Autogiro

De la Cierva claims a freely turning wing, with no engine, whose blades are articulated to move up and down. The hinge is what makes it work.

Let a maple seed fall, or a strip of paper folded into a blade. It comes down spinning, slowly. Nothing drives it: the air moving up through it makes it turn.

Cut 4 balsa strips 120 × 15 mm. They must weigh the same — if one is heavier the rotor shakes and is useless.

Sand each strip so it sits at about 4° to the plane of rotation, all tilted the same way. Without that angle the rotor will not start turning.

Glue the four blades to a balsa disc, fixed, with no articulation at all. This is the rotor of De la Cierva's first three autogiros.

Pin the hub to a rod so it spins without friction. Do not fit a motor of any kind.

Hold it up and release it straight. The rotor spins up by itself and descends turning. That is autorotation, and nothing more is needed to get it.

Time the fall three times and average. Compare with the bare rod dropped without a rotor. The difference is the lift the rotor is producing.

With the rotor spinning, aim a fan at it from the side to simulate forward flight. The rotor tilts and the whole assembly rolls over.

The blade moving into the wind meets more air and lifts more; the one moving away meets less. The rotor rises on one side and drops on the other. That is dissymmetry of lift.

Mount four identical blades, but hold each with a small strip of cloth tape acting as a hinge. Every blade must be free to rise and fall.

Same side wind, same spinning rotor. Now the blades flap — rising and falling once per revolution — and the assembly stays level.

Follow a single blade. It rises as it advances into the wind and drops as it retreats. Rising sheds lift, so it corrects itself.

Release both rotors with the fan running. The rigid one rolls over; the hinged one comes down straight. That is the whole distance between three crashes and a patent.

The patent. US 1,590,497, "Aeroplane with rotating wings", filed 31 March 1923 and granted 29 June 1926 to Juan de la Cierva y Codorníu, a Spanish engineer. He called the machine the Autogiro, capitalised, because it was his trademark; that is why the name is kept here.

The first-flight dates, carefully. It is widely repeated that the C.4 flew on 9 January 1923. That was the day testing began at Getafe aerodrome. The first real flight was on 17 January 1923, made by Captain Alejandro Gómez Spencer — the first person ever to fly a rotary-wing aircraft. Both dates are true and they do not mean the same thing.

The three failures it took. The C.1, C.2 and C.3 did not fly. All had the same fault: as soon as the machine gathered forward speed, the rotor lifted more on the advancing side than the retreating one, and rolled over. De la Cierva tried counter-rotating rotors and other fixes before arriving at the right one, which is also the simplest: if each blade can rise and fall freely, it balances itself. A blade producing too much lift rises, and in rising it changes its effective angle and stops producing too much. Nothing has to be measured or controlled. Step 12 of this guide is precisely that discovery.

An autogiro is not a helicopter, and the two should not be confused. An autogiro's rotor has no engine: it turns because air passes up through it. So an autogiro cannot hover or take off vertically the way a helicopter can — it has to move forward. In exchange, it cannot stall the way an aeroplane does, and if the engine quits it keeps descending under control. That was De la Cierva's obsession after one of his own trimotors crashed in 1919 from a stall.

What it left behind. The flapping hinge did not stay with autogiros: it is an essential part of nearly every helicopter that followed, because dissymmetry of lift is the same problem wherever there is a rotor and forward flight. Igor Sikorsky patented his helicopter in 1935 (US 1,994,488), twelve years after the C.4's first flight, and blade articulation already came from here. And autorotation is still the procedure by which a helicopter lands when its engine fails — the same thing your rotor does in step 7. De la Cierva died in 1936 in the crash of a commercial airliner at Croydon; he was not flying it.