Actual Flying Rotax Installs

[Skippydiesel]

Slightly pessimistic words of caution:

The manufacturer of the engine (Rotax) has gone to great lengths (money, science, experience/testing) to design a machine that will give reliable (safe) service over an extended operational life - what makes you think that you can do better?
The development of the engine exhaust system (extraction pipes & noise attenuator), is but one aspect of a complex relationship with other components & systems - have you considered the negative impacts/benefits if any, of your "enhancements" ?
The internal combustion engine can be likened to a pump. Air & fuel in/burnt fuel & gases out. You are modifying the burnt fuel/gases out side of the plumbing (gas flow) - what, if any changes, have you made to the inlet side ?
With the exception of a relatively small group in society, the noise emitted by a poorly muffled or straight through exhaust system is considered to be unpleasant & anti social - have you considered the noise impact, of your modifications, on those living within earshot ?

To make any significant gain in the power output of a given engine, you must increase the volumetric efficiency (the ability to process fuel/air). No simple task. In doing so you will inevitably increase, the amount of heat developed by the engine and the pressures being exerted on almost all internal components. These will need to be managed by other changes ( eg increased cooling capacity, greater oil flow, stronger/heavier components) if the engines expected in service life & reliability/safety is not to be significantly reduced.

[Scott Todd]

"Rotax bark" ? They tend to be pretty quiet with the stock muffler. No sense re-inventing the wheel.

[builderflyer]

Slightly pessimistic words of caution:
With the exception of a relatively small group in society, the noise emitted by a poorly muffled or straight through exhaust system is considered to be unpleasant & anti social - have you considered the noise impact, of your modifications, on those living within earshot ?

My old Jabiru 3300 powered Sonex has the original and rare one piece 3 to 1 collector exhaust. (not the later Jabiru supplied four piece collector exhaust system). My Sonex exhaust sound could be described with only one word,,,,,,,"loud". However, the one universal comment I get from almost everyone who has heard my airplane fly, and repeated over and over again until I'm tired of hearing it is, how great my airplane sounds. In 16 years of flying my Sonex, I have not received one negative comment on how it sounds, loud or otherwise. But I know you are correct in that my Sonex would be banned from the skies in certain other countries, just not here in the USA.

Art,,,,,,,,,,,,,,,,,Sonex taildragger #95,,,,,,,,,,,,,,,,,,Jabiru 3300 #261

Edit: come to think of it, however, there are some airports in some communities here in the USA whereupon departing from those airports may trigger a nasty letter regarding noise. This happened to me many years ago when I was flying a Bellanca Super Viking out of Torrance airport in southern California, as I recall.

[Skippydiesel]

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My old Jabiru 3300 powered Sonex has the original and rare one piece 3 to 1 collector exhaust. (not the later Jabiru supplied four piece collector exhaust system). My Sonex exhaust sound could be described with only one word,,,,,,,"loud". However, the one universal comment I get from almost everyone who has heard my airplane fly, and repeated over and over again until I'm tired of hearing it is, how great my airplane sounds. In 16 years of flying my Sonex, I have not received one negative comment on how it sounds, loud or otherwise. But I know you are correct in that my Sonex would be banned from the skies in certain other countries, just not here in the USA.

Art,,,,,,,,,,,,,,,,,Sonex taildragger #95,,,,,,,,,,,,,,,,,,Jabiru 3300 #261

Edit: come to think of it, however, there are some airports in some communities here in the USA whereupon departing from those airports may trigger a nasty letter regarding noise. This happened to me many years ago when I was flying a Bellanca Super Viking out of Torrance airport in southern California, as I recall.[/quote]

Hi Builderflyer,

I have little doubt that the admiring listeners (to your Jab) were predominantly aircraft/field people.
I too like a good throaty sound from an exhaust, however those non aircraft obsessed persons, living in the vicinity of an airfield, are all too often not so delighted by the sound of aircraft engines.
It behoves all pilots to "fly friendly" that is with consideration for the non aviation persons (the vast majority) in the community.
My little local grass strip, has self imposed noise & overfly limits due to neurotic community members who are convinced the noise of a small aircraft passing overhead, is but the precursor to the same aircraft crashing through their roof.
Do not forget they (the majority) have the voting power to have all sorts of restrictions imposed on aircraft movements and even have an airfield closed (happens here in Australia).
I try my best not to "prod" the beast (the voting community) least they form a committee to close us down.

[Zack]

We have experimented a little with what Tony Bingelis defines as a ‘swiss muffler’. It’s a fairly long outer tube, a perforated inner tube, the space between inner and outer filled with absorption material.

This layout was not a success for the high revving Rotax. To tame the Rotax bark we needed a very long (almost as long as the fuselage) unit. It worked so-so to start with, but after a short while the thing got louder and louder: the absorption material disappeared out of the muffler!

Kai, thanks for that input! I am indeed planning for a "swiss muffler" (I had to look that up https://www.piteraq.dk/flight/muffler.html) I hope it will be more robust! Do you happen to know how far the muffler was from the cylinder head? I may ask for an alternative straight pipe section to be included with the exhaust and try them both out. This is who I am having fabricate the exhaust: http://www.stradafab.net/exhausts You can see on the website the mufflers he fabricates for motorcycles. He perforates the inner pipe, then wraps it in steel wool then fiberglass absorption material before sliding a carbon fiber tube over it.

Skippy, I appreciate the words of caution. I don't have much choice, the stock system won't fit.

[Skippydiesel]

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Skippy, I appreciate the words of caution. I don't have much choice, the stock system won't fit.

Just checking - this is a Rotax 912 being fitted to a Sonex??

If yes, then I would draw your attention to, the now significant number of 912/Sonex, that have Rotax mufflers hidden within Sonex engine cowl.

Mine - yet to be cowled , painted & fly, should have no difficulty accommodating the Rotax standard muffler (hope I am not being overconfident).

[Kai]

Zack,

You are probably better than we were- but our swiss muffler experience was not a success. Actually, to our astonishment, because we have something similar on our club 180 HP O360 sailplane tug, where it works fairly well. Our R912 issues were insufficient noise attenuation, as well as the ‘stuffing’ disappearing. We first tried with glass wool, then with steel wool, and finally with stainless steel wool. We also tried a few diffrent lengths. Nogo :-(

Selecting the radiator and the oil cooler, their ducting, and their position, mainly governs the layout of the exhaust system. If you start looking on the web, I’m sure you can find a Sonex with the muffler under the cowling. I would not be surprised if the coolers are then either side mounted, or up front behind the cooling air intakes. And I have actually approved an installation where the cooler went behind the seat with a scoop à la a P51 and outside pipes to the engine (It never flew: destroyed during the final build stage in hangar fire).

Thx
Kai

[Skippydiesel]

Well most of the Rotax 912 installations, I have viewed, have the coolant radiator on the left rear side of the engine bay, oil cooler in front and muffler below the engine. There are other variations - my newly acquired, almost complete Sonex, has the oil cooler front left, muffler below and coolant radiator lower centre. The idea being that all the air entering the cowling exists through the radiator. Interesting concept, yet to be proven.

[Zack]

Exhaust update: We've started mocking up equal length primaries.

Exhaust Fabrication 1 small.jpg (58.68 KiB) Viewed 35342 times

We spend 90 minutes studying the available space and I'm happy to report we have come up with a compromise plan. We have a lot more room than we thought behind the radiator after we moved the wiring out of the way. We're going to run the mufflers at an angle back towards the center rear of the cowling near the firewall, behind the radiator. The exhaust will have one exit, directly downstream of the radiator exhaust duct. This new routing has a few advantages. It will allow for a larger diameter muffler for more sound attenuation. The distance is a "tuned" distance of approximately 21". I'll only have one exhaust exit in the cowling, not two. Finally, it will exit in the turbulent air from the radiator duct instead of clean air, leaving the airflow along the sides of the bottom fuselage undisturbed for hopefully a small decrease in drag.

Exhaust available space small.jpg (62.09 KiB) Viewed 35342 times

The area in light blue is available, further up the firewall is full with iS stuff (ecu, fuel pumps, fuse box etc). I also have a good 5-6 inches between the engine mount and the radiator exhaust vent. The fabricator feels confident in the durability of the "swiss style" muffler. He wraps the perforated core in stainless steel wool, then fiberglass. If this isn't durable, I have the space to do a "toucan" style exhaust muffler with a right and left entrance and a center outlet, and the primaries will already be in the right place. http://toucanexhaust.com/

The stock Rotax exhaust muffler will likely fit, but custom primaries will be needed. Maybe a turbo if you want to go wild.

[Skippydiesel]

Turbo charging a naturally aspirated engine, without significant & costly modifications, can only lead to a truncated service life.

If you ned high altitude performance, purchase a Rotax 914 - it will also give you improved take off & climb while giving Rotax reliability & durability.

Your aftermarket exhaust system & mufflers is unlikely to do much more than make an impressive sound - if this is your aim/expectation, go for it (& pay the cost).

The standard Rotax muffler and exhaust pipes will fit & give good service.