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  • 1
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    Helicopter far-field acoustic levels as a function of reduced main-rotor advancing blade-tip Mach number (1990)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: During the design of a helicopter, the weight, engine, rotor speed, and rotor geometry are given significant attention when considering the specific operations for which the helicopter will be used. However, the noise radiated from the helicopter and its relationship to the design variables is currently not well modeled with only a limited set of full-scale field test data to study. In general, limited field data have shown that reduced main-rotor advancing blade-tip Mach numbers result in reduced far-field noise levels. The status of a recent helicopter noise research project is reviewed. It is designed to provide flight experimental data which may be used to further understand helicopter main-rotor advancing blade-tip Mach number effects on far-field acoustic levels. Preliminary results are presented relative to tests conducted with a Sikorsky S-76A helicopter operating with both the rotor speed and the flight speed as the control variable. The rotor speed was operated within the range of 107 to 90 percent NR at nominal forward speeds of 35, 100, and 155 knots.
    Keywords: ACOUSTICS
    Type: NASA-TM-102684 , NAS 1.15:102684
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 2
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    Localized, Non-Harmonic Active Flap Motions for Low Frequency In-Plane Rotor Noise Reduction (2012)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: A first-of-its-kind demonstration of the use of localized, non-harmonic active flap motions, for suppressing low frequency, in-plane rotor noise, is reported in this paper. Operational feasibility is verified via testing of the full-scale AATD/Sikorsky/UTRC active flap demonstration rotor in the NFAC's 40- by 80-Foot anechoic wind tunnel. Effectiveness of using localized, non-harmonic active flap motions are compared to conventional four-per-rev harmonic flap motions, and also active flap motions derived from closed-loop acoustics implementations. All three approaches resulted in approximately the same noise reductions over an in-plane three-by-three microphone array installed forward and near in-plane of the rotor in the nearfield. It is also reported that using an active flap in this localized, non-harmonic manner, resulted in no more that 2% rotor performance penalty, but had the tendency to incur higher hub vibration levels.
    Keywords: Aircraft Design, Testing and Performance
    Type: AD-A558227 , ARC-E-DAA-TN4949 , American Helicopter Society 68th Annual Forum; May 01, 2012 - May 03, 2012; Fort Worth, TX; United States
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 3
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    Helicopter far-field acoustic levels as a function of reduced rotor speeds (1990)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: This paper will present far-field measured noise levels relative to tests conducted with a model S-76A helicopter. The project was designed to provide supplemental experimental flight data which may be used to further study reduced helicopter rotor speeds (and thus, advancing blade-tip Mach number) effects on far-field acoustic levels. The aircraft was flown in straight and level flight while operating with both the rotor speed and flight speed as test variables. The rotor speed was varied over the range of 107 percent of the main-rotor speed (NR) to 90 percent NR and with the forward flight speed varied over the range of 155 to 35 knots indicated air speed. These conditions produced a wide range of advancing blade-tip Mach numbers to which the noise data are related.
    Keywords: ACOUSTICS
    Type: NOISE-CON 90: National Conference on Noise Control Engineering; Oct 15, 1990 - Oct 17, 1990; Austin, TX; United States
    Format: text
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    NASA TECHNICAL REPORTS
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