Dynamic analysis of bearingless tail rotor blades based on nonlinear shell models

Abstract

The unique structural features of helicopter bearingless rotors call 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 article a nonlinear anisotropic shallow shell model is developed that accommodates transverse shearing deformations, and arbitrarily large displacements and rotations, but strains are assumed to remain small. The displacement-based shell model has six degrees of freedom at each node and allows for an automatic compatibility of the shell and beam models, the model is validated by comparing its predictions with several benchmark problems. A four-bladed composite bearingless tail rotor system is analyzed in detail using the shell model and compared with the predictions of a beam model. Significant differences are observed between the two models, especially in the torsional behavior.