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bob27s -> Jett 90L For 3d Work? (6/6/2003 6:15:19 PM)
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Hi,
Thanks for writing.....
The standard SJ-90L is not a very good choice for 3D
First, some background.
The SJ-90L is an RPM engine, and with the standard Jett-Stream muffler should be set up (prop selection) to run at 14K rpm or higher... preferably closer to 15K on the ground. The engines are shipped from Jett with the low range needle pre-set for perfect transition using a 12x6 prop in this RPM range.
Assuming the props you have are hitting the 14K and over range, the transition issues you are seeing may be due to the larger prop (likely a thicker blade section), and you may have to lean out the low/mid range needle a bit. Instructions are on the Jettengineering.com web site.
In summary, turn the low end needle in about 1/8 of a turn at a time until you get perfect transition, and a smooth idle. Only do this after you have at least 20 minutes of run time on the engine, preferably in a test stand. Let the engine run a full power, lean the top end to peak, back off 400-500 rpm. Then pull to idle. Let it sit at idle for 20-30 seconds. Then open the throttle quickly. If it loads up and then accellerates, you have to turn the low end needle in a bit more. Then repeat the process. Always let the engine run at full throttle for a few seconds (to clear it out) before pulling back to idle and checking.
Additionally, the tuned effect of the Jett-Stream muffler creates a step in the power curve as the engine comes on and off of the muffler. This happens just above mid-range, and if the engine is over-propped, it can create some of the transistion issues you are seeing. It is simply the nature of a tuned exhaust system.
The SJ-90T you may hear about is basically the same engine, with a longer 120 muffler so it is tuned for more around 11-12K rpm (larger 14x6 prop, slower).
Using the Turbo-Jett in-cowl muffler on the engine will help with the pipe-step situation, as it is a much broader-band tunning device.
So how has Jett addressed the 3D issue?
In 2002, Jett first introduced the BSE100. This engine was designed from the ground up to run at lower RPM, primarily with the Turbo-Jett muffler. This engine is AWESOME for 3D work, and
I fly one in my AeroWorks 90 size Edge 540. Turns 15x8 at about 9600 rpm. Great midrange torque, smooth transition. And like the 90L, it is in a '60' size package.
This year at the 2003 Toledo Show, Dub introduced the BSE120L. Same size as the 90L and 100L, with additional power. This is the ultimate "60" size 3D engine.
Let me know how things work out
Bob Brassell
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