how autogyros work

with our thanks to Jeff Lewis

Though they share the same basic controls as a fixed wing aeroplane - Stick, Throttle, and Rudder pedals - Gyroplanes are totally different. Though the same basic control inputs are needed as a fixed wing aeroplane, the Gyroplane is significantly more nifty and manoeuvrable. Gyro pilots will routinely perform manoeuvres such as steep turns which would leave their fixed wing counterparts gasping for breath. As if that wasn't unusual enough in itself, these manoeuvres are also performed without any of the gut-wrenching changes in G force "enjoyed" by fixed wing pilots


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 sail Is 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.

The vector diagram above illustrates this phenomenon. (The main diagram, and the diagram in the lower right have been rotated such that the plane of rotation forms the horizontal axis. The actual orientation of the rotor is illustrated in the small diagram in the top right.) The diagram in the lower right shows the winds relative to the rotor. Since the rotor is spinning, there will be relative wind due to this spin, which is labelled as Relative Wind due to Rotor. The Relative Wind due to Aircraft Movement is due to the fact that the aircraft is moving forward, and the rotor is mounted in such a way that the plane of rotation is at a slight angle to the direction the aircraft is moving in. The sum of these two vectors is the relative wind to the airfoil, and is labelled as Resultant Relative Wind. The main diagram shows a cross section of the rotor at a point in time where it is moving forward relative to the aircraft. The Resultant Relative Wind from the smaller diagram is shown on this as the Relative Wind. Any wind passing over an airfoil will create both lift and drag. The lift will be perpendicular to the airflow, and the drag will be parallel to the airflow. This is true for all airfoils, not just for the rotor in an autogyro. When the lift and drag vectors are added together, they create a Resultant Force. In autorotation, this resultant force is in front of the Axis of Rotation, so in addition to providing lift, it also pulls the rotor forward. This is in sharp contrast to the rotor of a helicopter in forward flight. A helicopter gets its propulsion by tilting the rotor forward. This angles the lift forward, giving the helicopter forward propulsion.


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.


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.