94122_hagen - NASA Technical Memorandum 108843 Measurements...

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National Aeronautics and Space Administration Martin J. Hagen, Gloria K. Yamauchi, David B. Signor, and Marianne Mosher NASA Technical Memorandum 108843 Measurements of Atmospheric Turbulence Effects on Tail Rotor Acoustics September 1994
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National Aeronautics and Space Administration Ames Research Center Moffett Field, California 94035-1000 NASA Technical Memorandum 108843 September 1994 Measurements of Atmospheric Turbulence Effects on Tail Rotor Acoustics Martin J. Hagen, California Polytechnic State University, San Luis Obispo, California Gloria K. Yamauchi, David B. Signor, and Marianne Mosher, Ames Research Center, Moffett Field, California
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Measurements of Atmospheric Turbulence Effects on Tail Rotor Acoustics MARTIN J. HAGEN,* GLORIA K. YAMAUCHI, DAVID B. SIGNOR, AND MARIANNE MOSHER Ames Research Center Summary Results from an outdoor hover test of a full-scale Lynx tail rotor are presented. The investigation was designed to further the understanding of the acoustics of an isolated tail rotor hovering out-of-ground effect in atmospheric turbulence, without the effects of the main rotor wake or other helicopter components. Measurements include simultaneous rotor performance, noise, inflow, and far- field atmospheric turbulence. Results with grid-generated inflow turbulence are also presented. The effects of atmospheric turbulence ingestion on rotor noise are quantified. In contradiction to current theories, increasing rotor inflow and rotor thrust were found to increase turbulence ingestion noise. This is the final report of Task 13A—Helicopter Tail Rotor Noise, of the NASA/United Kingdom Defense Research Agency cooperative Aeronautics Research Program. Notation a speed of sound, m/s A autocorrelation coefficient b number of blades (4) c blade chord (0.18 m) C T / σ rotor thrust coefficient divided by rotor solidity, rotor thrust/R ρ(Ω R) 2 bc dBA A-weighted Sound Pressure Level (referenced to 20 μ Pa) d separation distance between rods in the turbulence-generating grid (7.6 cm) M tip rotor tip Mach number, R/a N number of data samples per hot-film time record (2048) OASPL Over-all Sound Pressure Level, dB (referenced to 20 μ Pa) * California Polytechnic State University, San Luis Obispo, California. O r observer radial distance nondimensionalized by R R rotor radius (1.105 m) u' rms turbulence velocity, 1 N (U i -u) 2 i=1 N , m/s u average velocity, 1 N U i i=1 N , m/s U velocity measured by hot-film probe, m/s U atmospheric wind speed measured by cup anemometer, m/s V rot inflow velocity measured by pitot probe, m/s x distance downstream from grid, cm x 0 position of maximum grid-generated turbulence (5 x 0 d 15), cm z height above ground, m z hub height of rotor hub above ground, (6.1 m) t hot-film signal sample spacing (0.0125 s) γ atmospheric wind direction measured from rotor axis (positive, clockwise looking down), deg Λ eddy length, m rotor rotational speed, rad/s ρ air density, kg/m 3 τ autocorrelation delay, τ = j t, s θ rotor collective pitch, deg
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2 Subscripts f denotes far-field hot-film parameter n
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94122_hagen - NASA Technical Memorandum 108843 Measurements...

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