Max C. Kurbjun
Overall sound-pressure levels and frequency spectra have been obtained under static conditions from a modified supersonic propeller designed to operate efficiently at a high forward speed without the high noise levels associated with the supersonic propeller. The three-blade, 10-foot-diameter, 1,700-rpm propeller is powered by a turbine engine and is designed to operate at a Mach number of 0.95 at 40,000 feet. The results consist of overall sound-pressure levels and frequency spectra obtained from analyses made of recordings taken during ground runups of the modified supersonic propeller. These results are compared with similar results obtained with a conventional subsonic propeller reported in NACA Technical Note 3422 and with a supersonic propeller reported in NACA Technical Note 4059. The noise output of the modified supersonic propeller displays approximately the same overall sound-pressure level and frequency-spectrum characteristics, under static conditions, as the current subsonic transport propeller reported in NACA Technical Note 3422. The maximum overall sound-pressure level produced was 120 decibels at a distance of 100 feet. This overall noise output represents a lowering of the maximum overall sound-pressure levels by approximately 10 decibels at comparable engine horsepowers as compared with the output of the supersonic propeller reported in NACA Technical Note 4059. In general, it may be stated that a propeller may be designed to possess good aerodynamic performance at high forward speeds and still provide, under static conditions, an overall noise output not greater than that of propellers currently being used on transport airplanes, and with a similar frequency spectrum.
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