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bob27s -> RE: Jett engines and noise limits (7/28/2005 5:00:14 PM)
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Thanks for writing Dave.......
(and for the move FBD)
The short answer is .... NO
The longer answer is ....... well ...... maybe...
I only have some limited sound info in my personal records. Because the sound check levels vary so much due to engine/airframe/engine mount/prop/rpm/field type and surface/phase of moon/fuel etc.. Neither Dub nor myself feel comfortable giving specific answers on sound levels - especially since it is so important for many pilots and club fields. However, we have done comparisons with other engines/mufflers over the years, and we have a fairly good idea of how the jett products stack up in performance.
The SJ-50 engine typically turns a 10x6 prop or 9x8 prop well over 17,000 rpm. At this rpm and engergy level, in open air, hard mounted on the nose of a Q-500, the engine will produce something in the 101-102 db range.
The jett-stream muffler is tuned for an rpm band that runs from about 14,500 up to about 18,000 rpm. The SJ-50 is designed to run at full power "over" 15,500 rpm and has to be propped accordingly.
The muffler, considered by itself, operated at the same rpm with the same prop, is quieter than most factory stock mufflers. However, with the performance and rpm, you tend to get that noise back again.
I will assume that on the Viper, you want some speed. Typically you would want to run an APC 8.8x8.5 or 8.75 pylon prop turning over 17,000 in this application.
Your best bet toward hitting 98db is to try an APC standard blade 9x9 prop. Break the engien in first on the bench (per instructions). Get it running well with a 10x6 first. Then switch to the 9x9. Should pull the rpm down toward 15,500 rpm. At that point you have something that is making about the same sound level as a stock OS .46 FX. (maybe try a 9x10 too, but be very careful - its a lot of prop - dont let it get hot)
Be careful not to get the engine too lean. Use the needle setting of the 10x6 as a baseline. Find the peak ground rpm with the 9x9 and back off 500-600 rpm. That is where you keep it for flight.
Do not run it at full throttle on the ground for long. Do the initial ground warm up, set the needle, and then throttle back (once set, you should not have to do much adjusting). Once in the air, the Viper will allow the engine to unload and get up into the mid 16K rpm range where it will be much more happy.
In the past we have configured SJ-50 and FIRE-50 engines with full size quiet pipes. The big pipe and how it is tuned gives you much more control over the rpm range, torque curve so you can turn "bigger" props slower, and they are quieter to start with. Those configurations were succesful at getting below 98 db (I do not have specifics at hand).
Anyway..... Let me leave it here. Give it a try if you wish. Just be careful not to get the engine too lean/hot. Let me know how you make out.
Bob
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