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  • 1
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    The Effects of Crosswind Flight on Rotor Harmonic Noise Radiation (2013)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    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-15487 , 69th Annual American Helicopter Society Forum and Technology Display; May 21, 2013 - May 23, 2013; Phoenix, AZ; United States
    Format: application/pdf
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  • 2
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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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  • 3
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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
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  • 4
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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
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  • 5
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    Acoustics Reflections of Full-Scale Rotor Noise Measurements in NFAC 40- by 80-Foot Wind Tunnel (2012)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: The objective of current research is to identify the extent of acoustic time history distortions due to wind tunnel wall reflections. Acoustic measurements from the recent full-scale Boeing-SMART rotor test (Fig. 2) will be used to illustrate the quality of noise measurement in the NFAC 40- by 80-Foot Wind Tunnel test section. Results will be compared to PSU-WOPWOP predictions obtained with and without adjustments due to sound reflections off wind tunnel walls. Present research assumes a rectangular enclosure as shown in Fig. 3a. The Method of Mirror Images7 is used to account for reflection sources and their acoustic paths by introducing mirror images of the rotor (i.e. acoustic source), at each and every wall surface, to enforce a no-flow boundary condition at the position of the physical walls (Fig. 3b). While conventional approach evaluates the "combined" noise from both the source and image rotor at a single microphone position, an alternative approach is used to simplify implementation of PSU-WOPWOP for this reflection analysis. Here, an "equivalent" microphone position is defined with respect to the source rotor for each mirror image that effectively renders the reflection analysis to be a one rotor, multiple microphones problem. This alternative approach has the advantage of allowing each individual "equivalent" microphone, representing the reflection pulse from the associated wall surface, to be adjusted by the panel absorption coefficient illustrated in Fig. 1a. Note that the presence of parallel wall surfaces requires an infinite number of mirror images (Fig. 3c) to satisfy the no-flow boundary conditions. In the present analysis, up to four mirror images (per wall surface) are accounted to achieve convergence in the predicted time histories
    Keywords: Acoustics
    Type: ARC-E-DAA-TN741 , AHS Specialists'' Conference; Jan 20, 2012 - Jan 21, 2012; San Francisco, CA; United States
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  • 6
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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
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  • 7
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    A study of Propfan propagation noise (1993)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: A study of Propfan far-field noise is carried out based on geometrical acoustics theory. The analysis traces the acoustic rays and ray tube areas carrying the acoustic disturbances to the far-field. Sound attenuation due to nonlinear steepening, atmospheric absorption and turbulence scattering are also investigated. A comparison of our prediction methodology with experimental acoustics measurement shows good agreement. Geometrical decay and atmospheric absorption are identified as the primary noise attenuating mechanisms. Nonlinear effects are negligible. It is determined that the acoustic footprints of advanced propellers are dominated by caustics. Details of the formation of these caustics may provide a basis for future noise minimization efforts.
    Keywords: ACOUSTICS
    Type: AIAA PAPER 93-0602 , ; 13 p.|AIAA, Aerospace Sciences Meeting and Exhibit; Jan 11, 1993 - Jan 14, 1993; Reno, NV; United States
    Format: text
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  • 8
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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
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  • 9
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    Aeromechanics and Aeroacoustics Predictions of the Boeing-SMART Rotor Using Coupled-CFD/CSD Analyses (2010)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: This paper will highlight helicopter aeromechanics and aeroacoustics prediction capabilities developed by Georgia Institute of Technology, the Pennsylvania State University, and Northern Arizona University under the Helicopter Quieting Program (HQP) sponsored by the Tactical Technology Office of the Defense Advanced Research Projects Agency (DARPA). First initiated in 2004, the goal of the HQP was to develop high fidelity, state-of-the-art computational tools for designing advanced helicopter rotors with reduced acoustic perceptibility and enhanced performance. A critical step towards achieving this objective is the development of rotorcraft prediction codes capable of assessing a wide range of helicopter configurations and operations for future rotorcraft designs. This includes novel next-generation rotor systems that incorporate innovative passive and/or active elements to meet future challenging military performance and survivability goals.
    Keywords: Acoustics
    Type: ARC-E-DAA-TN910 , AHS 66th Annual Forum and Technology Display; May 11, 2010 - May 13, 2010; Phoenix, AZ; United States
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  • 10
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    Direct CFD Predictions of Low Frequency Sounds Generated by a Helicopter Main Rotor (2010)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: The use of CFD to directly predict helicopter main rotor noise is shown to be quite promising as an alternative mean for low frequency source noise evaluation. Results using existing state-of-the-art grid structures and finite-difference schemes demonstrated that small perturbation pressures, associated with acoustics radiation, can be extracted with some degree of fidelity. Accuracy of the predictions are demonstrated via comparing to predictions from conventional acoustic analogy-based models, and with measurements obtained from wind tunnel and flight tests for the MD-902 helicopter at several operating conditions. Findings show that the direct CFD approach is quite successfully in yielding low frequency results due to thickness and steady loading noise mechanisms. Mid-to-high frequency contents, due to blade-vortex interactions, are not predicted due to CFD modeling and grid constraints.
    Keywords: Acoustics
    Type: ARC-E-DAA-TN1528 , 2010 American Helicopter Society 66th Annual Forum; May 11, 2010 - May 13, 2010; Phoenix, AZ; United States
    Format: application/pdf
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