Challenger Modifications
Gary Middleton

Using the Grand Rapids EIS for flaperon position

LSA Aircraft


Using the Grand Rapids EIS for flaperon position indicator

A simple modification that uses the coolant temperature reading on the Grand Rapids Model 2000/2002 EIS to indicate the relative flaperon position. Of all the modifications I’ve done to my CH1LW, this is one of the most useful and most inexpensive. It requires one wire, a clamp and a $1.39 potentiometer.

One of the most commonly used displays shows the RPM, High CHT, High EGT on top and the Coolant Temp, Flight Time, and Aux (Fuel) on the bottom.

Before I came up with this, each time I flew I had to look at my ailerons and guess at the current flaperon setting. Once in the air I would turn it a few times up or down to get the flight configuration I wanted. On landing, I would crank in a few turns of flaperons but without knowing for sure where it was before my 2½ turns down, it was always a guess. After staring at the bogus water temp reading for a dozen hours or so I decided to see if there was a way to use this input to indicate my flaperon position. The EIS’s coolant reading input is designed to display the resistance of a temperature sender. Hmmm… If I knew the range of resistance of a water temp sender, used a variable resistor tied that to the flaperon system perhaps…It turned out to be much easier than I thought! (How often does that happen?!)

Well it works. Not only that, it works quite well. It is consistent and reliable.

NOTE: This is a relative flaperon position indicator. It doesn’t read the exact position, such as -22° instead it reads 100° when the flaps are at zero.  For example:

Flaperon Angle
EIS Coolant Temp Reading
Number of Turns
5 up
4 up
3 up
-2 1/2
2 up
1 up
1 dn
+3 1/2
2 dn
+4 1/2
3 dn
4 dn
+7 1/2
5 dn
+ flaperon angle represents a down angle

So here is what a flight is like for me now. I turn in 95° or so for take off (4 degrees flap or about 2½ turns down). As I reach the end of the runway and turn to heading I crank it up to 106° (2½ degrees reverse flap or about 2 turns up from zero) and back the RPM’s down to 5600 for a nice cruise climb. As I approach my cruising altitude, I crank back to 100° (zero flaperons) and back off the throttle to 5400 for level flight. As I near the airfield I crank to 95° again for a slow decent to pattern altitude. On the tail end of the downwind leg I lower the flaperons to 90° (about 4 turns down from zero) for landing.

I was quite surprised to find that 100° or zero flaperon consistently resulted in level, hands-off flight. Of course this does change slightly with altitude so that as I reach 8000-ft AGL she likes it at about 104° for level flight.

Sure, I can fly just fine without this little mod, but now I never have to look and guess where my flaperons are set – especially during landings. We all come up with nifty ideas at times. Some work better than others, some we are convinced are moments of utter brilliance, and most we end up not using after a time. It is 37.5 hours since I installed it and I have become absolutely spoiled to knowing my flaperon position. I also know my stall speeds at each flaperon setting.

So how do you implement this? It’s about as easy as a thing can be. You will need: 1000 ohm Slide Potentiometer ( #312-9201-1K) One wire from the pot to the EIS’s Coolant Input (white wire)
One wire from the pot to the frame (ground/common) A bracket to mount the potentiometer A small clamp and a short piece of stiff wire

The EIS’s coolant temperature is a sink input, meaning that it is looking for a resistance to ground/common. If you ground the white EIS wire your temp will read about 314° and around 59° for an open. Make a simple bracket for the pot. Run the potentiometer terminal #3 to the EIS’s white wire and #4 to the airframe to get a ground. Drill a small hole in the potentiometer’s tab to attach the wire and clamp the other end to the flaperon push rod. Adjust the wire such that zero flaperon equals 100° on the EIS. Double check that it does not interfere with full aileron control at all flaperon settings. Then go flying and stop guessing!

Gary Middleton

Potentiometer Photo EIS Potentiometer Photo Flaperon Position Indicator Photo

Wiring Diagram


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