Investigation of Rotor Loads and Vibration at Transition Speed

Blade section normal force and flap bending moment were investigated for UH-60 Black Hawk helicopter rotor operating at transition speed. The measured data from both flight and full-scale wind tunnel tests are compared with calculations obtained using the comprehensive rotorcraft analysis CAMRAD II. There is good agreement for the oscillatory flap bending moments between the flight and wind tunnel tests when the rotor in the wind tunnel was trimmed to match thrust and pitch and roll moments measured on the main rotor shaft in the flight test. The calculations were made using three free wake models: rolled-up, multiple-trailer, and multiple-trailer with consolidation wake models. Both multiple-trailer and multiple-trailer with consolidation models improve the normal force correlation qualitatively, and the magnitude of 1/rev and 2/rev harmonics of flap bending moments. However, none of these wake models appear to be adequate for the accurate prediction of rotor-induced vibration.