Free Wake Analysis Of Rotor Configurations For Reduced Vibratory Airloads

The prediction of the dynamic behavior of helicopter rotor blades is critically dependent on the correct determination of the aerodynamic loading. It is well known that the aerodynamic environment in which a rotor blade operates is to a large extent determined by the vortex wake of other blades as well as by its own wake, and the analysis of rotor wake behavior in forward flight has consequently been of great interest for many years. Until recently, however, rotor aerodynamic analyses have been executed using models that lack many of the important features of the actual wake structure and so have met with indifferent success in correlating measured loads. This paper will discuss the status of ongoing work at Continuum Dynamics, Duke, and Princeton which has been directed at developing a forward flight aerodynamic analysis employing a novel approach to free wake simulation. The model discretizes the sheet of vorticity that trails from each blade by laying out vortex filaments along contours of constant sheet strength; these filaments are composed of curved elements and distort freely in response to the local velocity field. This approach eliminates the artificial distinction between wake vorticity that is generated as a byproduct of spanwise or azimuthal variations in the blade load and also generates a visually meaningful wake representation. Results to date indicate that this wake model, when coupled with a lifting surface model of the blade aerodynamics, has considerable promise as a tool in the prediction of unsteady airloads. This paper describes not only the substance of the model as it currently exists and its performance in data correlation exercises but also focuses on its potential utility as an aid to rotor design for reducing vibratory excitation. In'addition, the work planned for the remainder of the development effort is outlined.