Had a Gentle Lady glider lying around.. hadn't moved in a couple of years, and there's a lot of interest in electrifying one, by those with them and not being excited about buying the Electra, the purpose-designed electrified version.

So took mine apart to the empty fuselage stage..First I found the c.g. of the fuselage with everything in it. (no need to consider the wing, as it won't be changed), and weighing the parts after removal and noting where they were in the fuselage.
5.1 ozs of nose weight
4x450 receiver battery over the nose weight
Futaba R-114H receiver behind the battery
CS-55 servo in front of the wing..rudder
CS-55 servo directly behind the forward wing bulkhead..elevator
The removed parts added up to 13.025 oz.
Collected the electric parts and weighed them..
A Graupner Speed Gear 480 w/3.34:1 gear box
A Graupner 11x8 CAM folding prop
A Jeti 25 ESC
A 8x500 battery under the wing
2 Futaba S-133 servos ..rear of the wing opening
..total came to 15.325 oz. Not a lot of change.

Presuming the motor battery would fit under the wing, installed the servos at the aft end of the wing cavity, with the motor screwed to the bulkhead where the nose block was on the GL.

Checked the c.g.

The c.g is WAY too far aft! Even moving the motor battery to directly behind the motor with the receiver and ESC directly on top of it, the c.g. is still too far aft!

Constructed the following table using standard moment computations to see exactly what was the problem, and how to fix it: The datum point for all the positions is 2 inches in front of the noseblock bulkhead so that all the parts have a positive dimension for the position. Makes things less complicated to use only positive values..
The weights are multiplied by the arms to get the moments. The sum of the moments is divided by the sum of the weights to get the position of the c.g., with an adjustment for the leading edge distance from the datum point...
 Gentle Lady glider E-Gentle Lady Speed 480 geared Item arm Weight moment Item arm Weight moment 0.00 0.00 0.00 motor/prop 3.00 4.63 13.88 nose weight 3.00 5.10 15.30 nose weight 0.00 0.00 0.00 servo (CS-55) 8.00 1.30 10.40 servo (S-133) 15.50 0.90 13.95 servo (CS-55) 10.00 13.00 10.40 servo (S-133) 15.50 0.90 13.95 battery(4x450) 4.00 3.20 12.80 battery(8x500) 7.0 5/75 40.25 Rcvr R-114H 7.00 1.10 7.70 rcvr RCD 555 10.0 .9 9.0 airframe 18.00 9.00 162.0 airframe 18.0 9.0 162.0 ESC 8.0 1.0 8.00 C.G. 1.53 21 221.2 C.G. 2.31 23.075 261.25

the basic GL computed c.g. is 1/8" aft of the measured c.g. but 3/4" forward of the proposed installation! It requires a couple of ounces of dead weight at the nose bulkhead to move it to where it was. As previous experience with the GL had shown that making a new fuselage a longer nose can eliminate almost all the ballast (5 oz is a LOT in a 26 oz plane), this might be the only reasonable choice with this power!
Using a Speed 600 without or with a gearbox might be a better choice.

E-mail to: Paul J. Burke

eGLcg.htm..10-01-2007