title_body: Why can't quadcopters handle failure of one rotor? Ardupilot can detect (and it logs) motor output mismatch. I think detecting in flight motor failure is possible with modern firmware and flight controllers. Then if one rotor fails, why can't we have the quadcopter fly like a tricopter, since we now have ESCs that can change rotation directions?
upvoted_answer: I saw a TED talk from a guy who had written quadcopter software to handle losing one motor. Perhaps you can find it.
  With nothing to counter the unbalanced torque, the quad spun in yaw quite rapidly. Most software would attempt to counter this by increasing power on the motor opposite the dead motor, and decreasing power to the other two. Instead they allowed the quad to yaw and concentrated on maintaining stability.
  Although the unbalanced lift would tend to tip the quad over, it would tip the quad back a fraction of a second later when it had spun 180 degrees. I guess the software also had to take account of gyroscopic effects, and the time taken to spin a motor up or down.
  The demonstration was indoors, so I couldn't tell if the quad was capable of 'forward' flight, but it hovered and descended under control.
  You may well ask why this isn't part of common quadcopter firmware. I can only speculate that it's because:
  
  It's rare to loose a motor in flight (failures are usually a result of crashes)
  The algorithm for 3 motor flight isn't public domain (maybe the above team sold it to the military or something)
  Landing while spinning rapidly does almost as much damage as crashing.
  People who care about redundancy just add more motors.