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  • 1
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    EBF noise reduction through nozzle/flap positioning (1976)
    In:  CASI
    Details
    Publication Date: 2016-06-07
    Description: Results are presented of an experimental and analytical study of the dependence of externally blown flap (EBF) noise on the relative position and shape of engine exhaust nozzle. Tests, conducted on a 1/15 scale model of a triple-slotted EBF system, indicate that a significant reduction (of up to 10 to 15 db for no forward speed case and of up to 5 to 10 db for forward speed case) is possible in the low frequency (around 63 Hz) region of the noise spectrum of the full scale device for small nozzle/flap separation distances. The overall acoustic performance, measured in PNdb, does not exhibit significant reductions. The analysis of the EBF noise is carried out for two limiting airfoil in a free jet. The analytical results also suggest that low frequency noise can be reduced by placing the nozzle close to the flow turning elements.
    Keywords: AERODYNAMICS
    Type: NASA. Langley Res. Center Powered-Lift Aerodyn. and Acoustics; p 307-324
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 2
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    Development, fabrication and calibration of a porous surface microphone in an aerofoil (1975)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: The development of a porous surface microphone in an airfoil intended to measure acoustic signals in a turbulent airflow and to minimize the flow noise is described. The sensor because of its airfoil operates over a wide range of yaw angles and flow velocities without excessive flow noise. The acoustic properties of the porous materials used in the airfoil sensor and their effects on the frequency response of the sensor were analyzed and tested. An accurate airfoil was selected, having a smaller thickness-to-chord ratio and an airfoil sensor was designed. The sensor was calibrated acoustically and its flow noise evaluated in the quiet BBN wind tunnel at flow velocities up to 70 m/sec. Results are presented.
    Keywords: INSTRUMENTATION AND PHOTOGRAPHY
    Type: NASA-CR-132636
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 3
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    A study of the variable impedance surface concept as a means for reducing noise from jet interaction with deployed lift-augmenting flaps (1972)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: The feasibility of quieting the externally-blown-flap (EBF) noise sources which are due to interaction of jet exhaust flow with deployed flaps was demonstrated on a 1/15-scale 3-flap EBF model. Sound field characteristics were measured and noise reduction fundamentals were reviewed in terms of source models. Test of the 1/15-scale model showed broadband noise reductions of up to 20 dB resulting from combination of variable impedance flap treatment and mesh grids placed in the jet flow upstream of the flaps. Steady-state lift, drag, and pitching moment were measured with and without noise reduction treatment.
    Keywords: AIRCRAFT
    Type: NASA-CR-112166 , REPT-2399
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 4
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    Small scale noise and wind tunnel tests of upper surface blowing nozzle flap concepts. Volume 2. Acoustic test results (1976)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: Results are summarized of acoustic tests on two advanced concepts of upper-surface-blowing propulsive lift devices.
    Keywords: AERODYNAMICS
    Type: NASA-CR-137748 , BBN-3130
    Format: application/pdf
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  • 5
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    Study of a porous surface microphone sensor in an aerofoil (1975)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: The porous microphone in an airfoil is described as a directional sensor which rejects flow noise. The airfoil allows the sensor to be rotated in the airflow over a wide range of yaw angles, 0 to 90 degrees, avoiding flow separation over the surface of the sensor and its associated additional flow noise. The microphone is discussed in terms of its acoustic properties, vibration sensitivity, effect of Mach number on the directivity function, and flow noise. Additional information on the acoustic calibration of the microphone, the acceleration sensitivity of the airfoil, stationary source and receiver in a moving gas, acoustic tests in airflow, and flow noise tests of the airfoil porous surface sensor is included.
    Keywords: INSTRUMENTATION AND PHOTOGRAPHY
    Type: NASA-CR-137652 , BBN-3022
    Format: application/pdf
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  • 6
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    Prediction of airframe noise (1975)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: Methods of predicting airframe noise generated by aircraft in flight under nonpowered conditions are discussed. Approaches to predictions relying on flyover data and component theoretical analyses are developed. A nondimensional airframe noise spectrum of various aircraft is presented. The spectrum was obtained by smoothing all the measured spectra to remove any peculiarities due to airframe protrusions, normalizing each spectra by its overall sound pressure level and a characteristics frequency, and averaging the spectra together. A chart of airframe noise sources is included.
    Keywords: AERODYNAMICS
    Type: NASA-TN-D-7821 , L-9912
    Format: application/pdf
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    NASA TECHNICAL REPORTS
  • 7
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    A preliminary evaluation of noise reduction potential for the upper surface blown flap (1972)
    In:  CASI
    Details
    Publication Date: 2019-06-27
    Description: An assessment of the basic acoustic characteristics and noise reduction potential of an upper surface blown flap consisting of a rectangular nozzle and a single turning flap which was designed to approximate a typical takeoff setting is presented. The noise reduction concepts studied were: (1) replacing a section of the flap trailing edge with a porous material and (2) active modification of the flow field using blowing near the trailing edge. Diagrams of the airfoil section and test equipment are provided. Photographs of airflow conditions across the aerodynamic surface are included.
    Keywords: AIRCRAFT
    Type: NASA-CR-112246 , BBN-2478
    Format: application/pdf
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  • 8
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    Diagnostic calculations of airframe-radiated noise (1975)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: Methods of calculating airframe noise due to the following components are presented: wings and stabilizers, flaps, landing gear 'self-noise', landing gear bay (wheel well) oscillations, separated flow interaction with edges of cavities, and doors associated with gear deployment. The predominant source mechanisms were dipole-like in nature, being related to the local fluctuating aerodynamic forces on struts, airfoil edges, cavity edges, etc. Available data are converted into semiempirical prediction methods to enable a tentative rank ordering of noise sources. A sample application of these prediction procedures is carried out for a typical CTOL passenger jet using actual aircraft parameters, where available.
    Keywords: AIRCRAFT DESIGN, TESTING AND PERFORMANCE
    Type: AIAA PAPER 75-485 , American Institute of Aeronautics and Astronautics, Aero-Acoustics Conference; Mar 24, 1975 - Mar 26, 1975; Hampton, VA
    Format: text
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