A preliminary comparison between the SR-3 propeller noise in flight and in a wind tunnel

The noise generated by supersonic-tip-speed propellers is a possible cabin environment problem for future airplanes powered by these propellers. Models of these types of propellers were previously tested for acoustics in the Lewis 8-by-6-foot wind tunnel. One of these propeller models, SR-3, has now been tested in flight on the Dryden JetStar airplane and preliminary noise data have been obtained. Preliminary comparisons of the maximum blade passing tone variation with helical tip Mach number taken in flight with those taken in the tunnel showed good agreement when corrected to the same test conditions. This indicated that the wind tunnel is a viable location for measuring the noise of these propeller models. Comparisons of the directivities at 0.6 and 0.7 axial Mach number showed reasonable agreement. At 0.7 and 0.8 axial Mach number the tunnel directivity fell off quicker towards the front than did the airplane data. A possible explanation for this is boundary layer refraction which could be different in the wind tunnel than in flight. This may indicate that some corrections should be applied to both the airplane and wind tunnel data at the forward angles. At the peak noise angle the boundary layer refraction does not appear to be significant and no correction appears necessary.