Paul-

        You indicated in a prior email that I was going to need to get
   my VAM
        muffler re welded at the flange connection due to the existing weld
        being
        on the wrong side (the outside vs. the inside).

        Does this mean I need to get the outside welds removed, or
   simply add
        inside welds?  If I'm removing the outside welds, then I likely
   have a
        problem, since cutting them out will remove much needed length
   on the
        exhaust tubes -- I dont have extra clearance on my mount tubes
   to allow
        them to be shortened.  What about simply adding support
   brackets from
        the slide throttle down to the VAM to dampen vibration loads?

        mrs

        Looks like you will have to replace the tubes as the heat affected
        zone would still be there. Also brace it back  up to the engine to
        take the cantilevered loads off the tubes.

        While you are at it use two smaller tubes instead of
        one large one to quiet it down.

        --
        Paul Lamar ...No rotor no motor.



   I'll need a 3D of what you're proposing with respect to  2 smaller
   tubes.

   What's the reason for the redesign?   If it is cantilever loads causing
   cracking, why not simply provide strapping to brace it & be done
   with it?

   mrs

   Its some of both. The cracking is occurring just out side
   the weld so what is happening is the molecular structure of
   the metal is probably being modified in that area. Perhaps it is
   becoming more brittle. The strap is insurance. Bad things
   happen when these pipes crack due to the intake system
   being so close. You have five years invested and $30,000
   so better safe than sorry. We now have at least two failures
   of the same kind.

   OK on the 3D's I'll do it tonight.

   --
   Paul Lamar ...No rotor no motor.

Barring other concerns/limitations, things should be designed with
structural efficiency in mind.  Doing so allows the use of  the lightest
material and yields the best strength to weight.  Cantilevering anything
is generally not good for structural efficiency..  Bracing _is_ good
however.

In the case of the exhaust system, the operating temperature is high
enough that the material is weakened, and it's attached to a vibrating
machine.  The physical characteristics of the system material has been
changed locally by welding - more brittle than the base metal.  Triple
challenge.  Plus, as Paul said, the consequences of a failure of the
exhaust system can be catastrophic - dangerous.

Supporting the muffler via straps/struts takes advantage of geometric
structural efficiency and can, by an order of magnitude, make the system
more rugged and trouble free...  Even the conventional naturally
aspirated aircraft engines often (usually?) have struts/braces
supporting the mufflers.

I am still convinced that slip/ball joints and or flex sections are
needed on a 2 into 1 (muffler) system.  If your system has two separate
stacks which aren't rigidly tied to each other (or something else), you
might get away with out it (for a while).

The locations of the flexible sections should be considered carefully to
allow for the pieces of the system to change dimensions.  Ideally, it
seems that one of the pipes that leads from the engine to the muffler
should have a flex coupling in two places plus a slip fitting, while you
could probably get a way with the other pipe not having any..  Doing
this would allow the muffler and the engine to change sizes pretty
dramatically but not put any stress on any of the welds.


Regards,

Matt-

Matt please take a look at this. I just updated it.
http://www.rotaryeng.net/exhaust.html

Paul Lamar

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