We decided to mount our ultrasonic sensors on the carbon fiber arms outside of the motor mounts so that the sensors would protrude beyond the tips of the propellers. This configuration means that the sensors don't have any wasted range and also that if something goes wrong the propeller is not the first thing to hit an obstacle. We originally planed on having the sensors between the propellers witch is a 45 degree shift we had to change the configuration of our copter from an "X" to a "+", but this was easily done in Cleanflight.
