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  • 2015-2019  (5)
  • 1
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    Joint Eglin Acoustics Week 2013 Data Report (2017)
    In:  CASI
    Details
    Publication Date: 2019-07-12
    Description: Far-field acoustic measurements were obtained for the AH-64D, HH-60M and CV-22B at the Eglin AFB, Test Area C-72, in July/August 2013. The primary purpose for this flight test was to obtain a benchmark database of detailed acoustic source noise characteristics for the aircraft operating at typical mission gross weights over a range of typical mission operating conditions. Data were acquired for a range of steady-state level and descending flight conditions, hover, and a variety of unsteady maneuver conditions. Between 30 and 37 microphones were deployed during these tests. Vehicle position and state data, as well as weather data were acquired simultaneously with the acoustic data. This paper describes the test aircraft, onboard instrumentation, ground instrumentation, and the data acquired. Data from this test are available upon request and review.
    Keywords: Aeronautics (General); Acoustics
    Type: NASA/TM-2017-219681 , AMRDEC-SR-RDMR-AD-17-02 , L-20832 , NF1676L-27413
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 2
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    Helicopter Non-Unique Trim Strategies for Blade-Vortex Interaction (BVI) Noise Reduction (2016)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: An acoustics parametric analysis of the effect of fuselage drag and pitching moment on the Blade-Vortex Interaction (BVI) noise radiated by a medium lift helicopter (S-70UH-60) in a descending flight condition was conducted. The comprehensive analysis CAMRAD II was used for the calculation of vehicle trim, wake geometry and integrated air loads on the blade. The acoustics prediction code PSU-WOPWOP was used for calculating acoustic pressure signatures for a hemispherical grid centered at the hub. This paper revisits the concept of the X-force controller for BVI noise reduction, and investigates its effectiveness on an S-70 helicopter. The analysis showed that further BVI noise reductions were achievable by controlling the fuselage pitching moment. Reductions in excess of 6 dB of the peak BVI noise radiated towards the ground were demonstrated by compounding the effect of airframe drag and pitching moment simultaneously.
    Keywords: Acoustics; Aeronautics (General)
    Type: ARC-E-DAA-TN28912 , American Helicopter Society (AHS) Technical Meeting on Aeromechanics Design for Vertical Lift; Jan 20, 2016 - Jan 22, 2016; San Francisco, CA; United States
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 3
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    Effects of Crosswind Flight on Rotor Harmonic Noise Radiation (2018)
    In:  CASI
    Details
    Publication Date: 2019-07-18
    Description: In order to develop recommendations for procedures for helicopter source noise characterization, the effects of crosswinds on main rotor harmonic noise radiation are assessed using a model of the Bell 430 helicopter. Crosswinds are found to have a significant effect on Blade-Vortex Interaction (BVI) noise radiation when the helicopter is trimmed with the fuselage oriented along the inertial flight path. However, the magnitude of BVI noise remains unchanged when the pilot orients the fuselage along the aerodynamic velocity vector, crabbing for zero aerodynamic sideslip. The effects of wind gradients on BVI noise are also investigated and found to be smaller in the crosswind direction than in the headwind direction. The effects of crosswinds on lower harmonic noise sources at higher flight speeds are also assessed. In all cases, the directivity of radiated noise is somewhat changed by the crosswind. The model predictions agree well with flight test data for the Bell 430 helicopter captured under various wind conditions. The results of this investigation would suggest that flight paths for future acoustic flight testing are best aligned across the prevailing wind direction to minimize the effects of winds on noise measurements when wind cannot otherwise be avoided.
    Keywords: Acoustics
    Type: NF1676L-18442 , Journal of Aircraft (ISSN 0021-8669) (e-ISSN 1533-3868); 55; 5; 2137-2148
    Format: application/pdf
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  • 4
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    The Effects of Ambient Conditions on Helicopter Harmonic Noise Radiation: Theory and Experiment (2016)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: The effects of ambient atmospheric conditions, air temperature and density, on rotor harmonic noise radiation are characterized using theoretical models and experimental measurements of helicopter noise collected at three different test sites at elevations ranging from sea level to 7000 ft above sea level. Significant changes in the thickness, loading, and blade-vortex interaction noise levels and radiation directions are observed across the different test sites for an AS350 helicopter flying at the same indicated airspeed and gross weight. However, the radiated noise is shown to scale with ambient pressure when the flight condition of the helicopter is defined in nondimensional terms. Although the effective tip Mach number is identified as the primary governing parameter for thickness noise, the nondimensional weight coefficient also impacts lower harmonic loading noise levels, which contribute strongly to low frequency harmonic noise radiation both in and out of the plane of the horizon. Strategies for maintaining the same nondimensional rotor operating condition under different ambient conditions are developed using an analytical model of single main rotor helicopter trim and confirmed using a CAMRAD II model of the AS350 helicopter. The ability of the Fundamental Rotorcraft Acoustics Modeling from Experiments (FRAME) technique to generalize noise measurements made under one set of ambient conditions to make accurate noise predictions under other ambient conditions is also validated.
    Keywords: Aerodynamics; Acoustics
    Type: NF1676L-22568 , American Helicopter Society (AHS) Annual Forum; May 17, 2016 - May 19, 2016; West Palm Beach, FL; United States
    Format: application/pdf
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  • 5
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    Sound Diffraction Modeling of Rotorcraft Noise Around Terrain (2017)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: A new computational technique, Wave Confinement (WC), is extended here to account for sound diffraction around arbitrary terrain. While diffraction around elementary scattering objects, such as a knife edge, single slit, disc, sphere, etc. has been studied for several decades, realistic environments still pose significant problems. This new technique is first validated against Sommerfeld's classical problem of diffraction due to a knife edge. This is followed by comparisons with diffraction over three-dimensional smooth obstacles, such as a disc and Gaussian hill. Finally, comparisons with flight test acoustics data measured behind a hill are also shown. Comparison between experiment and Wave Confinement prediction demonstrates that a Poisson spot occurred behind the isolated hill, resulting in significantly increased sound intensity near the center of the shadowed region.
    Keywords: Acoustics
    Type: NF1676L-25339 , AHS International Annual Forum and Technology Display; May 09, 2017 - May 11, 2017; Fort Worth, TX; United States
    Format: application/pdf
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    NASA TECHNICAL REPORTS
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