How to prevent cracked muffler on 912uls

[Delta Whisky]

Dustin - your question touched on the irony of the planned flight. That day, Monday, showed hopes of being a relative calm day aloft and was the reason for the planned flight. I had previously made two flights in hopes of obtaining decent data but the turbulence makes the data suspect. None-the-less, I've started an analysis and, as soon as a head cold clears, I'll see if any part of what I have is useful.

Jim - Your idea has merit but one detail of what a close look into the muffler reveals gives me pause. I'll try to get a better picture the next time I'm at the airport but it appears the exhaust pipe/stinger is (or was) an extension of an internal pipe that runs the length of the muffler. This pipe contains numerous holes the exhaust gases have to pass thru on their way to freedom. If that is the case, four breaks were extremely clean and normal to the center line of the pipe.

[Dave S]

Found a photo which shows the small stinger brace I mentioned before that was part of the original design on the muffler that came with my kit. This is an earlier design ca. 2005. Don't know if that is currently used but it seems like a good idea. No cracks in the muffler body or the stinger weld to date.

[jiott]

My kit was purchased in 2012 and the muffler did not come with provisions for the stinger brace. John McB said the internal construction had been beefed up and the brace was no longer required. Mine has been flying for 1050+ hours now without the brace and no weld problems with the stinger-knock knock.

[Delta Whisky]

Inside muffler.jpgExterior of muffler failure.jpg

Better (promised) pictures and some clarity of the internal "silencing" pipe. After looking more closely at the evidence today, I'm developing the theory that the fairly long stinger, in a vibrating environment, caused the cracking which in turn allowed it to then contact the cowl - not the other way around. I found an airflow deflector I had installed to better direct air to the oil cooler also cracked in two places. Clearly I have a vibration issue that has to be resolved and, yes, I did balance the prop during the first 10 hours of flight. (Now 180 hours.)

[avidflyer]

I wonder how long it would last if you were able to slide in an elbow that inside baffle pipe fit into fairly snug. Then weld the elbow and a patch on the end of the muffler. Might last for a bit till you get another muffler at least.

[AvDES LLC]

Do you have periods of excessive vibration with this engine? That vibration could be causing that pipe to smack that cowling and would explain a lot. My exhaust did the exact same thing as the first pictures in this thread and a buddy of mines exhaust did the same thing as the missing stinger photo. Appears this isn’t an uncommon issue.

[jrevens]

I may be wrong, but it seems to me that this issue could be due to the design &/or construction of the Kitfox-specific designed/built exhaust system. There doesn’t seem to be the same frequency of these kinds of failures with the genuine Rotax exhaust systems, for instance, used on many other designs.

[AvDES LLC]

you make a good point. I haven’t seen under the cowling of any planes with 912’s installed except kitfoxes. That said,it would be interesting to see how the cowl is shaped around the exhaust of other aircraft with the 912 as this would give us an idea as to how close other designers have placed the fiberglass.

It’s a bummer to see so many of us have had the same issue.

[avidflyer]

I have a different muffler that has ball joints near the muffler, and then shorter pieces go into the muffler. 2 pipes instead of one for each run. That would probably reduce some of the vibrations our Kitfox mufflers see. JImChuk

[AvDES LLC]

would you mind posting a photo?