Coaxial rotors

In a coaxial rotor system, two rotors of equal diameter spin in opposite directions around a common vertical shaft. The counter-rotation cancels the reactive torque that would otherwise yaw the aircraft, so no tail rotor or other anti-torque device is needed. This allows the full engine power to drive the rotors, improving hover efficiency by roughly 10% compared to a conventional single-rotor helicopter with a tail rotor.¹

The aerodynamic interaction between the upper and lower rotors is complex. In hover, the lower rotor operates in the downwash of the upper rotor, reducing its efficiency; however, the overall system can still be more efficient than a single rotor because the induced power is distributed. In forward flight, the advancing and retreating blade effects are partially balanced, but interference can cause vibration and control coupling. Modern designs use swashplates and differential collective pitch to manage these effects.

The concept of coaxial rotors dates back to the early days of helicopter development. In 1859, French inventor Gustave de Ponton d'Amécourt patented a steam-powered model with coaxial rotors, though it never flew. In the 1930s, Italian engineer Corradino D'Ascanio built a coaxial helicopter that achieved short flights, and in 1939, the German Focke-Wulf Fw 61 used a side-by-side configuration, but coaxial designs were pursued by several pioneers.

The most significant production use of coaxial rotors has been by the Soviet/Russian Kamov design bureau. Starting with the Ka-10 in 1949 and continuing with the Ka-25, Ka-27, and Ka-52, Kamov helicopters have used coaxial rotors for naval and attack roles, valuing the compact footprint and stability in shipboard operations. In the 21st century, coaxial rotors have become common in small unmanned aerial vehicles, such as the DJI Phantom series, due to their simplicity and stability.

The primary advantage of coaxial rotors is the elimination of the tail rotor, which reduces the aircraft's footprint and makes it easier to operate in confined spaces. This is particularly valuable for naval helicopters that must land on small decks. Additionally, the absence of a tail rotor reduces noise and vulnerability to damage, and all engine power is used for lift and propulsion, improving hover efficiency.¹

However, coaxial designs have drawbacks. The mechanical complexity of the rotor head, with concentric shafts and swashplates, increases maintenance costs and weight. The aerodynamic interference between the rotors can cause reduced performance in certain flight regimes and increased vibration. Control systems must be sophisticated to handle the coupling between collective and cyclic inputs. These factors have limited the adoption of coaxial rotors in commercial aviation, where conventional designs dominate.

Beyond Kamov, coaxial rotors have appeared in several notable but less-publicized aircraft. The American Gyrodyne QH-50 DASH, an unmanned anti-submarine helicopter used by the U.S. Navy in the 1960s, employed coaxial rotors and was one of the first operational UAVs. The Cierva W.9, a British experimental helicopter from the 1940s, used a coaxial rotor with a jet at the tip of the lower rotor to provide anti-torque, an unusual variation.

In the model aircraft world, coaxial rotors are popular in toy helicopters because they are easier to fly than single-rotor models. The design also appears in some human-powered helicopters, such as the AeroVelo Atlas, which won the Sikorsky Prize in 2013; its four coaxial rotor pairs were key to achieving stable hover. Additionally, the concept has been explored for Mars helicopters; NASA's Ingenuity uses counter-rotating rotors, though not on the same axis, but the principle of torque cancellation is similar.

Torque effect

The tendency of a helicopter to rotate in the opposite direction of its main rotor due to Newton's third law.

Swashplate

A mechanical device that translates control inputs into cyclic and collective pitch changes of the rotor blades.

Collective pitch

The simultaneous change of pitch angle of all rotor blades to control lift.

Cyclic pitch

The periodic change of pitch angle of rotor blades to control the direction of thrust.