How to prevent cracked muffler on 912uls

[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?

[jrevens]

I have a different muffler that has ball joints near the muffler... JImChuk

Absolutely. Ball & slip joints are “standard”, and necessary on many aircraft exhaust systems. A ball joint between a tail pipe & the rest of the system is very common on Lyc’s & Continental’s. Also, for several reasons, the generally accepted “correct” stainless steel alloy is 327. I have to assume & hope that’s what Kitfox uses.

[avidflyer]

I'll try to get a picture tomorrow and post it. JImChuk

[avidflyer]

Sorry guys, I forgot to get a picture of my muffler. I was even in the hangar. (probably was wondering why I had walked up there....) I'll try again tomorrow. JImChuk

[jrevens]

Absolutely. Ball & slip joints are “standard”, and necessary on many aircraft exhaust systems. A ball joint between a tail pipe & the rest of the system is very common on Lyc’s & Continental’s. Also, for several reasons, the generally accepted “correct” stainless steel alloy is 327. I have to assume & hope that’s what Kitfox uses.

I don’t know where my head was, but I meant to say 321, not 327. There are others that are good too, but I believe the people who build systems for aircraft in the homebuilt world generally choose 321, in my experience. I’m interested in what knowledge others have about this.

[Delta Whisky]

As I removed my muffler and got to looking more closely at other items of interest under the cowl, I have become more of a believer in the "vibration caused this" theory. I am walking cautiously here because all too often I jump on the first theory I come up with and it takes a big hammer and crowbar to get me to step back and look elsewhere. Having come clean, I will share that four parts I designed and installed, made from aluminum, have to be replaced due to cracking - and they are located near the center of rotation. That got me to thinking about the muffler and how far it is off the center and that situation is exacerbated by the stinger that hangs off quite a bit more. I compared these parts to an RV-12 and saw a bit of difference in location and size of the exhaust components and am still wondering (based on how often this has now been reported) if there is a resonant frequency issue that might be playing in the equation and could explain why many but not all have seen the issue. Added to the thought process - I had about one finger's width of clearance between the exhaust stinger and cowl and my cowl (round) is fairly flexible where the stinger passes thru. (Born out by the discovery that at flight speeds the cowl has been pushed up against engine components - a situation that will also be addressed soon.) I'm given to a thought that if the contest is between the cowl and the steel exhaust system - at least once in a while the cowl should lose. One final observation - I'm running a Luga prop - because it is so light, it offers almost no dampening in the system. Anyhow, just some thoughts in hopes that someone will be able to put the pieces together and come up with a cause that leads to an solution.