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bob27s -> RE: SJ 50 Prop???? (3/31/2008 8:56:27 PM)
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Hi,
Thanks for writing.....
Always happy to answer questions....
First, be certain to follow the run-in instructions - use a 9x6 or 9x7 for initial run-in, or a 10x6 cut down to 8.5" diameter.
Prop selection is all based on the engine's design rpm. For the SJ-50, you select a prop that allows the engine to turn over 16,500 peak ground rpm. It is happy with everything from a 9x6 through a 10x6 or 11x5 even... all hit the target range.
For flight, find peak rpm (briefly) then back down 800 rpm rich from there before launch.
After that, start flying with a standard blade APC 9x7 or 9x8.
That will give you lots of top end speed, will keep the engine in its proper rpm band, and will most importantly give you an execellent launch. Use those first flights to see how the engine responds to the mixture. You may have to lean/richen a tiny bit from where described above.
After you have some time on both the engine and airplane --- and understand how both work together .... try an APC 9x9. You may also want to try an APC 8.8x8.5 --- but beware the launch thrust with either of these will be a bit lower than with the 9x7 or 9x8.
The standard SJ-50 is one heck of an engine.... dont sell it short :) Prop it correctly and it will make that Magnum really move.
If you want to focus on speed a bit, you can contact Dub and get the LX muffler (higher rpm tuning).
This will force you to prop the engine to turn over 17,500 ground peak -- means lighter prop load, but better in flight rpm.
With this you would use a 9x7, 9x8 (largest), 8.8x8.5, 8.75x8W and also the 8.5x7.25 (yes, this works great...).
As for the engine....
Not economical to convert the engine. Everything except the carb, bearings, drive washer and muffler bolts is different.
You can update the engine to a SJ-50LX ---- higher timed, higher rpm muffler
VERY near the 60LX performance. Requires piston/sleeve/head and the above mentioned LX muffler. About 500 rpm more than just adding the muffler when using any of the props abouve (except the 9x9) and you will definately want to try the 8.8x8.5 prop.
And.. in this particular application ----- APC 8.8x9.25, clip to a finished diameter of 8". [&:]
Bob
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