Structural dynamic analysis of bearingless rotor blade

Conventional articulated helicopter rotor systems typically have mechanical hinges, dampers and bearings in the hub to relieve high blade root moments and to ensure dynamic stability. This hardware operates in a high stress field and experiences large cyclic and centrifugal loads. These devices require continuous inspection and frequent maintenance. In addition, they can significantly degrade the reliability of the rotor system. In order to improve helicopter reliability, reduce maintenance, and potentially improve rotor hub aerodynamic characteristics, the helicopter researchers have developed the hingeless and bearingless rotor by taking advantage of recent advancements in materials technology. The unique structural features of bearingless rotors calls for the development of design and modeling methodologies for laminated composite flex-structures. Indeed, the flex-structure should be flexible enough to replace the flap, lead-lag, and feathering bearings, while maintaining high strength and stiffness in the axial direction. Laminated composite materials are a material of choice for such an application. Chordwise deformations, transitional zones between different cross-sections and localized compressive stresses are all likely to be present in the flex-structure, rendering the validity of a beam model questionable.;In this research an anisotropic shallow shell model is developed that accommodates transverse shearing deformations and arbitrarily large displacements and rotations. However, strains are assumed to remain small. Two kinematic models are developed in this research: the first model uses two rotation parameters to locate the direction of the normal to the shell's mid-plane while the second one uses a rotation tensor which is composed of three parameters. The latter model, which has an in-plane rotation degree-of-freedom, allows for an automatic compatibility of the shell model with other three-dimensional structural models. A shell model is validated by comparing its predictions with several benchmark problems which include static and dynamic, linear and nonlinear, as well as isotropic and anisotropic conditions. The performance of the in-plane rotation degree-of-freedom of the shell model is tested also by solving some special configurations. In actual helicopter rotor blade problems, the shell model of the flex-structure is shown to give very different results when compared to beam models. The lead-lag and torsion modes are strongly affected, whereas flapping modes seem to be less affected. A study was also carried out to simulate a tail rotor system; the pitch actuator force is found to vary significantly when shell or beam models are used.