WHAT IS COAXIAL HELICOPTERS CONFIGURATION?
The coaxial helicopter configuration was frequently seen in the experimental period as the contra-rotation offered torque cancellation. However, the coaxial machine also has some idiosyncrasies in autorotation and once the single rotor machine was perfected the coaxial was simply too complicated except for special applications. For use on naval vessels, where hangar space is limited, the absence of the tail rotor allows a more compact hull for a given load capacity. The Kamov machines demonstrate that amply. The coaxial configuration has also been used with success on unmanned helicopters.
HOW IT WORK?
Provided that the two rotors are not too far apart, the coaxial rotor system in the hover behaves very much like a single rotor with the same total solidity. This is not true if the vertical separation is significant, because if the wake of the upper rotor has started to contract the lower rotor will encounter increased inflow velocity and will thus need more power. However, this power loss may be cancelled by the fact that the lower rotor recovers the swirl energy of the upper rotor.
The coaxial helicopter needs two swashplates. The first connects the stationary cyclic controls to the lower rotor, and the second allows the cyclic control to pass from the lower rotor to the upper rotor. The cyclic control thus tilts both rotors in the same sense. The collective control will apply the same pitch change to both rotors. Yaw control is commonly obtained by differentially changing the collective pitch. This requires rotating mixers to add the cyclic inputs from the swashplates to the collective inputs that generally pass up the centre of the mast. The resulting mechanism is incredibly complicated as a study of a Kamov rotor head will show.
To overcome the stability problem, coaxial helicopters need a significant amount of vertical tail area to give yaw stability in autorotation. The fins are often slatted to prevent stalling at high angles of attack. In some machines a mechanism is fitted so that the action of the pedals is automatically reversed when the collective is fully lowered.
In forward flight the coaxial helicopter has the advantage that the retreating blade of one rotor is on the opposite side to the retreating blade of the other rotor. This means that if a suitably rigid rotor head assembly can be built, the lift trough due to a retreating blade can be overcome by the lift from the advancing blade of the other rotor and higher forward speed becomes possible.
