the center of gravity of the prop remains unchanged when it is turning (or when it isn't turning).... the CG is the same regardless of the props RPMs...
But... a tractor configuration (prop in the front) has elements of Pitch and yaw instability whereas a pusher is stable... or tends to be stable'izing.....
of course... this is assuming newtonian physics.... as we all know... the mass of an object does increase with increasing velocity...
