ROTOR WAKE MODELING FOR HIGH SPEED APPLICATIONS

The effect of various wake models on the prediction of rotor vibratory airloading has been studied in an ongoing research program on wake aerodynamics in high speed flight. At high advance ratios, the rotor wake structure is particularly complicated and the simple tip vortex representation often used in the past is inadequate. On the advancing side the generating blade may experience large spanwise and azimuthal load variations, including negative tip loading. New analysis methods have been developed to represent the important features of high speed wakes. One approach involves multiple filament kinematic wake models. The necessary features of such models and the flight conditions under which they perform reasonably well are discussed. The development of a novel multi-filament, full span free wake analysis using curved vortex elements is also described. The curved filaments are laid down along contours of constant vortex sheet strength thereby providing a natural representation of the wake that automatically accounts for both trailed and shed vorticity components. This method also provides a visually meaningful representation of the wake since the filaments correspond to the actual resultant vorticity field. The advanced kinematic wakes and the new free wake show a substantial degree of agreement with the data for higher harmonic airloading, with the free wake giving good results over a wider range of conditions. The results of this work suggest that accurate wake modeling is the most important factor in the prediction of higher harmonic airloading.