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JonathanMilbank
Joined: 14 Apr 2012
Posts: 388
Location: Aberdeen area
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 Posted: Sat Mar 25, 2023 12:24 pm Post subject: Muffler (silencer) suspension puzzle |
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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 occasional 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 exhaust port studs 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 (Neville Eyre) 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 secured 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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Area-51
Joined: 03 May 2021
Posts: 391
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| Posted: Sat Mar 25, 2023 6:25 pm Post subject: Re: Muffler (silencer) suspension puzzle |
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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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peterz(at)zutrasoft.com
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| Posted: Sun Mar 26, 2023 3:57 am Post subject: Muffler (silencer) suspension puzzle |
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…or in my case a pocketed dog gearbox, heavy warpdrive prop, eaten plastic gearbox thrust washer (twice), and lightly loaded cruise prop resonance, resulting in dramatic exhaust cracking (three times).
Replaced the pocketed dogs and plastic washer, changed to a lighter prop, and no more cracked exhaust. Magic.
Cheers,
PeteZ
| Quote: | On Mar 25, 2023, at 10:39 PM, Area-51 <goldsteinindustrial(at)gmail.com> wrote:
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.
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=510499#510499
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Area-51
Joined: 03 May 2021
Posts: 391
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| Posted: Sun Mar 26, 2023 4:20 am Post subject: Re: Muffler (silencer) suspension puzzle |
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Very interesting!!
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