One way to think of autorotation is the effective use of stored energy to safely land the helicopter. Like the slow and careful release of the energy stored in a wound spring as opposed to allowing a quick high-energy release.
A helicopter sitting on the ground has no stored energy (battery excluded). After start up, the energy in the fuel is converted to motion via the engine. During lift off and climb out the engine continues to add energy to the system. Once established in cruise flight, the helicopter has three sources of stored energy: kinetic energy (from motion) in airspeed and rotor rpm, and potential energy (known as gravitational potential energy because of its position in a gravitational field) in altitude.
When an engine quits, the conversion of fuel to energy stops. When this happens the first step is to lower the collective control to reduce drag on the main rotor blades, which prevents rotor rpm from slowing down. This causes the helicopter to start descending and this begins the consuming of altitude energy to keep the rotor system spinning. In an autorotative descent, at a fixed airspeed, lowering the collective will increase rotor rpm. To spin faster, the rotor system requires energy, so energy is removed from stored altitude and the helicopter’s descent rate increases. In reverse, raising the collective takes energy from the rotor system (it slows down) and transfers it to altitude and the helicopter’s descent rate slows. Using the collective, the pilot can move energy between rotor and altitude to assist in maneuvering the helicopter to a landing spot.
However, it is extremely important to not let the rotor rpm get too slow. Allowing this to happen will cause the rotor blades to stall and completely eliminate the pilot’s ability to control and slowly use the stored energy. The helicopter will free-fall and release all its energy at impact–enough energy to destroy the helicopter and its occupants.
In an autoraotative glide the pilot can also control airspeed and the same energy transfer concepts apply. Increasing airspeed requires energy and it needs to come from somewhere. In this case from altitude and rotor rpm, so when increasing airspeed the helicopter will descend faster (loss of altitude energy) and rotor rpm will drop (loss of rpm energy). Basically, the pilot is transferring energy from altitude and rotor rpm to airspeed. Decreasing airspeed puts energy back into altitude and rotor rpm. It is the skillful manipulation of all this stored energy that will allow the pilot to make a successful power off landing.
As the pilot maneuvers to a landing spot the helicopter gets closer to the ground and is running out of stored altitude energy. That’s OK as the goal is to land. Maintaining approximately 60 knots airspeed leaves a healthy amount of energy in airspeed to stop the descent rate and this is done by flaring at a low altitude, normally less than 100 feet agl. During the flare, the rotor system will also absorb energy causing rpm to increase and can be controlled by raising the collective. Care must be taken to not flare too much or add too much collective as this can cause the helicopter to gain altitude. The objective is to bring the helicopter to about a 5- or 10-feet hover above the surface. Timed right, all or most of the airspeed energy will be consumed and the helicopter will momentarily be close to the ground with no descent rate and little or no forward speed. However, it will start descending again and here is where the pilot will raise the collective to provide a burst of lift to cushion the touchdown. Raising the collective uses the energy stored in the rotor system and rpm rapidly slows down. Done right the helicopter will once again be sitting on the ground with all of its stored energy depleted.
AS350 flaring during a practice autorotation