Abstract: Main rotor blades of the dual, counter-rotating, rigid coaxial rotor system exhibit a unique unconventional combination of positive and negative twist gradients in which the rotor system rotor Figure of Merit (hover efficiency) is improved by providing a dissimilar twist distribution between the lower rotor blade and the upper rotor blades. This improvement is specifically a result of reduced profile drag of the lower rotor system, achieved by driving the effective operating condition of the lower rotor blades to be similar to the upper rotor blade such that the tip drag losses of the lower main rotor have been reduced considerably using a mathematically vigorous approach. While minimal induced power consumption resulted due to the dissimilar lower main rotor twist, a significant profile power benefit is realized, resulting in the improved hover efficiency with essentially no reduction in rotor forward flight performance.

Abstract: Main rotor blades of the dual, counter-rotating, rigid coaxial rotor system exhibit a unique unconventional combination of positive and negative twist gradients in which the rotor system rotor Figure of Merit (hover efficiency) is improved by providing a dissimilar twist distribution between the lower rotor blade and the upper rotor blades. This improvement is specifically a result of reduced profile drag of the lower rotor system, achieved by driving the effective operating condition of the lower rotor blades to be similar to the upper rotor blade such that the tip drag losses of the lower main rotor have been reduced considerably using a mathematically vigorous approach. While minimal induced power consumption resulted due to the dissimilar lower main rotor twist, a significant profile power benefit is realized, resulting in the improved hover efficiency with essentially no reduction in rotor forward flight performance.