Auto-gyroing would require pitch control of the blades. The toy quadcopters control by differential thrust and torque. For attitude, they control the thrust of the different motors to get a vectored thrust effect. For orientation, the pairs of motors are spinning in opposite directions, so if you spin a pair faster and a pair slower, the torque imbalance between the CW and CCW spinning motors will spin the quadrotor body.
Autorotation in a helicopter is done by setting the blade pitch negative so that the rotor is driven by the air wooshing up through the rotor. When you are close to the ground, you pull collective, trading the energy stored in the spinning of the rotor blades for vertical thrust to slow your descent. If you do it right, you land gently with a much slower rotor speed. If you do it wrong, you fall the last five feet. :-O
(Auto-gyro works like a helicopter auto-rotation. I suspect auto-gyros typically don't have as much blade pitch control as a helicopter. As I understand it, it is much safer to land an auto-gyro with forward velocity and a roll-out rather than vertically like a helicopter.)
You would only need to auto-gyro in the event of fuel starvation or multiple engine failure. That's not the hard part. The hard part is recovering from a single engine failure. Now you have an unbalanced torque. If you don't notice the problem and do something about it VERY fast then you will flip over. It might be possible to develop a control system that can use the engine opposite the failed one for attitude control while slowing the descent (or maybe even maintaining altitude) with the other two still-balanced engines, but that is one gnarly control problem.
Twin engine airplanes have this problem too but it's much less severe. Adverse roll is a second-order effect in an airplane, it's a first-order effect in a quad. An airplane has the tail tending to stabilize it. Despite all that, losing an engine on takeoff in a twin engine aircraft is often catastrophic. In a quad, I'd wager good money (though not my entire life savings) that it's unrecoverable. But if you're going to put human passengers in it you have to plan for that because it will happen sooner or later.
Auto-gyroing would require pitch control of the blades. The toy quadcopters control by differential thrust and torque. For attitude, they control the thrust of the different motors to get a vectored thrust effect. For orientation, the pairs of motors are spinning in opposite directions, so if you spin a pair faster and a pair slower, the torque imbalance between the CW and CCW spinning motors will spin the quadrotor body.
http://en.wikipedia.org/wiki/Quadrotor
Autorotation in a helicopter is done by setting the blade pitch negative so that the rotor is driven by the air wooshing up through the rotor. When you are close to the ground, you pull collective, trading the energy stored in the spinning of the rotor blades for vertical thrust to slow your descent. If you do it right, you land gently with a much slower rotor speed. If you do it wrong, you fall the last five feet. :-O
(Auto-gyro works like a helicopter auto-rotation. I suspect auto-gyros typically don't have as much blade pitch control as a helicopter. As I understand it, it is much safer to land an auto-gyro with forward velocity and a roll-out rather than vertically like a helicopter.)