Explainer #8
How does a helicopter steer?
A helicopter's rotor blades are long, thin wings. The engine keeps them spinning at an almost steady speed, and each blade pushes air down, so the air pushes it up. To climb, the pilot tilts every blade a little more at the same time: more lift. To move forward, a plate under the blades makes each blade tilt a little more on one side of the circle and a little less on the other, so the whole spinning disc leans and pulls the helicopter that way. A small rotor on the tail pushes sideways to stop the body from spinning the opposite way to the big rotor.
The principle
1. Spinning blades are wings. Each rotor blade has the same shape as an aeroplane wing and is set at a small angle. As it whirls round, it pushes air downwards, and the air pushes the blade up. The rotor spins at an almost constant speed; what changes is the angle of the blades. (In the video the rotor is shown slowed right down.)
2. Up and down: all blades tilt together. With one lever, the pilot slides a plate around the rotor shaft up or down. When it goes up, every blade twists to a slightly steeper angle at the same time. Steeper blades push more air down, so there is more lift and the helicopter climbs. Plate down: flatter blades, less lift, and it sinks.
What is the plate? It is two rings around the shaft, one on top of the other. The lower ring does not spin: the pilot’s controls push it up, down or tilt it. The upper ring sits on it with a bearing and spins with the blades. A rod goes from the upper ring to each blade and twists it.
3. Steering: tilt the plate. With the stick, the pilot tilts the plate. Now, as a blade goes round, its rod rides up on one side of the plate and down on the other, so the blade twists a bit more on one side of the circle and a bit less on the other. One side lifts more, so the spinning disc of blades leans over. Its push now leans too, and pulls the helicopter in that direction. The shaft itself does not tilt, only the disc of blades.
4. The tail rotor stops the spin. Turning the big rotor one way pushes the body round the other way. A small rotor on the tail pushes sideways to cancel that. Pushing a bit more or a bit less with it (using the pedals) turns the nose left or right.
Common myth
A helicopter steers by tilting the whole rotor shaft
The shaft (the mast) stays fixed to the body. What tilts is the spinning disc of blades: each blade changes its angle a little as it goes round, so it rises on one side and dips on the other. That is why one small plate under the blades is enough to steer the whole helicopter.How this video was made: Rendered 3D model of a simplified single-rotor helicopter with three main blades. The rotor is slowed right down, the blade twist and the lean of the disc are exaggerated so you can see them, and the plate is drawn as two coloured rings.
