How to prevent cracked muffler on 912uls

[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

[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.

[jrevens]

Have you dynamically balanced the prop?

[Delta Whisky]

Yes and balanced the carbs ------- and get complements on how smooth the engine is. Although, it is smoother at 5200 than anywhere else - but you have to pay close attention to see any difference.

[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]

I don't really have any good pictures of the hole/clearance where my tailpipe exits the cowl, but maybe you can get a good idea from these, FWIW. The clearance is not a lot, but is probably a little more than 1/2". I haven't seen any evidence of the pipe contacting the cowl.

IMG_2091.jpg144 1st Bug splatter scrub - N27JE.jpgIMG_2077.jpgIMG_2079.jpg

[avidflyer]

Well I got a couple of pictures of my muffler with the ball joints on the pipes. It actually has two ball joints per pipe. Lots of springs to wear out, but maybe the stinger won't vibrate off. I got this muffler quite a while back, don't remember what it came off of. There is a cabin or carb heat source attached to the muffler as well. JImChuk

[AvDES LLC]

Thank you for those photos. That helped explain a lot. When I get the cowling back on babe I’ll be sure to take some photos of clearance as well since my exhaust tip broke the same way and I never payed attention to that clearance.