I am wondering how most AP / AV multikopters are currently layed out with respect to their CG. I started with the battery below the frame and the camera below that (CG would be centrallized and about 10-15cm below the propellor plane that way), then moved to the battery on the frame, with the camera below the frame (CG would probbably be just 5cm below the prop plane that way), and have also had various configurations with the battery below the frame behind the camera (CG 5-10cm below the propellor plane and more fore-aft spread of unequal weight which might impact on balance at certain pitch angles). I have also seen some people have the battery below the frame and the camera above the electronics bay (CG would be 0-5cm below propellor plane) or with the battery above the electronics bay and the camera below the frame (CG would probably be 0 to 5 cm at or above the propellor plane). I am now moving back to the vertical stack electronics bay - frame - battery - camera (lowest CG) as this seemed to be the most stable configuration I had.

I wish I'd paid more attention in physics classes at school! My thinking is that it's not the prop plane that's important, but rather the plane of the arms. The arms are applying "leverage" to the aircraft, right? Or wrong? Is there any difference between under-mounted motors (facing downwards) and top-mounted (facing upwards) in terms of response of the aircraft to thrust from the motors?

cg above the props will couse it to be a unstable ballance.
The cg below makes the craft more stable but slower to react on controll forces.

Here-s a post with an interesting article on the effect of vertical position of CG on stability: http://www.rcgroups.com/forums/showp...9&postcount=59. In theory CG a bit below center of the prop plane would be more resistant to control movement, wanting to stabilize, while a CG a bit above the center of the prop plane would be more resistant to wind induced movement.