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Analysis of Measured and Predicted Acoustics from an XV-15 Flight TestFlight acoustic and vehicle state data from an XV-15 acoustic flight test are examined. Flight predictions using TRAC are performed for a level flight (repeated) and four descent conditions (including a BVI). The assumptions and procedures used for TRAC flight predictions as well as the variability in flight measurements, which are used for input and comparison to predictions, are investigated in detail. Differences were found in the measured vehicle airspeed, altitude, glideslope, and vehicle orientation (yaw, pitch and roll angle) between each of the repeat runs. These differences violate some of the prediction assumptions and significantly impacted the resulting acoustic predictions. Multiple acoustic pulses, with a variable time between the pulses, were found in the measured acoustic time histories for the repeat runs. These differences could be attributed in part to the variability in vehicle orientation. Acoustic predictions that used the measured vehicle orientation for the repeat runs captured this multiple pulse variability. Thickness noise was found to be dominant on approach for all the cases, except the BVI condition. After the aircraft passed overhead, broadband noise and low frequency loading noise were dominant. The predicted LowSPL time histories compared well with measurement on approach to the array for the non-BVI conditions and poorly for the BVI condition. Accurate prediction of the lift share between the rotor and fuselage must be known in order to improve predictions. At a minimum, measurements of the rotor thrust and tip-path-plane angle are critical to further develop accurate flight acoustic prediction capabilities.
Document ID
20050042021
Acquisition Source
Langley Research Center
Document Type
Conference Paper
Authors
Boyd, D. Douglas, Jr. (NASA Langley Research Center Hampton, VA, United States)
Burley, Casey L. (NASA Langley Research Center Hampton, VA, United States)
Date Acquired
September 7, 2013
Publication Date
January 1, 2001
Subject Category
Acoustics
Meeting Information
Meeting: American Helicopter Society 57th Forum
Location: Washington, DC
Country: United States
Start Date: May 9, 2001
End Date: May 11, 2001
Sponsors: American Helicopter Society, Inc.
Distribution Limits
Public
Copyright
Public Use Permitted.

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