Reduction of High-speed Impulsive Noise by Blade Planform Modification of a Model Helicopter RotorNational Aeronautics and Space Administration, Scientific and Technical Information Branch, 1982 - Acoustical engineering - 58 pages |
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1-revolution sample 1/4-scale model 40-revolution average 7-Meter Tunnel 80 knots fig acoustic data Advanced blade advanced main rotor advanced rotor blades advanced rotor system advancing-blade-tip Mach number amplitude average from microphone axis presents blade planform Comparison of pressure-time dB SPL dynes dynes/cm² Effect of rotor Figure 12 Figure A1 forward speed fraction free-stream velocity fuselage high-speed impulsive noise histories for microphone HSI noise HSI-noise pressure spike HSI-noise spike Hz f main rotor system microphone 4 Comparison microphone 4 Narrow-band microphone 5 Effect Narrow-band spectrum negative pressure pulse noise signature noise-spike OASPL curve Open-cell polyurethane foam overall sound pressure plots pressure-time histories reduction of high-speed rotor hub rotor revolution rotor thrust sample from microphone sound pressure level spectrum for advanced spectrum for standard SRS data Standard blade standard rotor system system for microphone system on noise system were tested Tip-path plane Tunnel background noise UH-1H helicopter model upstream waveform Мт