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Coaxial rotors or co-rotors are a pair of helicopter rotors mounted one above the other on concentric shafts, with the same axis of rotation, but that turn in opposite directions (contra-rotation). This configuration is a feature of helicopters produced by the Russian Kamov helicopter design bureau.
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.
Design considerations 
Angular momentum 
One of the problems with any single set of rotor blades is the tendency of the helicopter body to begin spinning in the opposite direction to that of the rotors once airborne. This is due to the principle of conservation of angular momentum: The engines of the helicopter, by exerting torque on the rotor blades, give a sizable amount of angular momentum to the blades. With no external forces such as contact with the ground, the angular momentum of the blades must be offset by a change in angular momentum of the body for momentum to be conserved. While the engine spins up the blades, the blades apply equal yet opposite torque to the body, causing the body to gain angular momentum in the other direction. This phenomenon would cause the helicopter pilot to lose control if not checked. To counteract the effect, the tail rotor was introduced to provide a constant input of angular momentum to the body opposite in direction to that from the main rotor. Then, the helicopter's fuselage remains stationary and stable level flight becomes possible. Varying the torque exerted by the tail rotor on the body of the helicopter (hence changing the magnitude of the angular momentum input) facilitates controlled turning. This gives the helicopter extreme maneuverability, and the helicopter can hover and pivot about the rotor axis. Control of rotational motion with designs lacking tail rotors is achieved by using two sets of rotor blades rotating in opposite directions, canceling each other's angular momentum.
Coaxial rotors solve the problem of angular momentum by turning each set of rotors in opposite directions. The equal and opposite torques from the rotors upon the body cancel 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.
Dissymmetry of lift 
Dissymmetry of lift is an aerodynamic phenomenon that is caused by the rotation of a helicopter's rotors in forward flight. Rotor blades provide lift proportional to the amount of air flowing over them. When viewed from above, the rotor blades move in the direction of flight for half of the rotation (advancing half), and then move in the opposite direction for the remainder of the rotation (retreating half). During the advancing half, a rotor blade produces more lift. As a blade moves toward the direction of flight, the forward motion of the aircraft increases the speed of the air flowing around the blade until it reaches a maximum, when the blade is perpendicular to the relative wind. At the same time, a rotor blade in the retreating half produces less lift. As a blade moves away from the direction of flight, the speed of the airflow over the rotor blade is reduced by an amount equal to the forward speed of the aircraft, reaching its maximum effect when the rotor blade is again perpendicular to the relative wind. Coaxial rotors reduce the effects of dissymmetry of lift through the use of two rotors turning in opposite directions, causing blades to advance on either side at the same time.
Other benefits 
One other benefit arising from a coaxial design include increased payload for the same engine power — a tail rotor typically wastes some of the power that would otherwise be devoted to lift and thrust, whereas with a coaxial rotor design, all of the available engine power is devoted to lift and thrust. Reduced noise is a second advantage of the configuration — part of the loud 'slapping' noise associated with conventional helicopters arises from interaction between the airflows from the main and tail rotors, which in some designs can be severe. Also, helicopters using coaxial rotors tend to be more compact (occupying a smaller 'footprint' on the ground), though at the price of increased height, and consequently have uses in areas where space is at a premium — several Kamov designs are used in naval roles, being capable of operating from confined spaces on the decks of ships, including ships other than aircraft carriers (an example being the Kara Class cruisers of the Russian navy, which carry a Ka-25 'Hormone' helicopter as part of their standard fitment). Another benefit is increased safety on the ground; by eliminating the tail rotor, the major source of injuries and fatalities to ground crews and bystanders is eliminated.
A principal disadvantage of the coaxial rotor design is the increased mechanical complexity of the rotor hub. The linkages and swashplates for two rotor systems need to be assembled atop the mast, which is more complex because of the need to drive two rotors in opposite directions. Because of the greater number of moving parts and complexity, the coaxial rotor system is more prone to mechanical faults and possible failure. However, US Army experience from the Vietnam War indicates that damage to the tail rotor and the long driving shaft, from enemy fire or treetop collisions, was the most frequent cause of helicopter losses, so from a military viewpoint the issue may be more complex. Also Kamov maintains that its coaxial assembly is built to withstand direct hits from 20 mm cannon fire, with limited tolerance for 23mm hits, and presents a small target. Kamov does have a good mechanical reliability record from use in demanding conditions including offshore oil rigs and Russian Navy vessels.
Coaxial models 
The system's inherent stability and quick control response make it suitable for use in small radio controlled helicopters. These benefits come at the cost of a limited forward speed, and higher sensitivity to wind. These two factors are especially limiting in outdoor use. Such models are usually fixed-pitch (i.e., the blades cannot be rotated on their axes for different angles of attack), simplifying the model but eliminating the ability to compensate with collective input. Compensating for even the slightest breeze causes the model to climb rather than to fly forward even with full application of cyclic.
Reduced hazards of flight 
The U.S. Department of Transportation has published a “Basic Helicopter Handbook”. One of the chapters in it is titled, “Some Hazards of Helicopter Flight'. Ten hazards have been listed to indicate what a typical single rotor helicopter has to deal with. The unique coaxial rotor design either reduces or completely eliminates these hazards. The following list indicates which:
- Settling with power — Reduced
- Retreating blade stall — Reduced
- Medium frequency vibrations — Reduced
- High frequency vibrations — None
- Anti torque system failure in forward flight — Eliminated
- Anti torque system failure while hovering — Eliminated
List of coaxial rotor helicopters 
- Breguet-Dorand giroplane laboratoire (1936)
- Breguet GIIE (1949-1951)
- Cierva CR Twin
- Gyrodyne QH-50
- Gyrodyne 2C
- Gyrodyne XRON-1
- Gyrodyne GCA2
- Mil Mi-X1
- Kamov Ka-15
- Kamov Ka-25
- Kamov Ka-26
- Kamov Ka-226
- Kamov Ka-27
- Kamov Ka-31
- Kamov Ka-32
- Kamov Ka-50 Black Shark
- Kamov Ka-52 Alligator
- Manzolini Libellula
- Sikorsky S-69
- Sikorsky X2
- Sikorsky S-97
- Kamov-Kumertau Rotorfly
- Kamov Ka-92
See also 
- NASA Technical Paper 3675
- A History of Helicopter Flight , J. Gordon Leishman Professor of Aerospace Engineering, University of Maryland, College Park.
- Coaxial Benefits
- Aerodynamic Features of Coaxial Configuration Helicopters