title_body: Why do a quadcopter's opposite rotors spin in the same direction? All the quadcopters I've seen have four motors, arranged in a square-like shape. Two of these rotors spin one way, and two of them spin the other way so that the main body of the quadcopter doesn't rapidly spin as it flies. (Conservation of angular momentum.)
  
  I find it interesting that the two rotors that spin in the same direction are opposite, not adjacent. Why not put the two anticlockwise-spinning rotors on one side and the two clockwise-spinning rotors on the other side? That seems the natural way to do it.
  
  So what makes the opposites-spin-the-same design better?
upvoted_answer: This design is to prevent unwanted yaw when moving, or vice-versa.
  
  Consider the following two actions:
  
  
  When a quadcopter yaws, it does so by creating a speed difference between the opposite-rotating rotors; creates a torque that rotates the quad.  
  When a quadcopter moves left/right, it does so by slowing all the motors on one side of the frame and/or speeding those on the other, imparting a slight roll - the thrust is now slightly sideways, so it moves. (This is the same for front/back.)
  
  
  From this, you can infer that if the same-direction rotors were on the same side as each other, when creating a speed difference to yaw the aircraft you would also impart a roll. Similarly, when trying to roll the aircraft would yaw.
  
  The image below shows visually how an opposite-corner rotation quadcopter moves: