Indoor Gyro
Philly the reason a helicopter must effect a collective pitch change to acheive autorotation unlike an auto gyro is that the helicopter sees a 180 degree change of airflow through the rotor system upon loss of power. At all times in powered flight (this of course does not apply to an RC helicopter in an inverted hover) the airflow is down through the rotor and upon loss of power the the flow reverses to an angle from below therefore the pitch must be reduced to acheive the proper AoA so the inner portion of the blades can provide that thrust component which in turn allows the outer portions to provide lift.
John