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  • 1
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    Crustal thickening processes in the Central Andes and the different natures of the Moho-discontinuity (1999)
    In:  Journal of South American Earth Sciences 12: 201-220
    Details
    Publication Date: 1999
    Keywords: ANCORP 96, Andes, tectonics/crustal structure, refraction seismics, geodynamics, geological interpretation
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    CITATION DEKORP
  • 2
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    Rotor noise (1991)
    In:  CASI
    Details
    Publication Date: 2006-06-13
    Description: The physical characteristics and sources of rotorcraft noise as they exist today are presented. Emphasis is on helicopter-like vehicles, that is, on rotorcraft in nonaxial flight. The mechanisms of rotor noise are reviewed in a simple physical manner for the most dominant sources of rotorcraft noise. With simple models, the characteristic time- and frequency-domain features of these noise sources are presented for idealized cases. Full-scale data on several rotorcraft are then reviewed to allow for the easy identification of the type and extent of the radiating noise. Methods and limitations of using scaled models to test for several noise sources are subsequently presented. Theoretical prediction methods are then discussed and compared with experimental data taken under very controlled conditions. Finally, some promising noise reduction technology is reviewed.
    Keywords: ACOUSTICS
    Type: NASA. Langley Research Center, Aeroacoustics of Flight Vehicles: Theory and Practice. Volume 1: Noise Sources; p 65-149
    Format: text
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    NASA TECHNICAL REPORTS
  • 3
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    Experiment versus theory (1982)
    In:  CASI
    Details
    Publication Date: 2006-04-17
    Description: High speed compressibility noise and vortex interaction noise, which are aerodynamically generated noise sources, were investigated. Noise generating mechanisms were identified. Linear and nonlinear theory were compared and are in agreement with data on amplitude and wave forms. The interaction area between the acoustic planform and blade/vortex interaction lines are examined.
    Keywords: ACOUSTICS
    Type: NASA. Langley Research Center Rotorcraft Noise; p 319-330
    Format: text
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    NASA TECHNICAL REPORTS
  • 4
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    Rotary wing aerodynamically generated noise (1982)
    In:  CASI
    Details
    Publication Date: 2006-04-17
    Description: The history and methodology of aerodynamic noise reduction in rotary wing aircraft are presented. Thickness noise during hover tests and blade vortex interaction noise are determined and predicted through the use of a variety of computer codes. The use of test facilities and scale models for data acquisition are discussed.
    Keywords: ACOUSTICS
    Type: NASA. Langley Research Center Rotorcraft Noise; p 97-106
    Format: text
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    NASA TECHNICAL REPORTS
  • 5
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    Helicopter impulsive noise - Theoretical and experimental status (1986)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: The theoretical and experimental status of helicopter impulsive noise is reviewed. The two major source mechanisms of helicopter impulsive noise are addressed: high-speed impulsive noise and blade-vortex interaction impulsive noise. A thorough physical explanation of both generating mechanism is presented together with model and full-scale measurements of the phenomena. Current theoretical prediction methods are compared with experimental findings of isolated rotor tests. The noise generating mechanism of high speed impulsive noise are fairly well understood - theory and experiment compare nicely over Mach number ranges typical of today's helicopters. For the case of blade-vortex interaction noise, understanding of noise generating mechanisms and theoretical comparison with experiment are less satisfactory. Several methods for improving theory-experiment are suggested.
    Keywords: ACOUSTICS
    Type: Journal of Sound and Vibration (ISSN 0022-460X); 109; 361-422
    Format: text
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    NASA TECHNICAL REPORTS
  • 6
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    Helicopter impulsive noise - Theoretical and experimental status (1986)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: The theoretical and experimental status of helicopter impulsive noise is reviewed. The two major source mechanisms of helicopter impulsive noise are addressed: high-speed impulsive noise and blade-vortex interaction impulsive noise. A thorough physical explanation of both generating mechanisms is presented together with model and full-scale measurements of the phenomena. Current theoretical prediction methods are compared with experimental findings of isolated rotor tests. The noise generating mechanisms of high speed impulsive noise are fairly well understood - theory and experiment compare nicely over Mach number ranges typical of today's helicopters. For the case of blade-vortex interaction noise, understanding of noise generating mechanisms and theoretical comparison with experiment are less satisfactory. Several methods for improving theory/experiment are suggested.
    Keywords: ACOUSTICS
    Format: text
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    NASA TECHNICAL REPORTS
  • 7
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    Helicopter model rotor-blade vortex interaction impulsive noise - Scalability and parametric variations (1985)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: Acoustic data taken in the anechoic Deutsch-Niederlaendischer Windkanal (DNW) have documented the blade vortex interaction (BVI) impulsive noise radiated from a 1/7-scale model main rotor of the AH-1 series helicopter. Averaged model scale data were compared with averaged full scale, inflight acoustic data under similar nondimensional test conditions. At low advance ratios (mu = 0.164 to 0.194), the data scale remarkable well in level and waveform shape, and also duplicate the directivity pattern of BVI impulsive noise. At moderate advance ratios (mu = 0.224 to 0.270), the scalig deteriorates, suggesting that the model scale rotor is not adequately simulating the full scale BVI noise; presently, no proved explanation of this discrepancy exists. Carefully performed parametric variations over a complete matrix of testing conditions have shown that all of the four governing nondimensional parameters - tip Mach number at hover, advance ratio, local inflow ratio, and thrust coefficient - are highly sensitive to BVI noise radiation.
    Keywords: ACOUSTICS
    Format: text
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    NASA TECHNICAL REPORTS
  • 8
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    Explosive high-speed shutter. (1968)
    In:  Other Sources
    Details
    Publication Date: 2011-08-12
    Description: Explosive high speed spectrographic shutter noting opening, closing, triggering and delay times
    Keywords: INSTRUMENTATION AND PHOTOGRAPHY
    Type: ; ACE(
    Format: text
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    NASA TECHNICAL REPORTS
  • 9
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    Helicopter Model Rotor-Blade Vortex Interaction Impulsive Noise: Scalability and Parametric Variations (1987)
    In:  Other Sources
    Details
    Publication Date: 2011-08-23
    Description: Acoustic data taken in the anechoic Deutsch-Niederlaendischer Windkanal (DNW) have documented the blade-vortex interaction (BVI) impulsive noise radiated from a 1/7-scale model main rotor of the AH-1 series helicopter. Averaged model-scale data were compared with averaged full-scale, in-flight acoustic data under similar non-dimensional test conditions using an improved data analysis technique. At low advance ratios (mu = 0.164 - 0.194), the BVI impulsive noise data scale remarkably well in level, waveform, and directivity patterns. At moderate advance ratios (mu = 0.224 - 0.270), the scaling deteriorates, suggesting that the model-scale rotor is not adequately simulating the full-scale BVI noise. Presently, no proved explanation of this discrepancy exists. Measured BVI noise radiation is highly sensitive to all of the four governing nondimensional parameters--hover tip Mach number, advance ratio, local inflow ratio, and thrust coefficient.
    Keywords: Aerodynamics
    Type: Journal of the American Helicopter Society; Volume 32; No. 1; 3-12
    Format: text
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    NASA TECHNICAL REPORTS
  • 10
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    Aeroacoustic research: An Army perspective (1978)
    In:  CASI
    Details
    Publication Date: 2016-06-07
    Description: A short perspective of the Army aeroacoustic research program is presented that emphasizes rotary wing, aerodynamically generated noise. Exciting breakthroughs in experimental techniques and facilities are reviewed which are helping build a detailed understanding of helicopter external noise. Army and joint Army/NASA supported research programs in acoustics which promise to reduce the noise of future helicopters without severe performance penalties are included.
    Keywords: ACOUSTICS
    Type: NASA. Langley Res. Center Helicopter Acoustics, Pt. 2; p 797-817
    Format: application/pdf
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    NASA TECHNICAL REPORTS
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