The effect of blade aerodynamic modelling on the prediction of high-frequency rotor airloads

Interactions between the blades and vortical structures within the wake of a helicopter rotor are a significant source of impulsive loading and noise, particularly in descending flight. Brown's Vorticity Transport Model has been used to investigate the influence of the fidelity of the local blade aerodynamic model on the accuracy with which the high-frequency airloads associated with blade-vortex interactions can be predicted. The Vorticity Transport Model yields a very accurate representation of the structure of the wake, and allows significant flexibility in the way that the blade loading, and hence the source of vorticity into the wake, can be represented. Two models for the local blade aerodynamics are compared. The first is a simple lifting-line model and the second is a somewhat more sophisticated lifting-chord model based on unsteady thin aerofoil theory. A marked improvement in accuracy of the predicted high-frequency airloads of the HART II rotor is obtained when the lifting-chord model for the blade aerodynamics is used instead of the lifting-line type approach. Errors in the amplitude and phase of the loading peaks are reduced and the quality of the prediction is affected to a lesser extent by the computational resolution of the wake. Indeed, the lifting-line model increasingly over- predicts the amplitude of the lift response to blade-vortex interactions as the computational grid is refined, exposing clearly the fundamental deficiencies in this commonly-used approach particularly when modelling the aerodynamic response of the blade to interactions with vortices that are much smaller than its chord. In comparison, the airloads that are predicted using the lifting-chord model are relatively insensitive to the resolution of the computation, and there are fundamental reasons to believe that properly converged numerical solutions may be attainable using this approach.