Contra-rotating rotors

Coaxial rotors or "coax rotors" are a pair of helicopter rotors mounted one above the other on concentric shafts, with the same axis of rotation, but turning in opposite directions (contra-rotation). This configuration is a feature of helicopters produced by the Russian Kamov helicopter design bureau.

The idea of coaxial rotors originates with Mikhail Lomonosov. He had developed a small helicopter model with coaxial rotors in July 1754 and demonstrated it to the Russian Academy of Sciences.

In 1859, the British Patent Office awarded the first helicopter patent to Henry Bright for his coaxial design. From this point, coaxial helicopters developed into fully operational machines as we know them today.

Two pioneering helicopters, the Corradino D'Ascanio-built "D'AT3" of 1930, and the generally more successful French mid-1930s Gyroplane Laboratoire, both used coaxial rotor systems for flight.

Having two coaxial sets of rotors provides symmetry of forces around the central axis for lifting the vehicle and laterally when flying in any direction. Because of the mechanical complexity, many helicopter designs use alternate configurations to avoid problems that arise when only one rotor is used. Common alternatives are single-rotor helicopters or tandem rotor arrangements.

One of the problems with any single set of rotor blades is the torque (rotational force) exerted on the helicopter fuselage in the direction opposite to the rotor blades. This torque causes the fuselage to rotate in the direction opposite to the rotor blades. In single rotor helicopters, the antitorque rotor or tail rotor counteracts the main rotor torque and controls the fuselage rotation.

Coaxial rotors solve the problem of main rotor torque by turning each set of rotors in opposite directions. The opposite torques from the rotors cancel each other out. Rotational maneuvering, yaw control, is accomplished by increasing the collective pitch of one rotor and decreasing the collective pitch on the other. This causes a controlled dissymmetry of torque.

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