Today's Message Index:
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1. 07:51 AM - Re: Finishing tips (danbish99)
2. 01:25 PM - Muffler (silencer) suspension puzzle (JonathanMilbank)
3. 07:25 PM - Re: Muffler (silencer) suspension puzzle (Area-51)
4. 07:39 PM - Re: the Obsidian Files - So Much Noise!!! (Area-51)
Message 1
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| Subject: | Re: Finishing tips |
Any idea who wrote this article? Has some excellent tips but I have a few questions
for the author. It's posted on the Europa Club site.
Thanks,
Dan
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=510496#510496
Message 2
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| Subject: | Muffler (silencer) suspension puzzle |
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?
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=510497#510497
Message 3
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| Subject: | Re: Muffler (silencer) suspension puzzle |
Nearly all and everything contemporary produced is completely designed within a
digital computer specified environment; from electronics to mechanical; covering
design criteria such as Thermal Expansion & Conductivity, Modal Frequency
Vibrational Response, Dynamic Physical Loads & Gravity, Material Elasticity Stress
& Strain, Chemical Interactions... This is a very basic matrix of R&D analysis.
When a company such as Bombardier develop a power train it is run through the above
matrix before being produced and released for industry use.
The factory supplied 9XX exhaust system will have the above matrix applied along
with field testing results which are fed back into the design process upon a
"Continual Improvement" basis, which the end user sees as optional or mandatory
factory Service Bulletins.
This closed loop system quickly falls apart when third party aftermarket components
are brought into the mix; some may cause premature failure, others may prevent
it; it depends upon the applied knowledge of who ever is producing the component.
The whole exhaust is constantly vibrating at (x)hz at (x)deg and will be tested
at the factory based upon the typical operating ranges of the power unit being
in the vicinity of 5100rpm and (x)deg give or take 15% either side. Stress and
strain fatigue points of the exhaust system will naturally be found around
both bending points, and attachment points. Hard mounting the exhaust muffler
to the engine block moves any inertia forces of the exhaust into the engine block
which may or may not be designed to support such loads beyond (x)hrs of usage
at (x)rpm.
A rough running engine or loose fixing somewhere in the mix adds another dimension
of vibration into the fatigue failure equation.
This why its important to keep any motor well serviced and tuned and running smooth.
The short answer to the question is "Cycle Count" but that is just going to be
too cryptic for most to understand.
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http://forums.matronics.com/viewtopic.php?p=510499#510499
Message 4
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| Subject: | Re: the Obsidian Files - So Much Noise!!! |
Today is Sunday and the conditions are useless again for flying.
So its back into "Shut That Noise Up" project...
In this post 181's air box is getting analysed for harmonic resonance. The test
passes frequencies from 1hz to 20,000hz through the 1.5mm fibreglass component
to see where points of dynamic distortion will be at specific frequencies; blue
is zero red is maximum.
Four frequencies were isolated from the pack to review what is going on at 3600
/ 4000 / 5200 / 5600 rpm. The data provided correlates to where the air box body
is "resonating" at specific rpm. The purpose of this is to identify focused
points of resonance that can be individually attenuated as opposed to wrapping
the entire box.... image order correlates to rpm list above.
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=510500#510500
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