In my limited knowledge and experience of light aircraft having flat-four boxer
engines, particularly the Rotax 912 series, the various muffler designs have
something in common. The muffler boxes all hang by the four exhaust tubes
(headers)
---From the exhaust ports and this has always seemed strange to me. It hardly
seems surprising that aircraft owners get cracked tubes, broken springs and
occasionally cracks in the muffler boxes.
Before going further I want to assure anyone who might be interested that my
engine
mount rubbers are good, the carburettors and propeller are balanced "to within
an inch of their lives", the muffler and tubes get installed with extreme
care not to introduce stresses by tightening the manifolds haphazardly. In fact
CKT provides a telescopically jointed #1 cylinder tube which, if installed
last before tightening the joint clamp, should ensure that there aren't any
installation-induced
stresses.
Yes I've tried the alternative type of tube with the Rotax-provided header
pieces
which insert into the exhaust ports and it made no difference. It cracked after
fewer hours than the older type which it replaced.
A friend has an Evektor SportStar with an identical Rotax 912 and its header
tubes
look segmented, like a vacuum cleaner hose. He never has muffler related
problems.
Before converting my aircraft from Classic to a more XS-type configuration
forward
of the firewall, I had the original double-chambered heavy silencer provided
by Europa and it also had occasional broken springs and cracked header tubes.
Eventually I resorted to making two thick large eliptical "washers", each with
a large hole through which the lower engine mounting bolts were secured and
a small hole for attaching a bracket. These brackets, being one narrow strip
of metal each side going to the sides of the silencer, were then secure by
jubilee
clips to where the tubes entered the muffler box. That put an end to broken
tubes etc for a couple of hundred hours.
Now to my question. Why are mufflers allowed to hang unsupported by their tubes
and consequently vibrate laterally. Is there a risk that by preventing some or
most of these vibrations, cracks and/or damage might occur elsewhere?
I'm hopeful of an answer from someone who has formal engineering expertise to
offer,
relating to "dark arts" like harmonics, oscillations, nodes of vibration,
damping, crack prevention and all that esoteric stuff, but in terms that a
layman
can understand. In summary, what are the associated risks of anchoring the
muffler box like I did previously?
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