One sensor at the center of the bottom, looking to the ground.
One sensor at the center of the top, looking to the ceiling.
The third on one of the four sides of the quadcopter, as the face.
... and, yes, a gyro !
Turndirection of the 4 motors:
CW NCW
\ /
Center
/
NCW CW
CW= Clockwise, NCW = Not clockwise
A good idea is to implement a 10 seconds wait command. You start the robot and he waits 10 seconds before he powers the motors. Enough time to go some meters away, before he lift of ! To avoid a start into the face !