Steering by tilting a spinning disc
A HELICOPTER's ROTOR BLADES are long, thin wings going round in circles, and the aircraft moves by tilting the whole spinning disc they trace out. The mechanism that does it is a sliding collar around the shaft called a swashplate. Raise the collar and every blade takes a bigger bite of air at once, so the machine climbs. Tip the collar to one side and each blade changes its angle at the same point in every rotation — so the disc tilts, and the HELICOPTER leans and goes that way.
Flying one is famously awkward because every input upsets something else. Adding LIFT changes the twist on the airframe, correcting the twist changes the heading, and the PILOT is adjusting a stick, a lever and two pedals more or less constantly. Holding a steady hover is the hardest thing to learn.
Landing with the engine off
If the ENGINE stops, a helicopter does not drop like a stone. The PILOT immediately lowers the blade angle so that, as the aircraft descends, air rushing up through the ROTOR keeps the blades spinning — the same way a sycamore seed spins as it falls. The rotor becomes a windmill storing energy in its own weight.
Close to the ground the pilot flares and pulls the blade angle back up, converting all that spin into one last push of LIFT, and the machine settles onto its SKIDS. It is called autorotation, every trainee practises it, and it is the reason an engine failure in a helicopter is an emergency rather than a catastrophe.
Several answers to one problem
Spinning a big rotor tries to twist the body of the AIRCRAFT the opposite way, and different designers have solved that differently. Most fit a small TAIL rotor pushing sideways, which works but spends up to a tenth of the engine's power doing nothing except stopping the cabin revolving.
Others avoid the tax. Heavy-lift machines like the Chinook use two big rotors turning opposite ways at either end; some Russian designs stack two rotors on one mast; a few blow air down the tail boom instead of using a rotor at all. Our drones puzzle covers the version now most familiar, which cancels the twist with four small blades turning in opposing pairs.
What they are actually for
The point of all this complexity is that a HELICOPTER does not need anywhere to run up. It can HOVER over a mountainside, a motorway, a hospital roof or the deck of a ship, winch someone aboard and leave. Mountain and sea RESCUE, air ambulances, and an AIRLIFT into places with no road at all are jobs nothing else does as well.
The trade-off is speed. On the advancing side of the disc, a blade's own speed adds to the aircraft's; on the retreating side it subtracts, so that blade runs out of lift first. That imbalance puts a ceiling of roughly 400 kilometres an hour on conventional helicopters, no matter how much power you give them.