AUTOROTATION The motion of the rotor and the resulting upward thrust, or lift, depends entirely upon autorotation, resulting from the air flowing up and through the slightly tilted rotor blades as the machine moves forward. Nature has applied the principle of autorotation for millions of years, seen in the whirling flight of the sycamore seed as it falls to the ground. Auto rotation slows its descent and the wind has greater opportunity to disperse the seeds over a wider area. The windmill was probably the first human invention which used autorotation, by harnessing the wind to produce rotary motion. The idea of a flying windmill, where rotating sails produced a wind to lift the machine, had a certain fascination with inventors, and among Leonardo da Vinci's thousands of drawings is an idea for flight along these lines. The real possibility for achieving such a machine was, however, delayed until development of the airfoil and the airplane which embodied this device. A windmill is basically an airscrew or propeller working in reverse, such that the air flowing over the sails is deflected by them, and exerts a force on the sails pushing them around. The sails effectively 'give way' to the wind and are pushed round by it. As early as the Middle Ages, however, it was realized that if the saiIs were set at a verv flat angle to the wind they would be made to rotate against the airflow and thus be 'pulled' round into the wind. The principle here is the same as with a sailing ship which can 'tack' close to the wind, meaning it can move forward against the wind, at a shallow angle to it, if the sails are properly set. In much the same way a glider moves forward as it descends through the air. The rotor blades of an autogyro are shaped to achieve the same effect, and set at a shallow angle of about two degrees to the horizontal plane in which they rotate. The shape is that of an airfoil which enables the blades to turn into the airflow rather than be pushed round by it. When turning fast these rotor blades offer considerable resistance to the upward airflow, and it is their resistance that can be used to provide lift. The amount of lift created depends upon a compromise between the airspeed of the rotors, and the resistance the rotating blades offer to the airflow past them. In practice the desired lifting force is only produced when the blade speed greatly exceeds the forward speed of the machine.

TAKEOFF To take off the rotor must produce adequate lift and it is necessary therefore to bring the rotor up to the required speed. This can be done in two ways. The first and simplest way is to propel the machine forward and, by tilting the rotor system back, making use of the airflow through the blades to build up the rotor speed. This, however, requires a suitably long runway. A second method involves more complex machinery but makes possible very short takeoff distances. Here the rotor is brought up to speed by a linkage to the engine used to provide the forward motion. When the rotor has the correct speed, the linkage is disengaged. The machine is then allowed to move forward and take off is achieved by tilting back the rotor system. Some autogyros can 'jump start' by over-speeding the rotor using the engine. The drive is then disengaged, and the rotor pitch increased. The aircraft jumps, using the stored energy, and continues then in autorotation.

LANDING When the engine and propeller speed are reduced, the forward speed will decrease and the autogyro goes into a steady descent path. The autorotation principle still applies, as the air flowing up and through the rotor maintains the rotorspeed. A lifting force is therefore produced which, although insufficient to maintain the machine altitude, prevents it from falling like a stone. Even when the propeller is stopped, the autogyro will descend safely, under full control, from any altitude.

In this respect the autogyro is at some advantage over the helicopter since in the case of the helicopter's engine failure the 'climbing pitch' angle of the rotors (about 11 degrees) would quickly stop them, with disastrous results. To keep his rotors turning the pilot will have to quickly reduce the pitch angle of his blades to that which provides 'autorotation' for a safe forced landing, but some valuable height may be lost in the process.