I've just found quite simple method to detemine Z-axis position of COG: 1. Determine where is COG on X and Y axis. I think nobody has a problem to find it. 2. Tilt your copter on the edge of landing skids until you find a balance (copter doesn't tend to flip to right or left) 3. Make a picture. In my copter green line is where I found my COG on X and Y axies. 4. COG is where red line crosses green line What do you think about such a method?