A Comparison of Predicted and Experimental Rotor Loads To Evaluate Flap-Lag Coupling with Blade Pitch

An analytical model, for the prediction of rotor performance and structural loads, has been developed to include time dependent variations in blade stiffness using a modal approach. Comparison of predicted rotor loads with wind tunnel rotor test results revealed the importance of using coupled modes and in choosing the most appropriate co-ordinate system to satisfy the hypothesis of time-invariant mode shapes. The decision to employ modes, which are either defined in hub or blade axis co-ordinates, is discussed in relation to various rotor configurations. Further rotor load comparisons are presented for a distinctly different rotor using Lynx flight data. In this case, the inclusion of time varying structural coupling between flap and lag motions demonstrated an improved capability to predict the magnitude and phase of chordwise bending moments.