Forward Flight Stability Characteristics for Composite Hingeless Rotors with Transverse Shear Deformation

Abstract

The shaft-fixed aeroelastic stability behavior of a soft-in-plane, composite hingeless rotor blade in hover and in forward flight has been investigated by using the finite element method in both space and time. The non-classical structural effects, such as anisotropy, transverse shear, and torsion warping, are incorporated in the structural formulation. Timoshenko-type shear correction coefficients, which take into account the bending-shear and extension-shear couplings, are introduced to consider the nonuniform distribution of shear across the section of the blade. The aerodynamic model in the current aeroelastic analysis is formulated to allow either quasi-steady or unsteady two-dimensional aerodynamics. The Leishman-Beddoes model based on an indicial response method is employed to consider the unsteady aerodynamic effects. The effects of compressibility and reversed flow are also incorporated. Finite element equations of motion undergoing moderately large displacements and rotations are derived using the Hamilton's principle. Numerical simulations are carried out to validate the current analysis with other literature. The influence of composite couplings, transverse shear deformation, and unsteady aerodynamics on the aeroelastic behavior of soft-in-plane helicopter blades is investigated. Numerical results illustrating a great potential to improve the aeroelastic stability are presented for a blade with positive tension-pitch coupling. The transverse shear deformation is seen to have a stabilizing effect on the lag mode stability for cases with elastic couplings. Overall, the transverse shear effects become larger at higher forward speeds. It is also seen that the lag mode stability is significantly influenced by the unsteady aerodynamic effects.