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  • 1
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    Bounds on thickness and loading noise of rotating blades and the favorable effect of blade sweep on noise reduction (1978)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: The maxima of amplitudes of thickness and loading noise harmonics are established when the radial distribution of blade chord, thickness ratio, and lift coefficient is specified. It is first shown that only airfoils with thickness distribution and chordwise loading distributions which are symmetric with respect to midchord need be considered for finding the absolute maxima of thickness and loading noise. The resulting chordwise thickness and load distributions for these maximum noise conditions require infinite slope at some points along the chord but otherwise are uniform. It is shown that sweeping the blades reduces the thickness and loading noise, but there is no optimum sweep which generates the lowest noise.
    Keywords: ACOUSTICS
    Type: NASA. Langley Res. Center Helicopter Acoustics; p 373-385
    Format: application/pdf
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  • 2
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    A new capability for predicting helicopter rotor and propeller noise including the effect of forward motion (1977)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: The governing equation and computing technique for the prediction of helicopter rotor and propeller noise are described. The method which gives both the acoustic pressure time history and spectrum of the noise includes the thickness and the loading noise. It was adapted to computers resulting in a new capability in noise prediction by removing many of the restrictions and limitations of previous theories. The capability results from the fact that the theory is developed entirely in the time domain. The formulation and the technique used are not limited to compact sources, steady level flight or to the far-field. In addition, the inputs to the computer program are normally available or are amenable to experimental measurements. This program can be used to study rotor and propeller noise with the aim of minimizing the radiated noise to reduce annoyance to the public. Several examples demonstrating the features and capability of the computer program are presented.
    Keywords: ACOUSTICS
    Type: NASA-TM-X-74037
    Format: application/pdf
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  • 3
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    Development of a noncompact source theory with applications to helicopter rotors (1976)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: A new formulation for determining the acoustic field of moving bodies, based on acoustic analogy, is derived. The acoustic pressure is given as the sum of two integrals, one of which has a derivative with respect to time. The integrands are functions of the normal velocity and surface pressure of the body. A computer program based on this formulation was used to calculate acoustic pressure signatures for several helicoptor rotors from experimental surface pressure data. Results are compared with those from compact source calculations. It is shown that noncompactness of steady sources on the rotor can account for the high harmonics of the pressure system. Thickness noise is shown to be a significant source of sound, especially for blunt airfoils in regions where noncompact source theory should be applied.
    Keywords: ACOUSTICS
    Type: AIAA PAPER 76-563 , Aero-Acoustics Conference; Jul 20, 1976 - Jul 23, 1976; Palo Alto, CA
    Format: text
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  • 4
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    Prediction of light aircraft interior noise (1976)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: A computerized interior noise prediction method for light aircraft is described. An existing analytical program, development for commercial jets, forms the basis of some modal analysis work which is described. The accuracy of this modal analysis technique for predicting low-frequency coupled acoustic-structural natural frequencies is discussed along with trends indicating the effects of varying parameters such as fuselage length and diameter, structural stiffness, and interior acoustic absorption.
    Keywords: ACOUSTICS
    Type: NASA-TM-X-72838
    Format: application/pdf
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  • 5
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    Acoustic and aerodynamic performance investigation of inverted velocity profile coannular plug nozzles, comprehensive data report, volume 2 (1979)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: Volume 2 of a three volume report is presented. Volume 2 presents acoustic data comparisons in graphic form.
    Keywords: ACOUSTICS
    Type: NASA-CR-159575-VOL-2 , R79AEG166-VOL-2
    Format: application/pdf
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  • 6
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    The generation of sound by vorticity waves in swirling duct flows (1977)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: Swirling flow in an axisymmetric duct can support vorticity waves propagating parallel to the axis of the duct. When the cross-sectional area of the duct changes a portion of the wave energy is scattered into secondary vorticity and sound waves. Thus the swirling flow in the jet pipe of an aeroengine provides a mechanism whereby disturbances produced by unsteady combustion or turbine blading can be propagated along the pipe and subsequently scattered into aerodynamic sound. In this paper a linearized model of this process is examined for low Mach number swirling flow in a duct of infinite extent. It is shown that the amplitude of the scattered acoustic pressure waves is proportional to the product of the characteristic swirl velocity and the perturbation velocity of the vorticity wave. The sound produced in this way may therefore be of more significance than that generated by vorticity fluctuations in the absence of swirl, for which the acoustic pressure is proportional to the square of the perturbation velocity. The results of the analysis are discussed in relation to the problem of excess jet noise.
    Keywords: ACOUSTICS
    Type: Journal of Fluid Mechanics; 81; June 24
    Format: text
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  • 7
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    A study of partial coherence for identifying interior noise sources and paths on general aviation aircraft (1979)
    In:  CASI
    Details
    Publication Date: 2019-07-13
    Description: The partial coherence analysis method for noise source/path determination is summarized and the application to a two input, single output system with coherence between the inputs is illustrated. The augmentation of the calculations on a digital computer interfaced with a two channel, real time analyzer is also discussed. The results indicate possible sources of error in the computations and suggest procedures for avoiding these errors.
    Keywords: ACOUSTICS
    Type: NASA-TM-80197 , Acoust. Soc. of Am. Meeting; Nov 26, 1979 - Nov 30, 1979; Salt Lake City
    Format: application/pdf
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  • 8
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    Measurement, analysis, and prediction of aircraft interior noise (1976)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: Considerations of comfort of passengers and crew in light aircraft and helicopters indicate substantial benefits may be obtained by the reduction of interior noise levels. This paper discusses an ongoing research effort to reduce interior noise in such vehicles. Data from both field and laboratory studies for a light aircraft are presented. The laboratory data indicate that structural vibration is an efficient source of interior noise and should be considered in the reduction of interior noise. Flight data taken on a helicopter before and after installation of acoustic treatment demonstrate that over 30 dB of noise reduction can be obtained in certain portions of the spectra. However, subjective evaluations of the treated vehicle indicate that further reductions in interior noise are desirable. An existing interior noise prediction method which was developed for large jet transports was applied to study low-frequency noise in a light aircraft fuselage. The results indicate that improvements in the analytical model may be necessary for the prediction of interior noise of light aircraft.
    Keywords: ACOUSTICS
    Type: AIAA PAPER 76-551 , Aero-Acoustics Conference; Jul 20, 1976 - Jul 23, 1976; Palo Alto, CA
    Format: text
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  • 9
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    The large-scale organized structure in free turbulent shear flow and its radiation properties (1978)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: Modeling of the large-scale coherent structures in a turbulent free shear flow is described. The objective is to gain insight into the interaction between the large-scale structure and the fine-grained turbulence. Some aerodynamic sound radiation properties of the large-scale structure are examined. It is found that the dominant contributions to the radiation come from shear noise rather than self-noise. The fact that a pure tone excitation leads to broadband amplification is most probably due to the enhancement of the fine-grained turbulence by the coherent structure. Numerical modeling of the coherent structure is also discussed.
    Keywords: ACOUSTICS
    Type: In: Structure and mechanisms of turbulence II; Proceedings of the Symposium on Turbulence; Aug 01, 1977 - Aug 05, 1977; Berlin
    Format: text
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  • 10
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    acoustic and aerodynamic performance investigation of inverted velocity profile coannular plug nozzles, comprehensive data report, volume 3 (1979)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: Volume 3 of a three volume report is presented. Volume 3 contains the detailed aerodynamic test results plus the concept screening and model design report.
    Keywords: ACOUSTICS
    Type: NASA-CR-159575-VOL-3 , R79AEG166-VOL-3
    Format: application/pdf
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