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 anechoicwindtunnelsofmarkedlydifferentsize:theCEPRA19tunnelinFranceandtheDNWinTheNetherlands. The rotor was instrumented with miniature blade-mounted pressuretransducers and was tested on thesamerotor test stand located within the e ow of both open-jet wind tunnels. In-the-e ow rotor acoustic signatures and selected on-blade dynamic pressure signatures are presented for each facility and discussed in relation to the differences in tunnel geometry, tunnel e ow quality, and the aeroacoustic characteristics of each facility. This comparison shows that lowbackground noiselevels,a large in-the-e ow anechoicmeasurement space, and lowine ow turbulenceto the rotorarenecessarytoyieldhighqualityaerodynamicandacousticdata.Thebestmatchofacousticdatabetweenthe two facilitiesoccurswhen thetip-path planeoftherotorischosen tomatch closely dynamicblade-surface-mounted pressures. Achieving a good match between scaled wind-tunnel acoustics and in-e ight measurements are shown to be more dife cult: Agreement is reasonably good at low advance ratios but marginal at higher advance ratios.