Drivetrain Influence on the Blade Loads of Hingeless Helicopter Rotors

The impact of structural rotor-drivetrain interaction on the blade loads of the Bo105 helicopter is investigated by numerical simulation. For this purpose, the constraint of constant rotor hub speed is dropped and a drivetrain model, consisting of discrete inertia elements and intermediate flexible elements, is connected to the hub. The structural rotor-drivetrain system is coupled to an aerodynamic model consisting of an analytical formulation of unsteady blade element loads combined with a generalized dynamic wake. A time-marching autopilot trim of the rotor-drivetrain system in wind tunnel configuration is performed for a large blade loading flight state as well as a high advance ratio flight state. The comparison of the simulation results with those of a baseline case (constant rotor hub speed) reveals a major drivetrain influence on the blade lead-lag load harmonics at blade passage frequency. Beside the full drivetrain model, reduced models are shown to be capable of predicting the drivetrain influence on blade loads, if they yield the same eigenfrequency of the coupled rotor-drivetrain mode ωRDL2 (second collective lead-lag mode couples with drivetrain) as the full model. In a sensitivity analysis, ωRDL2 is varied by modification of the stiffness of a reduced drivetrain model. The resulting changes in blade loads are correlated to ωRDL2, which serves as a simple but accurate classification of the drivetrain regarding its influence on vibratory blade loads. Finally, the improvement of lead-lag load prediction by the application of a drivetrain model is demonstrated through comparison of simulated loads with measurements from a wind tunnel test.