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bob27s -> Sport Jett 46 PROP SIZE!!!!!! (12/5/2002 7:24:09 PM)
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Hi, thank you for writing about your SJ-46. From what you wrote, you have the stock jett-stream muffler as well.
The APC 10x6 is our baseline prop for the SJ-46 engine, and the engine loves it. We do our testing using 15% nitro fuel, and the RPM numbers quoted on the web site were established with this combination.
The engine will run on FAI fuel (0 nitro, basically what you outlined). However unless there is a reason to do so, I would recommend using at least 5% nitro.
With FAI fuel, you will lose about 300-500 rpm on the top end and your idle speed increase about 200 rpm as well.
Engine break-in is important. Put the engine on a test stand. Follow the instructions that came with the engine, or visit jettengineering.com and you can find these instructions in the TECH section. For break in prop size, if the engine is new, break it in with a standard APC 9x6, running at 17,000 rpm. It should still be slightly rich running at that RPM. Run the engine for approxmately 20 minutes (4 x 5 min) is best. On the last run, you can lean the engine out to find peak rpm with that prop, then back off to 17,000 and shut it down. Then put the 10x6 on it, back out the needle a bit, and then run it to find the peak RPM with the 10x6 and your fuel. (do keep it at peak, just get there and back off). You may also need to slightly adjust your low end mixture for proper transition with the 0% fuel. This is set by Dub before it leaves his shop, however it was set for 15% fuel. It will be real close, and may not need any adjustment at all. If needed, again, look at the TECH section of the web site for instructions on how to adjust the low end mixture.
When installed in the airplane, test run once again to briefly find peak rpm. When installed in the airplane, this is almost always lower by 100 rpms. Find that peak, then back off 400-500 rpm, and this is where you will want to set the mixture for flying.
APC makes a good 11x4 prop and also an 10.5x6 and 11x5. These have all shown good performance on the engine.
The APC 10x6 is the ideal prop for the engine, and most likely the best choice for your application
Keep in mind, that the 46 likes RPM. The faster it turns, the happier it is :)
The trick here (with the jett-stream muffler) no matter which prop you choose, is to keep the peak rpm to no less than 15,000 rpm on the ground, and set for launch no lower than 14,500.
Just for reference, I have my SJ-46 paired with the Turbo-Jett muffler, which is installed in a P-51 mustang. With this combination, I found that a 12x4 prop worked very well for over-all aircraft flight performance. The turbo-jett is a bit more broad-range in its tuning ability, and I can turn that 12x4 at up to 14,500 and launch it down near 13,900. With the stock muffler, it will not run well with the 12x4.
I trust this will be helpful
Be sure to send us a photo of the plane (and you too!) for our Hall Of Fame when you can. [email]hof@jettengineering.com[/email]
Bob Brassell
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