Structural dynamics modeling of rectangular rotor blades

This study investigates 1-D and 3-D methods for modeling helicopter rotor blade structural dynamics to better understand the accuracy of current structural modeling based on 1-D beam theory. Natural frequencies are calculated at various rotor angular speeds for a large variety of blades ranging from simple isotropic beams to a realistic composite blade. The blade shape is limited to rectangular planforms, but various lengths are considered. 1-D beam analysis is conducted using the RCAS rotorcraft comprehensive analysis with 2-D cross-sectional properties calculated from VABS. 3-D finite element analysis is based on the commercial code MSC/Marc. Accuracy of both 1-D and 3-D analyses have been assessed through analysis of discretization errors that originate from insufficiently refined meshing. There is very good agreement between 1-D and 3-D predictions for the eight lowest modes of a large variety of blades, when there is no coupling between modes of different nature (flap, torsion) induced by materials and when the blade length is greater than ten times chord. Effects of blade length for isotropic and composite beams with no coupling between modes are similarly predicted by 1-D and 3-D analyses, except for torsion frequency, where 1-D analysis closely follows classical beam theory. With the presence of flap–torsion coupling between modes, the two approaches differ on prediction of the torsion-dominant frequency and significantly on the flap-dominant frequency.