bob27s
Posts: 4637
Joined: 4/9/2002
From: Cleveland,
OH, USA
Status: offline
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Hi Rick...
Thanks for writing.
95 dB is gonna be very tough.
Tell me a bit about the aircraft and application, and I will try to make some recommendations.
Background info in the mean time.....
The Jett-Stream muffler supplied with Jett engines is actually pretty quite by most standards. Our experience is that they are quieter than factory stock mufflers at the same rpm and power output. For example, if you were to install an OS46 muffler on a SJ50, and run it at 13,000 rpm.... then install the JettStream muffler and run the combination at 13,000 rpm.... the jett muffler would test quieter by a small amount.
However, the SJ50 and its muffler are designed to run over 16,000 rpm, and in most speed applications the engine will turn well over 18,000 rpm. At that power output and rpm, there is a ton of energy being release, and the prop noise becomes a significant factor.
To get to a significantly lower sound level, the RPM must be reduced significantly.
Changing the muffler will help a little, but using a 'regular' muffler will yeild lower noise simply by preventing the engine from obtaining high rpm. Jett offers the stock JettStream muffler in a non-tuned version, which is offered for scale and 3D aerobatic use. You get good performance, reduced noise, lower rpm, and no pipe-step. The SJ50 and 3D muffler work extreamly well using an APC 12.25x3.75 prop.
To maintain maximum power, a MAC muffled tuned pipe is one of the better options.
The pipe/header can be tuned to a lower RPM. Using a larger prop (diameter and/or pitch) you can most of the output power.
On the jettengineering.com web site, take a look in the Hall Of Fame, and read the letter by Gene Greatti regarding his Diamond Dust which was set up as described above. He too was looking for a quieter set-up, while maintaining the speed and power.
In his case, the MACs quiet pipe was tuned fairly long, to run just above 16,000 rpm, which is 2000 to 3000 rpm slower than most diamond dust applications. This higher torque set-up allowed him to run 9x9 and 9x10 props, and maintained the airspeed he was looking for.
Gene posts on the SPEED section regularly. You may want to contact him for some additional information on the 'quiet' speed set-up.
If you are not looking for all out speed...
The tuned pipe set up will still work... it can be tuned longer for use with 10x6, 11x5 or 12x4 props.
Another alternative....
Our Turbo-Jett in-cowl tuned mufflers are quiet and they provide increased power with a broader rpm range - no 'pipe-step' effect. I run one of my SJ46 engines in a P-51 mustang combined with a Turbo-Jett muffler. This turns a 12x4 prop at only 14,400 peak rpm on the ground, I settle the needle for 13,800, and it unloads about 1500 rpm in the air. It is easy to needle, runs great, and at 10 feet away, on grass, mine tested at 97 dB last summer.
Anyway...
let me know a bit more about your application, and I will see if I can provide additional suggestions.
Bob Brassell
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Noise reduction guidance for Jett 50 - 
