It is stated that a disadvantage of twin rotor helicopters is the lack of authoritative yaw control. In addition, this problem intensifies during autorotation.
Coaxial helicopters rely on differential collective to provide all of the rotor induced yaw control and intermeshing helicopters rely on differential collective to provide some of the rotor induced yaw control. This differential collective necessitates pedal reversal during autorotation and this, in turn, probably entails some concern during flare.
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There is a future solution, which would appear to be totally effective and without cost.
At some point in the near future rotors will be produced that have
Active Blade Twist . This in-flight 'twistability' will result in greater payload/GW ratios during hover and forward flight. Soon afterwards, these blades will be further improved to provide
Reverse Velocity Utilization , and this will increase the helicopter's maximum forward speed.
The blades on the above rotors have the ability
to vary their root pitch and their tip pitch, independently . In addition, both of the cyclic pitch changes need not be greater than once per revolution.
Yaw Control for Twin Rotor Configurations can be effected by simply leaving one rotor in the conventional hover profile. (i.e. large positive root pitch and average positive tip pitch) and then placing the counterrotating rotor in a high drag profile (i.e. large
negative root pitch and reasonably high positive tip pitch). This difference in rotational drag will yaw the craft. In addition, the direction of yaw will stay the same during autorotation.
Any arguments?