Sky Arrow Annual - mostly ROTAX

[FastEddieB]

As mentioned elsewhere, I started the annual condition inspection on my Sky Arrow yesterday, and worked on it some more today:

 

 

Mostly engine stuff so far. The airframe and control stuff looks good so far - especially for a 7+ year old plane.

 

The good:

 

Friction torque in limits.

 

Oil change went well with magnetic plug very clean.

 

Float bowls pristine.

 

Float pairs both less than the maximum 7 grams:

 

 

The not-so good:

 

1) As mentioned before, a broken "curved tube" was causing friction which kept my left "choke" from closing all the way:

 

 

Advice - take a good close look at all the BING hardware. Last year it was a broken adjusting ferrule.

 

One of the drains from the air box had come loose and was also chewed up and/or wearing away along its length (look closely lower left). Replace the full length this time - had spliced in new hose last time.

 

 

Advice - take a good close look at all the small diameter hoses - especially if more that 5 years old or so.

 

Finally, I'm stumped by this:

 

 

When I pulled the #2 plugs and saw how sooty they were, I assumed I was running rich on that side and the #4's would look similar. Nope. The soot does not seem like oil. What else might it be? I can't think of any way #2 can be rich while #4 is right on.

 

Thanks in advance, and I'll report any more findings.

 

Oh, on a side note, I'm ashamed to say I only put about 26 hours on the poor thing last year. There are few reasons why, but darn I need to fly more!

[Roger Lee]

Hi Eddie,

 

The dry black soot is normal and nothing you can do about it except don't idle. Idling is what causes it. Run it at 4K rpm for a few minutes with an immediate shut down and it won't be there.

[FlyingMonkey]

Is the #2 cylinder closest to the carb?  Maybe it's getting more fuel in the mixture?

[Anticept]

#2 is left front.

 

The arrangement is in this manner on lycomings and rotax, looking top down:

 

Front

-------

2 -- 1

4 -- 3

-------

Back

 

Just as a side note, TCM likes to number them starting back right, but same even left odd right pattern.

[FastEddieB]

#2 is left front.

Well, since I'm all backwards, for me it's right rear!

 

BTW, up to now my plugs have all been pretty similar in color, so this is new.

 

Changing gears...

 

Roger, did you say one could get good compression testing with a cold motor?

[Jim Meade]

Conveniently, the cylinder number is stamped on the intake manifold.

[Roger Lee]

Yes you can do a cold motor compression test on a Rotax because it is so tight  toleranced. The numbers hot or cold will either be the same or 1 psi difference. Normal for the leak down test with an 87 psi test pressure is 83-85 psi. Cold is the same. With an 80 psi test it should be 76-78. I have seen some mechanics write in the log they were 80 over 80. Can't happen. Nothing is that air tight and if it was then no oil could get past the rings. Air pressure will escape into the oil tank which can be heard as a gurgle. If your gauge really said 80/80 time to get new gauges. 

[Anticept]

Yes you can do a cold motor compression test on a Rotax because it is so tight  toleranced. The numbers hot or cold will either be the same or 1 psi difference. Normal for the leak down test with an 87 psi test pressure is 83-85 psi. Cold is the same. With an 80 psi test it should be 76-78. I have seen some mechanics write in the log they were 80 over 80. Can't happen. Nothing is that air tight and if it was then no oil could get past the rings. Air pressure will escape into the oil tank which can be heard as a gurgle. If your gauge really said 80/80 time to get new gauges. 

 

The gauges have a calibrated orifice between the two gauges. If the leakage in the engine is low enough, you can still have an indistinguishable differential pressure on both sides of the orifice, which is very possible with small bores even in the lycs and TCM. Sometimes the oil can do a hell of a job sealing the rings at the right moment too.

 

In advisory circular 43.13, for most engines, the orifice size is .040 inches. But, if you try to use it on a large engine, you could fail the test even though the engine is perfectly fine, so an orifice of .060 inches is used on bore diameters larger than 5 inches (used to be based on engine displacement for the big jugs on radials, but was changed due to some engines having few cylinders but very large bores).

 

It is also possible to have low compression numbers on a low time engine, but be functioning perfectly normal. Rings sometimes don't like to seat until you get the engine running. Had an instructor from school who would talk about a goofy engine that always ran fine, but would always score in the low to mid 60's during compression checks on one of the cylinders. The rings just wouldn't seat for the test, but fuel usage, power, and temps were normal for as long as he did inspections on it.

 

Point being, I wouldn't just throw away a set of gauges if I see 80/80s . If you play with the prop enough you can sometimes get the air to stop getting by the rings (except maybe a tiny tiny little bit), which is how I will often have 80's on the same cylinder several inspections in a row.

[FastEddieB]

Conveniently, the cylinder number is stamped on the intake manifold.

My spark plug leads are also conveniently marked.

[FastEddieB]

Thanks, Roger and Corey.

 

If I have some help later today I may throw the gauges on there cold.

 

I'll let you know the results. If they're low, I'll try again once everything is put back together.

[Tom Baker]

Really you don't need for help on the Rotax like you did for the engine in the Cirrus. You shouldn't try and turn the prop with pressure applied because of the gearbox will amplify the force of the pressure. Get the cylinder to top dead center and turn the pressure on by increasing the regulator valve, and not just plugging the compression tester into the air source. Stand clear of the prop, because if you are off TDC the propeller will spin when you get around 30-50 PSI. After your check shut off the pressure by turning the regulator to stop the air pressure.

[Roger Lee]

I agree with Tom. I never have help during a leak down compression test. If the cyl. is at exact TDC it won't move when pressure is applied.  I use a very thin soft aluminum rod and just put it just inside the spark plug hole and can feel and see the rod move up or down when the piston touches and you'll then know the exact TDC position. It is fast and easy. Compression test takes me about 15 minutes.

 

 

Just to alleviate any fears here:

The rod is very flexible and will not break off in the cyl. It will not damage or scratch the cyl. It is just sitting there able to move up or down to get a visual on TDC. You do not insert it all the way down the cyl. Just insert it enough to feel the last few mm of piston travel. Never use wood or plastic. They can break off then it would be retrieval time. A soft thin copper rod would work too. You don't want anything in there that could damage or break off in the cyl. Some common sense must prevail here.  I have been doing it this way for almost 20 years.

 

I'll take some video at the next inspection.

[Anticept]

Even easier way: start with 20psi, move prop a little untill it requires no force, and slowly crank up after that until you hit test pressure.

 

Works with all engines so you aren't in the path of a high speed fan

[FlyingMonkey]

How fast/far  does the prop move from just 87psi air pressure?

[FastEddieB]

I've been using a wooden dowel for TDC. Hard to imagine it breaking off - I could easily feel if it was cocking sideways.

 

I've also been doing it that way for decades!

[Runtoeat]

Eddie, regarding your two plugs looking sooty.  I just watched my friend rebuild a cylinder on a Franklin engine (helicopter application).  All other cylinders showed tan plugs except for the one with sooty plugs which looked like yours.  The suspect cylinder needed new rings - end gaps too large and oil ring lost tension.  Hope your rings aren't worn or one is stuck and that this isn't causing your dirty plugs.

[Anticept]

Sooty isn't the same as oily. If it shines and looks like this, it's an oil problem. This is just carbon deposits.

[Runtoeat]

Bad plug wire or boot with coincidence it would be both upper and lower wires/boots?  Seems too much of a long shot to involve both.

[Anticept]

It's unlikely to be related to any of that. More than likely it's a mixture or temperature issue with the cylinder.

 

Spark plug indications:

http://www.triumphspitfire.nl/plugs.html

 

More about spark plugs, including a chart on temp ranges (applies more to air aspirated engines, but can still apply to rotax if there's a cooling problem):

http://www.avweb.com/news/maint/spark_plugs_198595-1.html

[Roger Lee]

Don't over think this. This has been normal on the 912 since 1990. It has to do with unequal air flow at low rpm's due to the unequal length of the air intake to each cyl. which is and never will be equal. It isn't a problem and has nothing to do with the rings. It can move from one plug to another. It isn't there at high rpm's.

[Anticept]

I will say I have it on mine, front right cylinder, and never really cared because it's been like that forever

[FastEddieB]

Did compression check solo, cold today. Used 87 psi for the first time, instead of 80.

 

Results:

 

#1 - 87/80

#2 - 87/80

#3 - 87/76

#4 - 87/78

 

Do these seem low? They're well within ROTAX limits. Would you check them again warm?

[Roger Lee]

It's lower than anything I have ever seen other than an engine that I already suspected of a problem. A typical 912ULS has a compression test result of 85-83 (with a test pressure of 87) for normal. I haven't even had a 80. Has your plane sat for a while? Most engines I do cold are  same day or next day, but cold. I did a 20 engine study and found little to no difference in engine temp test, but I never had one that sat for days, weeks or months.

 

Since yours seem to be lower  across the board than what I would have expected then a warm engine test is certainly warranted. Even the 4 psi difference makes me want to double check. We teach in school that a 10-11 psi difference like yours may need further checking, but yours is across the board. 

 

For now I wouldn't sweat it and wait until you start up again.

 

Rotax will give that 25% number for an acceptable loss, but we tell everyone if it is even a 10 psi drop there has to be something out of whack. Make sure you have no leaks at the plug fitting or hoses. The only air leak you should hear is a gurgling in the oil tank. You can sometimes put your ear to the air intake or exhaust and IF there is a leak you can hear it.

[Anticept]

If you did it cold, warm the engine up and try it again. Oil on the piston rings seal better and if it's been sitting, some of it may settle off.

[Roger Lee]

I went back and checked something I couldn't pull from memory. Rotax says a drop of 2% - 2.5% is normal. That means 2% of 87 psi would be 85 psi (rounded). and 2.5% would be 84 psi (rounded). Figure many of these gauges aren't as accurate as many would like them to be you may see something around 82 psi and if it hit 80 psi or lower I would want to know why.

 

Engine warm up first before any worrying. My bet is they will come up.