Assessment of Shell and Beam Elements for Modeling Rotorcraft Blades

A geometrically exact shell element is developed within the finite element, multi-body dynamics based, Rotorcraft Comprehensive Analysis System (RCAS). The shell element accommodates transverse shearing deformation as well as arbitrarily large displacements and rotations. However, strains are assumed to remain small. The shell element is formulated using an approach that allows for compatibility with other structural elements in RCAS. It is validated by comparing its predictions with benchmark problems. The 2-D shell and 1-D beam finite element analyses are compared for three typical blade configurations of varying slenderness ratio: a swept-tip blade, a blade with discontinuous chordwise elastic axis and center of gravity locations, and a blade with a flexbeam. The purpose is to quantify the differences between 2-D shell 1-D beam finite elements for modeling rotor blades. There is good agreement between the 1-D and 2-D analyses in predicting the natural frequencies of straight blades with a slenderness ratio of greater than 12.5. However, there are considerable differences in frequency predictions for a slenderness ratio of 17.5 or less and a tip sweep angle of 30 degrees or higher. The differences increase with decreasing slenderness ratio and increasing tip sweep angle. For a blade with a discontinuous center of gravity offset, frequencies predicted using the 2-D and 1-D analyses show considerable difference and increase with the center of gravity offset. For a blade with a discontinuous elastic axis offset, the frequencies predicted using the two analyses are much closer. There is good agreement in the blade structural loads (bending moment) and deformations with an elastic axis offset of 0.125C. However, the differences become significant for an elastic axis offset of 0.25C and increase as the blade rotation speed increases.