any time.

 

 

[BirdDog]

Thanks Lads.

 

20 hours ago was May this year.  She gets flown regularly. It's easy for me to do this again - no probs.

 

Did it do it before???  Well, As I said in my post, I think am now more attuned to it, because I do my own maintenance, and am paying a lot more attention now.

 

And yes, It is NOT stiff.  That's not the issue.  The issue is, after sitting for a few weeks without a flight, it becomes easier (read less stiff) than if it's warm.  Make sense?

 

 

[cscotthendry]

Recently exposed to a 912 that had 1900 hours that I had been flying in with no issues. When it was ferryed back to base the pilot who is a L2 noted that it was tight to turn- real tight.

 

It was found it was in a batch that had pick up on the crank - this is listed in a Rotax notification - just have the serial number checked, this was one real unhappy crankcase and was condemned based on the internal damage.

 

Gary

 

Gary:

 

Can you post the SB number for that fault pls.

 

 

[facthunter]

Why not do what I said   Turn it over with a plug out of each cylinder. Or try it on another similar motor. It's only a comparison and if you want numbers on piece of paper it won't do it. There's still value in getting a feel for these things, but I find unless I'm there with you I can't suggest much more..  Compression resistance is a series of hopefully equal forces or lumps in the turning process .  Conventional aircooled  aero engines have larger piston/bore clearances and respond to more recent operation than engines like the Rotax 912. Even the disposition of the ring gaps will affect the feel in some instances.  Nev

 

 

[Yenn]

If there is a mechanical problem causing stiffness you will feel a constant drag. If there is stiffness caused by good compression you will be able to feel a springiness in the turning force. By that I mean that compression will try to turn the prop in reverse, you  will be able to bounce it, but mechanical stiffness will be constant.

 

 

[BirdDog]

Thanks again lads.

 

My LAME tells me I am all OK!  LOL! - But he is not the one with his arse in the seat in the sky!  LOL!

 

I am a little of the.... if it ain't broke, don't fix it mentality.  Everything checks out OK.  But, for peace of mind, this weekend, I will go hit up another leak down test, and see what I get.

 

I will report back the findings.

 

 

[BirdDog]

If there is a mechanical problem causing stiffness you will feel a constant drag. If there is stiffness caused by good compression you will be able to feel a springiness in the turning force. By that I mean that compression will try to turn the prop in reverse, you  will be able to bounce it, but mechanical stiffness will be constant.

 

Yep.  No drag!  Just the "lump" of compression feels less after sitting around.

 

 

[skippydiesel]

Thanks again lads.

 

My LAME tells me I am all OK!  LOL! - But he is not the one with his arse in the seat in the sky!  LOL!

 

I am a little of the.... if it ain't broke, don't fix it mentality.  Everything checks out OK.  But, for peace of mind, this weekend, I will go hit up another leak down test, and see what I get.

 

I will report back the findings.

 

Listen to your LAME - It is logical that an engine will "tighten" up when warm - this what they are designed to do. Engines are not efficient when "cold" All engines (not just aero's) should be operated with consideration/gently until full or near full operating temp has been reached. Your 912 ULS sounds just like my 900+ hr beauty - nothing to worry about (but I still do).

 

 

[facthunter]

 A settled in engine should not tighten up when hot. The cold clearances are set at a figure that allows good running "safe" fits when at running temps and with a bit of a margin as well.' Some new cylinder kits in the tropics have burnished some pistons occasionally, but really you should NEVER get to that stage It's too close to locking up. Any extra frictional heat will rapidly tip the scales to too hot AND TOO TIGHT.

 

 IF your motor was in that state (which is most unlikely) within seconds of being shut down or even operated at idle , it would have cooled enough for the situation to not exist (under those conditions).  (UNLESS some surface has dragged and had metal pick up) Any motor that's done more than say 25 hours should be free from this problem, unless you operate it at a higher load and temp than you have up till that time.  A new engine should take full power after just the normal warm up, safely as that's what every aero engine is expected to do.  They don't get the luxury of a running in period like new cars used to do.  Everything has enough clearance to be operated normally from the first trip. I wouldn't suggest it be a 12 hour race around Mt. Panorama but even that has been done. Nev.

 

 

[skippydiesel]

Sorry to disagree Nev. It could just be we are arguing terminology/semantics  however In my opinion it is quit normal for a cold engine to have greater/wider tolerances than a warm (normal running temperature) fully internal,temperature expanded engine. Even the circulation/distribution of oil may "take up" some movement and give a better "seal" to piston rings, etc. The combination of all these factors, may make a warm engine fee tighter/stiffer, on hand cranking, than a cold engine - mine certainly does.

 

 

[Thruster88]

 A settled in engine should not tighten up when hot.

 

I said at the beginning of this thread that my 1500hr Lyc 0-320 feels tighter (not just on compression)  when hot than cold. Checked another at work yesterday and it was the same. Can anyone explain exactly why this is so.

 

 

[spacesailor]

Piston EXPANSION, when warm.hot.

 

spacesailor

 

 

[BirdDog]

So I have done  lot of research in the last few days, and I have come to the conclusion that what I am experiencing is most likely normal.  I will still do another leakdown test, but I am not much worried anymore.

 

Firstly - Why would the procedure ask you to warm the engine to operating temps before doing the leakdown.  If there was no difference between hot and cold, why bother warming it up!  LOL!

 

Secondly, heat will cause things to expand - it can't be any other way!

 

Anyways... I will do the leakdown (hopefully in the next few days) just to be sure!