From RC Helicopter Wiki
An autorotation is a procedure where a helicopter can make a controlled approach and landing after motor failure, or deliberately cutting the power mid-air with the throttle hold switch on the transmitter.
An autorotation consists of two phases. Immediately power is lost, the collective is lowered and the pilot establishes an autorotative glide, where air flowing through the main rotor from bottom to top causes the rotor to maintain RPM (much like a windmill) and generate sufficient lift to make a controlled descent. As the helicopter approaches the ground, the pilot executes a flare, converting the helicopter's forward speed into additional rotor velocity and lift, and allowing the helicopter to establish a brief hover before landing, or perform a gentle running landing.
The main rotor is able to freewheel during autorotation due to a one-way bearing in the transmission. Depending on the location of the one-way bearing in the transmission, the tail rotor may (or may not) be driven from the energy gained by the main rotor, making it easier to maintain or change the helicopter's heading throughout the autorotation.
Autorotation performance is greatly affected by the size of the helicopter. 450-size helicopters can be autorotated but there is very litte spare energy at the end of the approach. Larger helicopters, such as 600 size machines, are much easier to autorotate and have much more hang time---the amount of time the helicopter can hover before the main rotor loses useful lift---making the procedure much more tolerant of error.
Autorotations can also be performed inverted (with positive collective pitch added): often the inverted section will be flown backwards, to enable the flip to upright to be made a part of the flare while minimizing the risk of hitting the tail on the ground. Another variation are blade stop autos, where the pilot deliberately adds positive pitch to slow the main rotor almost to a stop, then gives negative pitch to spin the rotor again---hopefully giving enough energy to complete the autorotation with the helicopter in on piece.
Autorotation practice is typically started by performing hovering autorotations from no more than 1 meter above the ground so the pilot can learn how much energy is stored in the main rotor. As the pilot becomes more confident, short practice autos can be performed from 1-3 meters in slow forward flight in approach for landing. Ideally, autorotations are performed into the wind, as the increased airspeed will give the helicopter increased translational lift. In practice, the engine may fail at inopportune times!
An autogyro is an aircraft that generates all its lift by a constantly autorotating main rotor, with forward power provided by a pusher prop.
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