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Method of Developing Helicopter Source Noise Models using Parameter Identification Techniques

Description:

TECHNOLOGY AREA(S): Air Platform 

OBJECTIVE: Develop a semiempirical tool for generating time-domain based, nondimensionally scaled, acoustic spheres from limited flight test data. 

DESCRIPTION: Accurate helicopter source noise models are required by the US Army in order to estimate the acoustic impact of proposed helicopter operations. Conventional helicopter source noise models used by current mission planning tools are empirical in nature, relying on measurements of helicopter noise captured by ground based microphone arrays during steady flyovers [1-2]. These models are entirely empirical, which limit their capability to estimate the noise produced by the helicopter at operating conditions inside the limited measurement database. Therefore, inaccurate estimates are provided when vehicle operations occur at different altitudes, gross weights, and external store configurations than those measured. These models are further incapable of accurately predicting effects of maneuvering flight conditions that are difficult to measure with a ground-based array. First-principles helicopter noise prediction models exist, but do not have the validated accuracy sufficient to produce reliable estimates of helicopter noise spheres required by mission planners. This topic proposes the development of a time-domain based hybrid method, where a mid-fidelity helicopter aeroacoustic prediction method is calibrated to measured data using a parameter identification approach. Accuracy comparable to empirical models is assured by calibrating the model to the available data; however, the model can be applied to predict noise at conditions that were not measured because it contains a physical model of the helicopter noise sources. Prior research has proven the viability of this concept through the development of the Fundamental Rotorcraft Acoustic Modeling from Experiments (FRAME) method of developing source noise models for helicopters and other rotorcraft [3]. The FRAME technique has been used to make accurate helicopter noise predictions from limited sets of vehicle data; for example, validated predictions have been made at different airspeeds, descent rates [4], and density-altitudes [5]. Validated predictions have also been made for a variety of horizontal and vertical maneuvers with load factors ranging from 0.5 g to 2 g [6]. However, the FRAME software is at a low TRL and is primarily oriented towards community noise prediction. The goal of this proposed topic is to prompt the development of a commercial source noise modeling method that can support acoustic predictions for civilian and military helicopter operations. 

PHASE I: The objective of phase I is to create a proof-of-concept semiempirical tool for generating, time-domain based, nondimensional scaled acoustic data for an isolated main rotor using wind tunnel acoustic measurements, or flight test measurements, as the source of model calibration data. Validate the tool by demonstrating that when the tool is calibrated to a subset of the measured data, the tool can accurately predict the time-domain main rotor harmonic noise radiation for rotor operating conditions both inside of (interpolation) and outside of (extrapolation) the range of data used to calibrate the tool. Develop technology transition plan and initial business case analysis. 

PHASE II: The objective of phase II is to further develop the tool to accurately model the acoustics of helicopters in free flight. Extend the tool to produce rotor harmonic time-domain noise data for both the main and tail rotors. Develop a method to calibrate the tool using ground-based microphone acoustic data collected during the flight testing of helicopters. Validate the tool by demonstrating that when the tool is calibrated to a subset of measured data, accurate rotor harmonic noise predictions can be made for flight conditions both inside of and outside of the range of calibration data. Extend the tool to generate acoustic data spheres suitable for use as input to existing acoustic propagation software used to assess the acoustic impact of helicopter operations. Refine transition plan and business case analysis. 

PHASE III: The objective of phase III is to further validate and finalize the tool for routine use in Government and commercial applications. Incorporate noise predictions for non-rotor-harmonic noise sources, such as broadband and engine noise. Validate the tool by demonstrating that accurate noise predictions can be made under atmospheric conditions different from those under which the calibration data were collected. Validate the tool by demonstrating that accurate noise predictions can be made under maneuvering flight using only steady flight noise data for calibration. Integrate the tool with a user interface and develop end-user documentation. The resulting tool is applicable to both military and commercial rotorcraft. Key military applications include predicting vehicle acoustic footprints during flight operations. The validated tool will be useful for accurate land use models for both military and civilian community operations. 

REFERENCES: 

1: Lucas, M. J. and Marcolini, M. A., "Rotorcraft Noise Model," AHS Technical Specialists’ Meeting, Williamsburg, VA, October 1997.

2:  Conner, D. A. and Page, J. A., "A Tool for Low Noise Procedures Design and Community Noise Impact Assessment: The Rotorcraft Noise Model (RNM)," Heli Japan, Tochigi, Japan, 2002.

3:  Greenwood, E., Fundamental Rotorcraft Acoustic Modeling from Experiments (FRAME). Ph.D. University of Maryland, 2011.

4:  Greenwood, E., and Schmitz, F.H., "A Parameter Identification Method for Helicopter Noise Source Identification and Physics-Based Semi-Empirical Modeling," presented at the American Helicopter Society 66th Annual Forum, Phoenix, AZ. May 2010.

5:  Greenwood, E., Sim, B.W., and Boyd, D.D., "The Effects of Ambient Conditions on Helicopter Harmonic Noise Radiation: Theory and Experiment," presented at the American Helicopter Society 72nd Annual Forum, West Palm Beach, FL. May 2016.

6:  Greenwood, E., Rau, R., May, B., and Hobbs, C., "A Maneuvering Flight Noise Model for Helicopter Mission Planning," presented at the American Helicopter Society 71st Annual Forum, Virginia Beach, VA. May 2015.

KEYWORDS: Rotorcraft, Helicopter, Acoustics 

CONTACT(S): 

James Stephenson 

(757) 864-9475 

james.h.stephenson23.civ@mail.mil 

Eric Greenwood 

(757) 864-9541 

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