HHC aeroacoustic rotor tests in the German-Dutch wind tunnel: Improving physical understanding and prediction codes

Within an international research program jointly conducted by the US Army's Aeroflightdynamics Directorate (AFDD), NASA Langley, DNW, ONERA and DLR, the effects of a higher harmonic blade pitch angle variation on the rotor noise and vibrations were intensively studied. The program, denoted with Higher Harmonic Control Aeroacoustic Rotor Tests (HART) comprised both theoretical and experimental activities, the latter ones being represented by a comprehensive wind tunnel test campaign. Besides aeroacoustic measurements it comprised flow visualization tests and flow field measurements which were applied along with optical methods to determine the blade deflections. The measurements were preceded by extensive predictions performed with the analytical models of each organization. Within these simulations different models were used, leading to a very comprehensive theoretical data base. It gave an impression of the rotor codes' uncertainties which were tried to be eliminated by means of the wind tunnel findings. Although these activities yielded general improvements of the simulation codes they demonstrated also that further work is required in order to accurately predict the effects of a higher harmonic blade pitch angle on the rotor noise and vibrations.