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The Effects of Ambient Conditions on Helicopter Harmonic Noise Radiation: Theory and ExperimentThe effects of ambient atmospheric conditions, air temperature and density, on rotor harmonic noise radiation are characterized using theoretical models and experimental measurements of helicopter noise collected at three different test sites at elevations ranging from sea level to 7000 ft above sea level. Significant changes in the thickness, loading, and blade-vortex interaction noise levels and radiation directions are observed across the different test sites for an AS350 helicopter flying at the same indicated airspeed and gross weight. However, the radiated noise is shown to scale with ambient pressure when the flight condition of the helicopter is defined in nondimensional terms. Although the effective tip Mach number is identified as the primary governing parameter for thickness noise, the nondimensional weight coefficient also impacts lower harmonic loading noise levels, which contribute strongly to low frequency harmonic noise radiation both in and out of the plane of the horizon. Strategies for maintaining the same nondimensional rotor operating condition under different ambient conditions are developed using an analytical model of single main rotor helicopter trim and confirmed using a CAMRAD II model of the AS350 helicopter. The ability of the Fundamental Rotorcraft Acoustics Modeling from Experiments (FRAME) technique to generalize noise measurements made under one set of ambient conditions to make accurate noise predictions under other ambient conditions is also validated.
Document ID
20160009056
Acquisition Source
Langley Research Center
Document Type
Conference Paper
Authors
Greenwood, Eric (NASA Langley Research Center Hampton, VA, United States)
Sim, Ben W. (Army Aviation Research and Development Command Moffett Field, CA, United States)
Boyd, D. Douglas, Jr. (NASA Langley Research Center Hampton, VA, United States)
Date Acquired
July 15, 2016
Publication Date
May 17, 2016
Subject Category
Aerodynamics
Acoustics
Report/Patent Number
NF1676L-22568
Meeting Information
Meeting: American Helicopter Society (AHS) Annual Forum
Location: West Palm Beach, FL
Country: United States
Start Date: May 17, 2016
End Date: May 19, 2016
Sponsors: American Helicopter Society, Inc.
Funding Number(s)
WBS: WBS 664817.02.07.03.01.01
Distribution Limits
Public
Copyright
Work of the US Gov. Public Use Permitted.

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