Dave, thank you for going through the extra effort to confirm the CG). So went flying with the Platter balanced at 3 inches from the motor mount. Tim, we experinced your recommendation regarding the CG sensitivity, half inch is too much.
So here is what we experienced. The Platter went straight up into a hover (reminding me of my 3D RC flying) . It was not leaning forward or to its back. Straight up. So we placed the battery at 2.5 CG that I had already marked. Now it went to a direct dive. CG sensitive.
Thinking out loud (please help us think it through..), I think the CG at 3 inch behind the motor mount as you drew it is absolutely correct.
Next, trying to assure that the Platter is not so sensitive. The control rod is in the closest hole next to the pivot screw on the bell crank, the stab horn it is in the outer most hole away from stab, the line at the handle is about 2.5 inches apart. The stab deflection is 10 degrees up or down. The bell crank is 2 inches wide, not the small one. I do think these are least sensitive positions that I can get.(?) The controls are free, no binding or hesitation.
The ready to fly Platter, weight is 5.8 ounces. The motor is Arrowing 2205- 22 that calls for a 7 by 5 prop or larger on 800mah, 30 C 2S battery. Moreover I have the motor set at zero , no down thrust, just a bit of right thrust with washers. We also tried at 3 inches CG and a smaller prop of 6 by 4 on the 2S battery but not enough punch to sustain flight.
At this time my gut feeling is to put a Cox Blackwidow with 6 by 3 prop and prove all is correct and called for in the original design. However the school baseball field is very close to our housing development (our flying field is shut down) and these days all are home. Too much noise to even loose the baseball field too.
So we are stuck with this motor, 2S battery and a 7 by 5 prop. Next will try, putting a single washers on the upper bolts to get a downthrust and try it at CG of 3 inches behind the motor mount. Option three, install a slower motor with a 6 by 3 prop.
Appreciate any other thoughts and recommendations. Thanks again.