A counter rotating coaxial main rotor does not require an anti-torque device as the upper and lower rotor’s collective pitch are rigged such that the driving torques are equal and opposite. Because the coaxial rotor does not require an anti-torque device such as a tail rotor, Fenestron, or Notar, a coaxial rotor helicopter requires about 10% less power than a single rotor/tail rotor helicopter.
In addition to the power saved by eliminating the anti-torque device, a coaxial rotor requires less power due to favorable interaction of the upper and lower rotor wakes. The coaxial rotor’s reduced power required relative to other rotor configurations can be used in several ways:
For the same rotor diameter and power the coax will lift approximately 11% more weight (9% net of weight empty increase). This equates to 30% greater useful load, which means more payload or fuel. The coax helicopter can hover at a 6000 ft higher altitude, or at a higher temperature.
- The diameter of the coax rotor can be reduced by up to 15% to reduce rotor and drive system weight resulting in a lower total aircraft weight.
- The diameter can be reduced by something less than 15% to both reduce weight and increase hover capability.