Coaxial rotors are a pair of rotors turning in opposite directions, but mounted on a mast, with the same axis of rotation, one above the other. This configuration is a noted feature of helicopters produced by the Russian Kamov helicopter design bureau.
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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 sizeable amount of spin angular momentum to the blades. Absent 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 in order 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 spin angular momentum in the other direction. This explains the tendency of the body to start spinning. This phenomenon would cause the helicopter to be out of 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, cancelling each other out in terms of angular momentum. Rotational maneuvering is a more complex topic with respect to these designs, and involves engineering beyond the scope of this article.
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.
Once a single-rotor helicopter is in forward flight, a second phenomenon manifests itself, called dissymmetry of lift, which possesses the potential to disrupt stable flight at speed. Dissymmetry of lift imposes an upper speed limit (known as the Never-Exceed Speed or VNE) upon single-rotor helicopters, by virtue of the fact that during one rotation of the rotor disc, a rotor blade experiences, in extreme parts of the flight envelope, two widely contrasting unstable conditions. On one side (the advancing side) of the rotor disc, rotor blades travel through the air sufficiently quickly for the airflow over them to become transonic or even supersonic, which causes fundamental changes in the airflow over the rotor blades, while on the other (retreating) side of the rotor disc, the rotors travel through the air much more slowly, possibly slowly enough to enter the stall condition, thus failing to produce lift. Both aerodynamic régimes result in (frequently catastrophic) flight instability.
Coaxial rotors solve the problem of dissymmetry of lift because one set of rotors is cancelled by the corresponding increased lift on the same side of the other set of rotors, and vice versa, resulting in a helicopter that can fly, theoretically at least, faster than a single-rotor design, and more stably in extreme parts of the flight envelope. Coaxial-rotor helicopters still possess a never-exceed speed, however, because the problems arising from rotor tips entering the supersonic aerodynamic régime still apply, and typically, even a coaxial-rotor helicopter is limited to flight at any speed which would result in the rotor tips reaching an airspeed in excess of approximately Mach 0.8.[1] In practice coaxial-rotor helicopters are slower than conventional helicopters for a given power simply because the twin rotors have higher drag.[verification needed]
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 the case of some designs can be severe (the UH-1 Iroquois or 'Huey' is a particularly loud example). Also, helicopters using coaxial rotors tend to be more compact (occupying a smaller 'footprint' on the ground) 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).
A principal disadvantage of the coaxial rotor design is the increased mechanical complexity of the rotor hub - linkages and swashplates for two rotor discs need to be assembled around the rotor shaft, which itself is more complex because of the need to drive two rotor discs in opposite directions. In an elementary engineering sense, the coaxial rotor system is more prone to failure because of the greater number of moving parts and complexity, though the engineering tolerances in aerospace are usually sufficiently precise to mitigate this somewhat. Additionally, while the resulting design has the capacity to be even more maneuverable than a conventional helicopter, achieving this in practice requires some ingenuity. As an example, the Kamov Ka-50 Werewolf (NATO reporting name 'Hokum') took a long time for Kamov to develop from prototype to operational status (though part of this long development time was because of additional complexities, such as the unique K-37-800 ejector seat mechanism on the Werewolf).
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 items of hazards have been listed to indicate that 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:
The reduction and elimination of these hazards are the strong points for the coaxial rotor safety design.[2][3]
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