DYNAMIC STABILITY OF HINGELESS AND BEARINGLESS ROTOR BLADES IN FORWARD FLIGHT (ROTORCRAFT, FINITE ELEMENT)

The aeroelastic stability of hingeless and bearingless rotor blades in forward flight is examined. Two types of structural modelling are used; spring-restrained hinged model and elastic beam model. With a hinged model, the blade is assumed to undergo three degrees of motion; rigid body flap, lag, and feather rotations about hinges at the blade root, with hinge springs to obtain arbitrary natural frequencies. Quasisteady strip theory is used to evaluate the aerodynamic forces and the unsteady aerodynamics effects are introduced approximately through a dynamic wake induced inflow modelling. The nonlinear time dependent periodic blade response is calculated using an iterative procedure based on Floquet theory. The periodic perturbation equations are solved for stability using Floquet transition matrix theory as well as constant coefficient approximation in the fixed reference frame. Results are presented for both soft-inplane and stiff-inplane blade configurations and the effects of several parameters on blade stability are examined.;For an elastic hingeless model, the blade is assumed to undergo flap bending, lag bending and torsional deflections. A finite element formulation based on Hamilton's principle is used. The blade is discretized into beam elements and each beam element consists of fifteen nodal degrees of freedom. The blade finite element response equations are transformed to the modal space in the form of a few normal mode equations. These nonlinear response equations containing periodic terms are solved iteratively using Floquet theory. The periodic perturbation equations linearized about the nonlinear response position, are solved for stability using Floquet transition matrix theory as well as constant coefficient approximation in the fixed reference frame. The effects of several parameters on the blade stability are examined, including the dynamic inflow, reverse flow, blade structural modelling, fuselage c.g. offsets, blade section pitching moment, blade c.g. offsets, lag stiffness, torsional stiffness, precone and trim conditions.;Stability results are also obtained for a bearingless blade configuration consisting of single flexbeam with a wrap-around type torque tube, and the pitch links located one on the leading edge and the other on the trailing edge of the torque tube. (Abstract shortened with permission of author.)...