How Do Helicopters Fly? Rotors, Torque and Why They Are So Hard to Control

Each rotor blade is a long, narrow wing, and as it sweeps round it generates lift in the same way an aeroplane's wing does, by meeting the air at an angle and deflecting it downward. The total lift of the rotor depends on how fast it spins, which is kept nearly constant, and on the angle of the blades, which the pilot changes. Raise the angle of all the blades together, with a lever called the collective, and the rotor makes more lift and the helicopter climbs; lower it and it descends. Because the blades push a large column of air downward, the downwash under a hovering helicopter is a gale, and the machine is in effect standing on it.

To move sideways, forwards or backwards, the pilot tilts the whole disc of the rotor in that direction with the cyclic, the stick between the knees, and the tilted lift pulls the aircraft along. The tilt is achieved not by moving the rotor shaft but by changing each blade's angle as it goes round, through a mechanism called the swashplate: a blade is given more angle on one side of the circle and less on the other, so that it climbs on one side and descends on the other, and the disc as a whole tilts. There is a complication, worked out in the 1920s, which is that a spinning rotor behaves like a gyroscope and responds to a force a quarter of a turn late, so the swashplate is rigged to apply the change ninety degrees before the effect is wanted. The controls:

• •Collective: left-hand lever; raises or lowers the angle of all blades at once, for climbing and descending
• •Cyclic: right-hand stick; tilts the rotor disc, for moving in any direction
• •Pedals: control the tail rotor, to point the nose
• •Throttle: on the collective's grip, mostly automatic in modern machines, keeping rotor speed constant as the load changes

Newton's third law says that if the engine turns the rotor one way, the rotor turns the helicopter the other way, and without something to stop it the fuselage would spin under the blades. The usual answer is the tail rotor, a small propeller on the end of the tail boom that pushes sideways to cancel the torque, and its thrust is what the pedals adjust, so that pressing one pedal swings the nose round. It consumes about a tenth of the engine's power and is the most vulnerable part of the aircraft; the fenestron, a shrouded fan in the tail fin, and the NOTAR system, which blows air out of the boom, are safer alternatives. Twin-rotor helicopters such as the Chinook avoid the problem by having two rotors turn in opposite directions, and coaxial designs stack two counter-rotating rotors on one shaft.

In a hovering helicopter nothing is stable. Raise the collective and the rotor demands more torque, so the nose swings and the pedals must move; the extra power changes the airspeed over the blades, so the cyclic must move; every correction requires another, and the pilot is making small movements with both hands and both feet continuously. Forward flight brings a further problem: the blade advancing into the airflow meets it faster than the blade retreating from it, so it makes more lift, and without correction the aircraft would roll. Juan de la Cierva solved it in 1923 by hinging the blades so that they flap up and down freely, and it is the flapping that limits a helicopter's top speed, since at around 400 kilometres an hour the retreating blade stalls. Modern machines add stability augmentation and autopilots, but the basic aircraft is one that must be flown every second, which is why learning takes longer than learning a fixed-wing aircraft and why the accident rate is higher.

A helicopter with a dead engine does not fall like a stone. The pilot lowers the collective at once, so that the blades are flat, and as the aircraft descends the air rushing up through the rotor keeps it spinning, like a sycamore seed, a state called autorotation. The spinning rotor stores enough energy that, near the ground, the pilot can raise the collective and use it to cushion the landing. Every helicopter pilot practises the manoeuvre, and it works from any height above a few hundred feet; the dangerous zone is low and slow, where there is no time to establish the glide. It is the reason the machine is certified to fly at all, and the reason a rotor is designed to spin freely of the engine the moment the engine stops driving it.

A helicopter hovering close to the ground needs noticeably less power than one hovering high up. Near the surface, the downwash cannot spread freely; it hits the ground and forms a cushion of air, and the swirling vortices at the tips of the blades are partly blocked, so the rotor works more efficiently. This ground effect is strongest within about one rotor diameter of the surface and fades above that. Pilots of heavily loaded helicopters sometimes rely on it, lifting a few feet into a hover and then gaining forward speed before climbing, because they could not hover higher with the power available.

The opposite hazard is called vortex ring state. If a helicopter descends fairly steeply while moving slowly forward, it can sink into its own downwash, and the rotor ends up recirculating the same disturbed air in a doughnut-shaped vortex around the blade tips. Lift falls away, and raising the collective to add power only makes the ring stronger, so the aircraft sinks faster. The recovery is to fly out of it, by pushing the cyclic to gain forward speed, or by other techniques depending on the type, into clean air. It is one reason instructors teach careful approach angles and speeds for landing.

A rotor's lift depends on the density of the air it moves, and air thins with height and with heat. On a hot day at a mountain airfield the blades must meet the air at a steeper angle to make the same lift, which needs more power, just when the engine, also breathing thin air, produces less. Pilots combine altitude and temperature into one figure called density altitude: at an airfield 1,500 metres up on a 35 degree day, a helicopter performs roughly as it would about a kilometre higher on a standard day. Hot, high and heavy is the combination that limits what it can lift.

That is why the most famous high-altitude landing was such a feat. In May 2005 the French test pilot Didier Delsalle landed a standard production Eurocopter AS350 on the summit of Mount Everest, where the air has only around a third of its sea-level density. The aircraft had been stripped of everything unnecessary to save weight, and he kept the skids on the snow for a few minutes before taking off again. Rescue helicopters in the Himalaya work close to similar limits, often lifting climbers on a long rope slung below the aircraft because there is nowhere safe to land.

The idea of lifting off with a spinning rotor is old. Leonardo da Vinci sketched an aerial screw in the 1480s, and Chinese children played with bamboo spinning tops that rose into the air centuries earlier. Turning the idea into a controllable machine took until the twentieth century, because of the problems of power, stability and torque.

Igor Sikorsky, a Russian-born engineer working in the United States, flew his VS-300 in 1939, and its single main rotor with a small tail rotor became the layout used by most helicopters ever since. Helicopters were soon used for rescue, and their ability to hover and land almost anywhere made them indispensable for medical evacuation, mountain rescue and work at sea.

A helicopter flies because its rotor blades are wings that generate lift as they sweep round, controlled by the collective, which changes the angle of all blades to climb or descend, and the cyclic, which varies the angle around the circle to tilt the rotor and move the aircraft. A tail rotor cancels the torque that would spin the fuselage, hinged blades correct the imbalance between advancing and retreating sides, and a rotor that keeps spinning in the airflow lets the machine glide down if the engine fails.