Kim D., Song K., Kim S., Joo G.

Hingeless main rotor systems for helicopters offer fairy simple hub structures compared to conventional articulated rotor systems, and composite materials are increasingly used in the rotor systems because of their high specific stiffness, specific strength, excellent fatigue characteristics, and so on. Composite structures can be favourably manufactured to have expected static or dynamic behaviours through the tailoring of anisotropic characteristic of composite materials. For hingeless rotor system, the flap and lag hinges are removed and the flexures allow flap and lag motions through elastic deformation. Large amount of aerodynamic moments can be transmitted to rotor shaft because of the absence of hinges, accordingly aeromechanical and aeroelastic instability problems of rotor system can occur. In order to reduce vibration and increase stability, sufficient in-plane (lead-lag mode) damping is required. In this study, composite tailoring technique has been applied to the flexures of a small-scale hingeless rotor system in order to enhance structural dynamic and aeroelastic characteristics. Composite flexures can be made to have different stiffness and strength according to stacking sequences, and they can have coupling stiffness in contrast to isotropic cases. When the flexures with different coupling stiffness are applied to a rotor system, even if they have the same axial stiffness, the structural dynamic and aeroelastic characteristics can be greatly different. In order to analyze the effects of flexures with different coupling stiffness on the aeroelastic stability of the rotor system, CAMRAD II (Comprehensive Analytical Model of Rotorcraft Aerodynamic and Dynamic) has been used. Analytical results show the in-plane damping can be remarkably increased through adjusting coupling stiffness. The composite flexures are applied to the 4-bladed, 2.13-meter diameter, Froude-scaled, soft in-plane hingeless rotor system. The rotating tests are in progress. In addition, application of viscoelastic damping material is also under investigation in order to get more increased in-plane damping.