STRUCTURAL DYNAMIC MODELING OF ADVANCED COMPOSITE PROPELLERS BY THE FINITE-ELEMENT METHOD (HELICOPTER ROTORS, SHEAR CENTER, ANISOTROPIC MATERIALS)

An analytical model is presented for determining the free vibration characteristics of conventional and advanced propellers composed of generally anisotropic materials (i.e., composite construction). It is assumed that the propeller is discretized into a series of straight beams (beam-type finite element), where the elastic axis of each beam element is aligned with the line of shear centers of the propeller. Blades of arbitrary shape and definition can be analyzed, since this line of shear centers is represented by a general space curve.;The beam-type finite elements are derived, using Hamilton's principle, with allowances for general anisotropic material behavior, cross sections of arbitrary shape, beam pretwist, cross section warping, and nonlinear behavior based on the moderate deflection theory (small strains and moderate rotations). The effects of blade sweep and general pitch change settings are accounted for. Numerical results, aimed at illustrating the characteristics and the behavior of the model, are presented. These results include; (1) verification studies that were done with known analytical and experimental solutions, and (2) the determination of the structural dynamic characteristics of a conventional propeller and an advanced propeller. Results from these studies show very good agreement with experimental data.;A model, which is used to determine the shear center, structural constants, and shear stress distribution of an arbitrary shaped cross section composed of multiple generally anisotropic materials is also presented. This model is derived based on two-dimensional elasticity theory and it includes three displacement functions that are used to define the warping of the cross section. Special finite elements, based on variational principles, are used to solve for these displacement functions. Numerical results are presented to illustrate the capability of the model, by locating the shear center on typical isotropic and composite airfoil cross sections. These results show that the shear center location is highly dependent on the ply-angle orientation.