This study of noise reduction concepts for helicopter main rotors has quantified the noise reduction potential and design implications of both conventional and novel acoustic design technologies. In the preliminary assessment of the concept, the first level analyses indicate significant noise reduction is possible for some observer locations and flight conditions with some impact on performance. Evaluation of the modulated blade spacing concept included both a detailed design of the rotor and the acoustic analysis. In the detailed design it was determined that the noise reduction features of the SILENT rotors (both modulated and evenlyspaced) would come with a 16 percent payload penalty, a 6.2 percent reduction in airspeed and a 17.2 percent maximum airspeed reduction. These performance penalties are balanced, however, by some rather significant reductions in rotor noise. On an A-weighted sound pressure level basis, the reduction in peak noise levels (three-microphone average) is predicted to be roughly 8, 7 and 3 dBA during takeoff, flyover and approach, respectively. The noise reductions up-range (15-20 seconds before overhead) are even greater: 20, 16 and 10 dBA during takeoff, flyover and approach, respectively. When compared to an evenly-spaced five-bladed main rotor, the peak noise levels of the SILENT rotor with modulated blade spacing are on average quieter by 2-3 dBA during flyover and approach, but on aver age louder by 3 dBA during takeoff. Up-range the SILENT rotor with modulated blade spacing is consistently quieter, by 2 to 5 dBA, than the evenly spaced rotor for the flyover and approach conditions although the levels are higher for the take-off condition. The results for all three flight conditions for down-range microphone locations is somewhat mixed; yet the noise levels are consistently lowest for the SILENT evenly spaced rotor. In addition to being quieter, the acoustic pressure time histories for the SILENT rotor with modulated blade spacing are strikingly different from those of evenly spaced five-blade rotor and the four-blade baseline rotor. Such a difference indicates a fundamental change in how the modulated blade spacing rotor will be perceived and its sound judged, both from a community acceptance standpoint and from a military perspective. This feature of the SILENT rotor with modulated blade spacing appears to offer a unique way to favorably alter a helicopter's acoustic signature. The SILENT rotor's predicted acoustic pressure time histories have been converted into computer files suitable for aural evaluation. A follow up study of the psychoacoustic response to the noise generated by main rotors with modulated blade spacing is presented in a companion paper


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    Title :

    Predicted noise for a main rotor with modulated blade spacing


    Additional title:

    Geräuschvorhersage für einen Hubschrauberrotor mit moduliertem Blattabstand


    Contributors:

    Published in:

    Publication date :

    2005


    Size :

    8 Seiten, 9 Bilder, 1 Tabelle, 19 Quellen




    Type of media :

    Article (Journal)


    Type of material :

    Print


    Language :

    English




    Predicted Noise for a Main Rotor with Modulated Blade Spacing

    Brentner, Kenneth S. | Online Contents | 2005


    Subjective Test of Modulated Blade Spacing for Helicopter Main Rotors

    B. M. Sullivan / B. D. Edwards / K. S. Brentner et al. | NTIS | 2002


    The effects of modulated blade spacing on static rotor acoustics andperformance

    SHAHADY, P. / LYON, C. / SCHAUER, J. et al. | AIAA | 1973


    A subjective test of modulated blade spacing for helicopter main rotors

    Sullivan, Brenda M. / Edwards, Bryan D. / Brentner, Kenneth | Tema Archive | 2005


    A Subjective Test of Modulated Blade Spacing for Helicopter Main Rotors

    Sullivan, Brenda M. / Edwards, Bryan D. / Brentner, Kenneth S. et al. | NTRS | 2002