Sport Pilot Talk

Went out flying today to do a little pattern work to knock off some rust after not flying for a few weeks. All started out well, although I did think that the Rotax was running a little rough will I cut power to idle abeam the numbers. Had 4 pretty good landings but as I was adding power for take-off number 5, I heard a 'clanking/chattering' sound. I immediately cut power and the engine sounded OK then added a bit of power and the noise was back. Also, had the faint smell of gas so I taxied back to the hangar.

I decowled the Sting and noticed a complete break in the right side exhaust system, right next to the cylinders. I'm guessing the 'chattering' was the exhaust system rattling (hope so!). I've called my local mechanic and hopefully he can take a look tomorrow. Rotax mechanics -- any thoughts on difficulty/time to repair?

Right rear cylinder? Same one I had break two years ago...

Yup.

Yours also broke at the weld, just like mine. Probably some voids or other defects in the weld in both cases.

That, or there is some minor misalignment stress on that pipe which over heat/cool cycles and engine vibration are cracking it at its weakest location?

I dunno, but when something happens once it's a fluke but to occur again for some one else in the same place raises some extra concern.

I think cleaning and coating the ball joints helps prevent stresses on the pipes. I think this is often neglected when the exhaust is removed.

The exhaust system is the most neglected, and possibly the most vulnerable, part of a Rotax installation. I've had a couple fail, and done weld repairs only to have them fail again. It's worst when you're using 100LL and trying to put the lead into suspension by using tri-cresyl phosphate (TCP or Decalin). What happens is that the lead vaporizes, blows out the exhaust ports, hits the cold tin, condenses and thus solder-coats the insides of the pipes. This compromises their flexibility, and adds mass, leading to cracking. If you must use 100LL, I find it's better to forgo the fuel additive, and just double up on your oil changes. Otherwise, you're replacing exhaust pipes far too often.

This is usually caused by poor stress relief during the welding and of course sometimes people have an engine that vibrates a lot from several source causes and eventually something gives.It is stainless and can be welded.st pipe and had header wrap on how would I see it.
This picture is the perfect example for people who say; If I had a crack in my exhaust pipe how would I see it. The pictures posted is proof that it won't be an issue. When these pipe break it is all the way across. There is no question.

drseti wrote:The exhaust system is the most neglected, and possibly the most vulnerable, part of a Rotax installation. I've had a couple fail, and done weld repairs only to have them fail again. It's worst when you're using 100LL and trying to put the lead into suspension by using tri-cresyl phosphate (TCP or Decalin). What happens is that the lead vaporizes, blows out the exhaust ports, hits the cold tin, condenses and thus solder-coats the insides of the pipes. This compromises their flexibility, and adds mass, leading to cracking. If you must use 100LL, I find it's better to forgo the fuel additive, and just double up on your oil changes. Otherwise, you're replacing exhaust pipes far too often.

What “cold tin” do you mean? The exhaust pipes are many hundreds of degrees, especially close to the cylinders. I could see what you are saying at the end of the exhaust, past the muffler, but not in the main pipes.

BTW, when mine failed, in addition to the broken pipe I had numerous cracks radiating from the exhaust stack weld at the muffler that had to be repaired. I don’t think anything other than poor weld quality and/or excessive vibration could explain it.

I've broken, I think, every one of my exhausts on my 2007 Sting--and a couple of them, several times. I think I can safely say that all of my breaks were due to aberrant vibrations--specifically when I had two successive Woodcomp SR200 props, which had one bad (warped) blade each, resulting in vibrations. It took us forever to discover that! So, I would look to vibration as being the source, first. As far as welding, yes they can be done and they will hold (I've been running two welded ones for nearly 1,000 hours now), but you cannot just weld over in the same place, because it will just break again; you have to weld a sleeve in place over the broken or damaged area. This takes a really good welding technician and equipment (thankfully, I have both nearby). Your best bet is to get a brand new exhaust from SportairUSA. That's what I did when I was having all my trouble. In fact, I got a whole set! I still have them all waiting on the shelf, in case I need them, although since putting on my DUC Swirl prop the problem has not recurred.

Stand by and tomorrow I will photograph and post the "sleeves" that were welded, when my #3 and #4 exhausts broke repeatedly.

Meanwhile, I would look for the source of a "new" vibration that you may not have even noticed! Are the six engine rubber blocks all pristine? If one becomes cracked all the way through, the engine is no longer attached there because the bolts do not go all the way through the rubber. Check the muffler carefully. Are the two "bands" holding it in place tight? What about the three "arms" that hold the muffler bracket to the back of the engine. Are any of the three bolts to the back of the engine loose or missing? Has one of the three "arm" welds broken? Just some of the most obvious things to check.

One final thought: (Since you have the same prop as mine) Did you fly to high altitude (>5,000) recently, then descend rapidly and get throttle-back harmonics which you didn't alleviate (by changing pitch and/or throttle position) quickly? That'll do it--and the one thing I don't like about the DUC Swirl prop...

Notice that in both pictures the actual break takes place outside of the welded area. Known as the heat affected zone, it does need to be annealed/stress relieved. Both pre and post heating would have helped.

It is a manufacturing flaw but good luck getting the manufacture to own up to it!

More Info, http://www.thefabricator.com/article/sh ... ected-zone than you probably want to know

My #3 and #4 "sleeves." Note, it took two sleeves on #3 (second picture) to finally get it right--after the source of the vibration was found and corrected. My experience is that they just don't break unless there is an aberrant vibration; that's why Sportair usually doesn't have them on the shelf, and new ones will likely have to come from TL Ultralight. I'm not in the "bad part" camp. Just my 2 cents.

I do agree, however, that failure to properly lube the exhaust sockets can cause these issues. And there is another possible issue with how the muffler is attached to the plate which holds it to the back of the engine on Stings. But I won't go there, until you've exhausted (pun intended) the other more likely issues.

Well, my mechanic found a welder who repaired the exhaust for $80 to "better than new" (as per the welder). The story doesn't end there, though. In order to put the part back, when needed to loosen the muffler bracket so we could move it a bit. Rich was quite prophetic:

What about the three "arms" that hold the muffler bracket to the back of the engine. Are any of the three bolts to the back of the engine loose or missing?

One of the "arms' was broken right where it attaches to the engine -- basically where it was flattened out and drilled to bolt on. What a PITA to repair! Took about 8 hrs to do both repairs.

Some visible porosity in the repair welds does not instill my confidence!

In the picture above on the welded coupled weld it shows the EGT about 1" - 1.25"" away from the head exhaust port. These should be 3.9" (4" works) or 100mm away from the exhaust port.