HINGELESS ROTOR BLADE STABILITY USING A COUPLED AEROELASTIC ANALYSIS WITH REFINED AERODYNAMIC MODELING

A non-linear unsteady aerodynamic model based on an indicial method is incorporated into a comprehensive coupled rotor aeroelastic analysis to investigate hingeless rotor blade stability. The aerodynamic analysis consists of three phases; a linear attached flow solution, a non-linear separated flow solution, and a non-linear dynamic stall solution. A free wake model is also incorporated into the analysis. Blade responses and loadings are calculated using a finite element formulation in space and time. A modified Newton iterative method is used to calculate blade response and trim controls as one coupled solution. Aeroelastic stability is determined utilizing Floquet theory for a linearized system. Linear unsteady aerodynamics as well as dynamic inflow are included into the Floquet analysis. Blade stability for the complete non-linear system, including non-linear unsteady aerodynamics is determined from transient response data. Damping estimations are calculated using a Moving-Block analysis. Damping predictions are correlated with data from a model hingeless rotor in forward flight. Results show that linear unsteady aerodynamics significantly improve regressing lag mode damping estimation at low speeds. At high speeds, inclusion of non-linear unsteady aerodynamic effects is important to capture the trends and magnitudes of lag damping. Results of a parametric study, of soft and stiff in-plane hingeless rotors, show a significant impact of flow separation and dynamic stall on rotor lag damping at high speed or high thrust flight conditions. Compressibility effects are also important for accurate damping predictions. In addition, results using a free wake analysis depict its importance in the determination of rotor trim, which in turn affects lag damping predictions.