Refined structural modeling and structural dynamics of elastically tailored composite rotor blades

A refined analytical model based on Vlasov theory is developed to (1) predict the cross-sectional stiffness constants for thin walled multi-cell composite rotor blades, (2) determine the location of the shear center for thin walled multicell composite rotor blade crosssections, and (3) investigate the effects of using spanwise non-uniform lay-ups to produce a desired twist distribution. The model uses an expanded Vlasov theory that includes transverse shear deformation of the cross-section, a warping function that captures the variation of shear stiffness along the contour of the cross-section, and the effect of two-dimensional ply elasticity. Analytical results are validated with both experimental data and detailed finite element results. The effects of ignoring two dimensional inplane ply elasticity, inplane warping, and local bending moments and curvatures were investigated using the new structural model. The influence of the skin and web thickness on the shear center location and torsion rigidity was also studied. The results show that the ignoring the two-dimensional inplane ply elasticity has a significant effect on the cross-sectional stiffness. Neglecting the local shell bending moments and twists has a significant effect on torsional rigidity for relatively thicker walled cross-sections. It was found that in-plane warping is not important for thin walled closed cell cross-sections. The shear center position is ' insensitive to the airfoil skin thickness but is sensitive to the web thickness. Torsional rigidity is influenced by the airfoil skin thickness but not influenced by the web thickness for relatively thick skin airfoils. The cross-sectional model was also integrated with a structural dynamic analysis of composite rotor blades. By using spanwise uniform and non-uniform ply-layups, a preliminary investigation on the behnvior of the composite blades was conducted. The results reveal that mode shapes are sensitive to the spanwise lay-up, however, natural frequencies remain essentially unchanged. Background And Introduction The advantages of using composites for rotor blades are well documented. Elastically tailored composite materials provide significant improvements in fatigue strength, make hingeless and bearingless designs feasible, and enhance stability, reduce vibration, and improve performance of rotor blades. Rotor blades are currently being manufactured with composite materials but elastic couplings have not yet been commercially exploited. There is a need for a simple and accurate structural model for multicell composite rotors and an accurate comprehensive aeromechanical analysis. During the past five years, continued advances have been made in the area of cross-sectional modeling of composite beams. Rapp and Worndle of Eurocopter Deutschland developed a two-dimensional crosssection analysis for composite rotor blades and compare the 2D results for cross-section properties to 3D solid finite element results (Ref. 1). Several interesting observations regarding shear center position for blades with non-uniform cross-sections are discussed. Restraint effects related to spanwise nonuniformities are also addressed. A new beam theory for laminated composites was derived from shear deformable plate theory by Sankar (Ref 2). Department Of Defense Fellow; Student Member, AHS f Assistant Professor; Member AIAA, AHS * Graduate Research Assistant Presented at The 37th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, Salt Lake City, UT, April 15-17, 1996 Copyright © 1996 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.