Coaxial rotor

Preamble

In the field of helicopter design, there are three principal arrangements of the rotor blades, namely:

Single rotor with tail rotor - the 'conventional' arrangement found on the majority of extant helicopter designs currently in service (the AH-64 Apache attack helicopter is a typical example);

Tandem twin rotors - two separate sets of rotor blades, turning in opposite senses, usually separated by a physical gap to avoid the two sets of rotor blades coming into contact in flight (which would be catastrophic for the helicopter and its occupants) - the CH-47 Chinook is a prime example of this design. Tandem twin rotors are usually a design feature of heavy-lift helicopters, intended to carry large payloads;

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.

This article covers the last of these configurations, hereby shortened to 'coaxial rotors', and the reasons for the adoption of the design.

Theoretical and Practical Considerations

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) seeks to remain at zero total angular momentum, the body begins to pivot about the rotor axis in the opposite sense to the rotors.

Since angular momentum is a directional quantity, the two components of the helicopter system, while possessing equal magnitudes of angular momentum, possess angular momentum in opposite senses, which cancel each other out, and thus the condition of zero total angular momentum is achieved.

However, this phenomenon is catastrophic from the point of view of the pilot, who wishes to maintain stable flight. To counteract this effect, the tail rotor was introduced to provide a constant input of angular momentum to the body in the opposite sense to that which it acquires 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 achieved, 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 senses, 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.

Once a single-rotor helicopter is in forward flight, a second phenomenon manifests itself, called dissymetry of lift, which possesses the potential to disrupt stable flight at speed. Dissymetry of lift imposes an upper speed limit (known as the Never-Exceed Speed or V_NE) 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 both problems (the problem of angular momentum and the problem of dissymetry of lift) in the one engineering package. By turning each set of rotors in opposite senses, the helicopter maintains zero total angular momentum until the pilot varies the angular momentum inputs in a controlled fashion to facilitate turning. The dissymetry of lift of 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 0.7.

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 manouvreable 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).

Hazards of helicopter flight

As with any moving vehicle, operation outside of safe regimes could result in loss of control, structural damage, or fatality. For helicopters the hazards are particularly acute since they are flying at relatively low altitude, with little time to react to a sudden event. The following is a list of some of the potential hazards of "conventional" helicopters:

Settling with power
Retreating blade stall
Ground resonance
Low-G condition
Operating within the shaded area of the height-velocity diagram
Vortex ring state, a problem the V-22 Osprey was associated with.

Reduction & Elimination of Common Helicopter Flight Hazards

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:

1. Settling with power - Reduced

2. Retreating blade stall - Eliminated

3. Ground resonance - Eliminated

4. Low-frequency vibrations - None

5. Medium frequency vibrations - None

6. High frequency vibrations - None

7. Transition from powered flight to autorotation - Eliminated

8. Anti torque system failure in forward flight - Eliminated

9. Anti torque system failure while hovering - Eliminated

10. Height-Velocity Curve - Eliminated

The reduction and elimination of these hazards are the strong points for the Coaxial rotor safety design.

SEE *Coaxial Benefits + *Aerodynamic Features of Coaxial Configuration Helicopters