Unsteady vortex lattice aerodynamics for rotor aeroelasticity in hover and in forward flight

An unsteady vortex lattice method has been applied to the rotor aeroelasticity problem in hover and in forward flight. This procedure is an improvement over the classical two-dimensional quasi-steady aerodynamics which can predict neither the three-dimensional tip-relief effect, nor the unsteady wake dynamics beneath the rotor, both of which play an important role in rotor aeroelastic analysis. A number of code validation studies have been carried out including comparison of the present method with experimental data and other methods.;To develop a computational model of the rotor and its wake, a thin lifting surface and wake have been discretized into bound, shed and trailing vortex filaments fixed in a prescribed wake geometry. Also, to consider the unsteady wake aeromechanism among the blades, each blade's motion and deformation has been treated.;To verify the numerical modeling, Theodorsen's analytical prediction is compared with the result of the present method for a high aspect ratio wing undergoing an unsteady pitching motion; this shows excellent agreement. Also, the unsteady induced inflow was calculated and compared with experiment and other inflow models. The results for a two-bladed hovering rotor clearly show the effects of the three-dimensional tip-relief effect and the unsteady wake dynamics effect of the near and returning wake undergoing various unsteady motions. The correlation of the predicted unsteady loadings by the present method with those obtained by a finite-state wake inflow theory is excellent in most cases.;The present thin lifting surface theory also has been applied to a coupled flap-lag-torsion stability analysis for the hovering flight condition. The perturbed time histories of coupled flap-lag-torsional motions were analyzed by Fourier analyses to predict the damping and frequencies of particular modes. The results were compared with those from quasi-steady Greenberg theory which uses a constant induced downwash from simple momentum theory. The overprediction of lead-lag damping by the two-dimensional quasi-steady aerodynamics (coupled with simple momentum theory) is shown to be due to a lack of both three-dimensional tip-relief effects and unsteady wake dynamics of the near and returning wake.;Comparison with the results of a previously published lifting surface theory shows good agreement in predicting unsteady loading in forward flight. In forward flight, the present method is applied to aeroelastic response predictions for both low and high advance ratios. In low speed flight, there exists a strong blade-wake interaction which is diminished in high speed flight.