Coaxial rotors
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In the field of helicopter design there are three principal arrangements of the rotor blades, the traditional single rotor with tail rotor, the tandem twin rotors (two separate sets of rotor blades turning in opposite senses), and coaxial twin rotors, AKA "Dual Rotor" two sets of rotor blades, turning in opposite directions, but mounted upon 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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[edit] Theoretical and practical considerations
[edit] 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 sense to that of the rotors once airborne. This is described by the principle of conservation of angular momentum: initially, the helicopter possesses zero total angular momentum (i.e., is not spinning about the rotor axis). The engines of the helicopter, by turning the rotor blades, input a sizeable amount of angular momentum into the rotor blades. Since the helicopter as a system (treating the rotor blades and the body as two components of that system) remains near zero total angular momentum, the body begins to pivot about the rotor axis in the opposite direction to the rotors. In other words the torque exerted by the engine, as well as turning the blades as intended, also turns the helicopter body in relation to the rotor.
This phenomenon is catastrophic from the point of view of the pilot, who wishes to maintain stable flight. To counteract the effect, the tail rotor was introduced to provide a constant input of angular momentum to the body in the opposite direction to that from the engine. Since angular momentum is a directional quantity, the two components of the helicopter system, while possessing equal magnitudes of angular momentum, possess it in opposite directions, which cancel each other out. Thus, the condition of zero total angular momentum is maintained, but the helicopter's fuselage remains stationary and stable level flight becomes possible. Varying the torque exerted by the tail rotor upon the helicopter's tail boom (which controls the magnitude of the angular momentum input) facilitates controlled turning, and contributes to the helicopter's extreme manoeuvrability, due to the fact that in the hover condition (no lateral movement relative to the ground) the helicopter can be pivoted about the rotor axis independently of other flight controls. Control of rotational motion with the other two designs is achieved by the simple expedient of ensuring that the two sets of rotor blades rotate in opposite directions, cancelling each other out in terms of angular momentum. Rotational manoeuvring is a more complex topic with respect to these designs, however, and involves engineering features that are beyond the scope of this article.
Coaxial rotors solve the problem of angular momentum by turning each set of rotors in opposite directions, allowing the fuselage to maintain zero angular momentum until the pilot varies the angular momentum inputs in a controlled fashion to facilitate turning.
[edit] Dissymmetry of lift
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 designated not to fly at any speed which would result in the rotor tips reaching an airspeed in excess of Mach 0.8.[verification needed] In practice coaxial-rotor helicopters are slower than conventional helicopters for a given power simply because the twin rotors have higher drag.[verification needed]
[edit] 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 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).
[edit] Disadvantages
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 manoeuverable 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).
[edit] Hazards of helicopter 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 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:
- Settling with power - Reduced
- Retreating blade stall - Eliminated
- Ground resonance - Eliminated
- Low-frequency vibrations - None
- Medium frequency vibrations - None
- High frequency vibrations - None
- Transition from powered flight to autorotation - Eliminated
- Anti torque system failure in forward flight - Eliminated
- Anti torque system failure while hovering - Eliminated
- Height-Velocity Curve - Eliminated
The reduction and elimination of these hazards are the strong points for the Coaxial rotor safety design.[1][2]
