Off Topic Resonator Question

I am building a flying machine that I want to be as quiet as possible and I'm willing to sacrifice weight and power to get there.

No one uses resonators on aircraft that I know of but they work so well on our cars I wonder if it would be worth doing on the aircraft.

The enging id a 4 cylinder Suzuki http://www.aeromomentum.com/am15.html

Thank you for any ideas.

"Resonaters" are really nothing more than "straight-though" mufflers that are often specifically sized and shaped for the application. Any addition to the exhaust system for the engine will create some back pressure, to some degree and additional added length may change the "tuning" of the engine, even if very slightly.

For this application, I'd consult the engine manufacturer, tell them about your idea, and see what they have to say about it.

Russ:

During the 50s through to the mid 80s, many of the light twin manufacturers like Cessna and Piper, used a thing called a "Ram Tube" to give these largely underpowered aircraft a little boost. The tube is actually a venturi with the exhaust pipe running down the middle. In theory, on the smaller Lycoming and Continental engines it could add up to 25 lbs. of thrust. The added benefit of these ram tubes was they were significantly quieter than the normal straight pipes. Might be something to look into.

Bill

Once you get past the headers, and into the collector, you no longer have to deal with the "tuned" length. A resonator is just a straight through glass pack muffler. There should be almost no exhaust pressure added when going through one. Another possibility is a chamber pipe like the Corvette side pipes. Some early planes with inline engines ran a long exhaust pipe with a muffler, or at least an expansion chamber. I have seen pics of them with it going along the right side of the fuselage.

As an added note, most of the noise you hear when flying is the propeller. There was an experiment where a prop was affixed to an electric motor, and the same prop was affixed to a piston engine. The difference in overall volume was about 10% at the same RPM. If you can find a quiet prop, then I would worry about a muffler.

You might even consider electronic noise attenuation. Some of the new trucks have it, and it eliminates the exhaust sound by inducing a 180 degree out of phase signal into the exhaust pipe with a transducer. That would not eliminate the prop noise though.

The noise from the prop is caused by the tips approaching the speed of sound, and making a shock wave. Simitar props are supposed to be quieter than a straight blade prop. That may or may not be true, as I have not checked personally. The props with the small "winglet" air dam on the end may be quieter as well. They are more efficient, and you can run a smaller prop at the same RPM, with the same thrust, so the tips do not get as close to the speed of sound.

The Russian "Bear" bomber ran its props so fast that they substantially broke the sound barrier at the tips. It was so loud, that someone flying next to them in another plane would be extremely uncomfortable because of the volume of the prop noise.