Answer: Not Helicopters
A gyrocopter (also called autogyro) is a rotary-wing aircraft that flies by means of autorotation. Unlike a helicopter, the rotor of a gyrocopter is not powered directly by a motor but turns by the action of the relative airflow on the blades.
The inventor of the Gyrocopter, Juan de la Cierva was a Spanish engineer, inventor, pilot, and aeronautical enthusiast. In 1921, he participated in a design competition to develop a bomber for the Spanish military. De la Cierva designed a three-engined aircraft, but during an early test flight, the bomber stalled and crashed. De la Cierva was troubled by the stall phenomenon and vowed to develop an aircraft that could fly safely at low airspeeds. The result was the first successful rotorcraft, which he named autogiro in 1923. De la Cierva’s autogiro used an airplane fuselage with a forward-mounted propeller and engine, an un-powered rotor mounted on a mast, and a horizontal and vertical stabilizer. His aircraft became the predecessor of the modern helicopter.
The gyrocopter’s main advantages originate in it being a microlight driven by autorotation. Therefore, it is able to fly extremely slowly while being very agile. In addition, due to the rotating blades it is impossible to stall. Short take-off and landing distances (10 to 50 meters) enable the gyrocopter to take-off almost vertically and also to land with high approach angles.
The gyrocopter is thought to be one of the safest aircrafts in the world. Should the thrust-generating propeller fail, the rotary-wing stays in autorotation. This allows the pilot to slowly and safely land the gyrocopter. In fact, landing the gyrocopter in an emergency situation is exactly the same procedure as landing it under normal conditions.