AEROELASTIC STABILITY OF BEARINGLESS ROTORS IN FORWARD FLIGHT

Aeroelastic stability characteristics for selected bearingless rotor configurations are calculated and correlated to experimental data. Bearingless rotors provide a simplified mechanical configuration since the flap, lag, and pitch bearings are all eliminated. However, the analysis of the bearingless blade is more involved because of the possible redundancy of load paths and non-linear coupling of the torsion and bending modes.;The bearingless rotor analysis is correlated to hover lag mode stability data for a simple three-bladed rotor tested in three different pitch link configurations. The analysis is extended to forward flight although no experimental data are available in these cases. A second more complicated bearingless configuration which includes precone, blade twist, blade sweep, and a lag shear restraint is then analyzed and correlated to experimental data for both hover and forward flight. The bearingless rotor program is flexible in analyzing rotor blades with a variety of physical constraints and the analytical results demonstrated good correlation to the experimental results in both hover and forward flight.;The rotor blade is analyzed by a finite element formulation based on Hamilton's principle. The element model has fifteen degrees of freedom in axial, bending and torsion deflections. Quasi-steady strip theory is used for the aerodynamic calculations while non-circulatory forces and dynamic inflow are included to approximate the unsteady effects. The analysis consists of three stages: trim solution, blade steady response, and stability calculations. The trim solution is calculated for a simple rigid articulated blade for either wind tunnel or propulsive trim as the control input to the response calculations. The periodic response is calculated by a time finite element method after the non-linear finite element in space equations are transformed to normal mode equations using the first few vacuum rotating modes. Then the stability is calculated from the perturbation equations of motion linearized about the steady response solution. These equations are transformed with the first few coupled rotating modes and solved for stability of Floquet transition matrix theory.