Abstract
The forward propeller of a model counterrotation propeller was tested with its original aft propeller and with a reduced diameter aft propeller. Noise reductions with the reduced diameter aft propeller were measured at simulated cruise conditions. Reductions of up to 7.5 dB were measured for the aft propeller blade passing tone, and reductions in the harmonics were also measured. Reductions in the interaction tones were observed as the result of the reduced diameter aft blades no longer interacting with the forward propeller tip vortex. Significant reductions in the total noise at each harmonic were observed. The chief noise reduction at each harmonic came from reduced aft propeller-alone noise, with the interaction tones contributing little to the totals at cruise. Total cruise noise reductions, as much as 3 dB at given angles for the blade passing tone, and as high as 10 dB at some of the harmonies were observed. These reductions would improve the fuselage interior noise levels and represent a cruise noise benefit for using reduced diameter aft propeller blades.
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