Firefly Slide Show






















































Feeding the Rotax 447 - Throttle Position Indicator
After completing the shim study, I reset the propellor to limit engine rpm to 6000 rpm (cruise prop) to see if I could get a little better cross country range. After doing so the engine was running much too rich, and I was able to remove the shim and run the 15K2 needle in it's normal #2 slot. In doing so, I discovered that the EGT's would spike at 4200 rpm. To discover what the throttle opening was at 4200, rpm I installed a throttle position indicator.

It consists of a little bit of small diameter aluminum tubing, some woven fish line, a bead, a spring, and per cent scale.


The fish line is tied to the cotter key that holds the throttle wire clevis to the throttle lever, and the line is fed through the aluminum tubing under the upper fuselage longeron and the up to just below the throttle per cent scale on the left side of the instrument panel. Small plastic wire ties hold the tubing in place below the longeron.

The fish line is looped so that it passes through the center of a white bead twice, and it then through another short piece of aluminum tubing inserted through the instrument panel surface where the line is tied to a spring which is connected to the upper fuselage longeron. Looping the fish line through the bead lets one move the bead relative to the line and it will remain in place due to the spring tension on the line. Using a word processor, scales were printed out using various font sizes, and the one that matched the throttle movement was selected and attached to the instrument panel. A piece of clear packaging tape was cut to be slightly larger than the scale and was placed over the scale to protect the scale surface from the bead rubbing up and down. Fish line passes around the ends of the tubing, through holes drilled in the instrument panel and tied to prevent the tubes from moving.

After flying the FireFly, I found that the throttle position for 4200 rpm was 20% and the minimum throttle position for 6000 rpm was just a little less than 40%. I copied and modified the chart that explains the Bing carburetor to show percent throttle opening.

As one can see, there is no need to ever change the main jet because the 447 is running at maximum rpm at a point where the main jet has little influence. Between 20 to 40 percent is the rpm range that I use the most and this region is influenced most by the throttle cut away valve and the needle and needle jet. This makes one wonder about the size selection for the Bing carburetor. Why is it so large?