A study on control law and hardware applying to flexible control rotor blades for flight performance improvement

Abstract

Flexible control systems for helicopter blades (e.g. HHC, IBC, Active Twist and Active Flap) have the potential to improve flight performances of rotorcraft. This study selects Active Flap as the flexible control system because it is more feasible than the others and it has been abundantly experienced in author’s organizations. CAMRAD II analyses are conducted to evaluate the effect of Active Flap on hover and forward flight performances. The hover analysis demonstrates that the required rotor power is reduced by 2.5 % with the positive 0/rev (quasistatic) flap deflection, which twists the blade in the nose down direction to reduce the local angle of attack and the local drag. The required power for hover changes non-linearly with respect to the flap deflections because of the independent behaviors of CM and CL variations. In forward flight analysis, the Active Flap is controlled by 2/rev, 3/rev and ideal flap deflections. The 2/rev and ideal controls can reduce the required rotor power for forward flight by 4% resulting from suppressing the negative angle of attack on the advancing blade. Furthermore, ideal control results in lower rolling and pitching moments acting on the hub than other harmonics, thus it can reduce re-trim control which is required when blades are actively controlled and can prevent significant increase of pilot workload. A conceptual design study on the Active Flap system to realize the developed flexible control law is carried out based on the previous research activities which had demonstrated the sufficient achievements. The system consists of two flaps and two drive mechanisms to share aerodynamic loads and prevent force fighting between the drive mechanisms.