Model helicopter rotor aerodynamics and acoustics as measured in two anechoic wind tunnels

The qualities that are important for helicopter impulsive noise wind-tunnel testing are evaluated by comparing aerodynamic and acoustic data gathered on a single-scaled model main rotor that was tested in two open-jet wind an echoic willd tunnels of markedly different size: the CEPRA 19 tunnel in Fra nce and the DNW in The Netherlands. The rotor was instrumented with miniatu re blade-mounted pressure transducers and was tested on the same rotor test stand located within the flow of both open-jet wind tunnels. In-the-Bow ro tor acoustic signatures and selected on-blade dynamic pressure signatures a re presented for each facility and discussed in relation to the differences in tunnel geometry, tunnel flow quality, and the aeroacoustic characterist ics of each facility. This comparison shows that low background noise level s, a large in-the-Bow anechoic measurement space, and low inflow turbulence to the rotor are necessary to yield high quality aerodynamic and acoustic data. The best match of acoustic data between the two facilities occurs whe n the tip-path plane of the rotor is chosen to match closely dynamic blade- surface-mounted pressures. Achieving a good match between scaled wind-tunne l acoustics and in-Eight measurements are shown to be more difficult: Agree ment is reasonably good at low advance ratios but marginal at higher advanc e ratios.