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  • 1
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    A decade of aeroacoustic research at NASA Ames Research Center (1988)
    In:  CASI
    Details
    Publication Date: 2013-08-31
    Description: The rotorcraft aeroacoustic research accomplishments of the past decade at Ames Research Center are reviewed. These include an extensive sequence of flight, ground, and wind tunnel tests that have utilized the facilities to guide and pioneer theoretical research. Many of these experiments were of benchmark quality. The experiments were used to isolate the inadequacies of linear theory in high-speed impulsive noise research, have led to the development of theoretical approaches, and have guided the emerging discipline of computational fluid dynamics to rotorcraft aeroacoustic problems.
    Keywords: ACOUSTICS
    Type: NASA, Washington, NASA(Army Rotorcraft Technology. Volume 2: Materials and Structures, Propulsion and Drive Systems, Flight Dynamics and Control, and Acoustics; p 1066-1090
    Format: application/pdf
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  • 2
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    The acoustics of a small-scale helicopter rotor in hover (1989)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: A 2.1 m diameter, 1/6-scale model helicopter main rotor was tested in hover in the test section of the NASA Ames 40- by 80-foot wind tunnel. Performance and noise data on a small-scale rotor at various thrust coefficients and tip Mach numbers were obtained for comparison with existing data on similar full-scale helicopter rotors. These data form part of a data base to permit the estimation of scaling effects on various rotor noise mechanisms. Another objective was to contribute to a data base that will permit the estimation of facility effects on acoustic testing. Acoustic 1/3-octave-band spectra are presented, together with variations of overall acoustic levels with rotor performance, microphone distance, and directivity angle.
    Keywords: AERODYNAMICS
    Type: NASA-TM-101058 , A-89015 , NAS 1.15:101058
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  • 3
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    Analysis of small-scale rotor hover performance data (1990)
    In:  CASI
    Details
    Publication Date: 2019-06-28
    Description: Rotor hover-performance data from a 1/6-scale helicopter rotor are analyzed and the data sets compared for the effects of ambient wind, test stand configuration, differing test facilities, and scaling. The data are also compared to full scale hover data. The data exhibited high scatter, not entirely due to ambient wind conditions. Effects of download on the test stand proved to be the most significant influence on the measured data. Small-scale data correlated resonably well with full scale data; the correlation did not improve with Reynolds number corrections.
    Keywords: FLUID MECHANICS AND HEAT TRANSFER
    Type: NASA-TM-102271 , A-90046 , NAS 1.15:102271
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  • 4
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    Comparison of Full-Scale XV-15 Wind Tunnel and In-Flight Blade-Vortex Interaction Noise (1997)
    In:  Other Sources
    Details
    Publication Date: 2019-07-18
    Description: An isolated full-scale XV-15 rotor was tested in helicopter mode in the NASA Ames 80 by 120-Foot Wind Tunnel. Extensive acoustic data were obtained to define the rotor operating condition for maximum blade-vortex interaction (BVI) noise. Additional data were obtained at operating conditions simulating flight up to 80 knots. An XV-15 aircraft was also tested under operating conditions corresponding to landing approaches for which BVI is expected to be a maximum. In-flight acoustic data were obtained using the YO-3A acoustic research aircraft. An attempt was made to closely match wind tunnel and flight test operating conditions. Details of the two tests are described and some representative acoustic results are presented. Comparisons are shown between the wind tunnel data and corresponding flight test data. Preliminary results indicate very good correlation of the BVI-related features. However, some differences between flight test and wind tunnel results exist away from the BVI event, thought to arise from differences in the two flow environments.
    Keywords: Acoustics
    Type: 53rd American Helicopter Society Annual Forum; Apr 29, 1997 - May 01, 1997; Virginia Beach, VA; United States
    Format: text
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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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    Three-dimensional blade vortex interactions (1991)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: A three-dimensional time dependent Navier-Stokes analysis was applied to the rotor blade vortex interaction problem. The numerical procedure is an iterative implicit procedure using three point central differences to represent spatial derivatives. A series of calculations were made to determine the time steps, pseudo-time steps, iterations, artificial dissipation level, etc. required to maintain a nondissipative vortex. Results show the chosen method to have excellent non-dissipative properties provided the correct parameters are chosen. This study was used to set parameters for both two- and three-dimensional blade vortex interaction studies. The case considered was the interaction between a vortex and a NACA0012 airfoil. The results showed the detailed physics during the interaction including the pressure pulse propagating from the blade. The simulated flow physics was qualitatively similar to that experimentally observed. The BVI phenomena is the result of the buildup and violent collapse of the shock waves and local supersonic pockets on the blade surfaces. The resulting pressure pulse build-up appears to be centered at the blade leading edge.
    Keywords: AERODYNAMICS
    Type: AHS International Specialists'' Meeting on Rotorcraft Basic Research; Mar 25, 1991 - Mar 27, 1991; Atlanta, GA; United States
    Format: text
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  • 8
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    Acoustical Tests Of A Scale-Model Helicopter Rotor (1988)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: Data obtained in simulated hovering flight in open environment. Report discusses measurements of sound generated in outdoor hoovering tests of 1/6-scale, four bladed helicopter rotor. Information of delineation between accoustic near field and far field and on effect of simple boundary-layer-tripping device. Also covers rotor accoustics at low thrust and at high thrust.
    Keywords: MECHANICS
    Type: ARC-11773 , NASA Tech Briefs (ISSN 0145-319X); 12; 7; P. 65
    Format: text
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  • 9
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    Comparing Test Data on Scale-Model Helicopter Rotors (1987)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: Hovering data correlate well, but forward-flight data do not. Report compares acoustics and performance of small-scale helicopter rotor with those of full-scale rotor in both hovering and forward flight.
    Keywords: MECHANICS
    Type: ARC-11722 , NASA Tech Briefs (ISSN 0145-319X); 11; 7; P. 76
    Format: text
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  • 10
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    A Study of Acoustic Reflections in Full-Scale Rotor Low Frequency Noise Measurements Acquired in Wind Tunnels (2010)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: Difficulties in obtaining full-scale rotor low frequency noise measurements in wind tunnels are addressed via residual sound reflections due to non-ideal anechoic wall treatments. Examples illustrated with the Boeing-SMART rotor test in the National Full-Scale Aerodynamics Complex (NFAC) 40- by 80-Foot Wind Tunnel facility demonstrated that these reflections introduced distortions in the measured acoustic time histories that are not representative of free-field rotor noise radiation. A simplified reflection analysis, based on the method of images, is used to examine the sound measurement quality in such "less-than-anechoic" environment. Predictions of reflection-adjusted acoustic time histories are qualitatively shown to account for some of the spurious fluctuations observed in wind tunnel noise measurements
    Keywords: Acoustics
    Type: ARC-E-DAA-TN1156 , American Helicopter Society Specialists'' Conference on Aeromechanics; Jan 20, 2010 - Jan 22, 2010; San Francisco, CA; United States
    Format: application/pdf
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