A Tool for Low Noise Procedures Design and Community Noise Impact Assessment: The Rotorcraft Noise Model (RNM)

David A. Connerd.a.conner@larc.nasa.govAerospace EngineerAeroflightdynamics Directorate (AMRDEC), U.S. Army Aviation and Missile CommandHampton, Virginia, U.S.A.andJuliet A. Pagejpage@arl.wylelabs.comSenior Acoustical EngineerWyle LaboratoriesArlington, Virginia, U.S.A.AbstractTo improve aircraft noise impact modeling capabilities and to provide a tool to aid in the development oflow noise terminal area operations for rotorcraft and tiltxotors, the Rotorcraft Noise Model (RNM) wasdeveloped by the NASA Langley Research Center and Wyle Laboratories. RNM is a simulation program thatpredicts how sound will propagate through the atmosphere and accumulate at receiver locations located on flatground or varying terrain, for single and multiple vehicle flight operations. At the core of RNM are the vehiclenoise sources, input as sound hemispheres. As the vehicle _flies" along its prescribed flight trajectory, thesource sound propagation is simulated and accumulated at the receiver locations (single points of interest ormultiple grid points) in a systematic time-based manner. These sound signals at the receiver locations may thenbe analyzed to obtain single event footprints, integrated noise contours, time histories, or numerous otherfeatures. RNM may also be used to generate spectral time history data over a ground mesh for the creation ofsingle event sound animation videos. Acoustic properties of the noise source(s) are defined in terms of soundhemispheres that may be obtained from theoretical predictions, wind tunnel experimental results, flight testmeasurements, or a combination of the three. The sound hemispheres may contain broadband data (sourcelevels as a function of one-third octave band) and pure-tone data (in the form of specific frequency soundpressure levels and phase). A PC executable version of RNM is publicly available and has been adopted by anumber of organizations for Environmental Impact Assessment studies of rotorcraft noise. This paper providesa review of the required input data, the theoretical framework of RNM's propagation model and the outputresults. Code validation results are provided from a NATO helicopter noise flight test as well as a tiltrotor flighttest program that used the RNM as a tool to aid in the development of low noise approach profiles.IntroductionTo more accurately estimate the noise footprintfor rotorcraft and tiltrotor operations, and to provide atool to aid in the development of low noise terminalarea operations, Wyle Laboratories developed theRotorcraft Noise Model 1 (RNM) under contract to theNASA Langley Research Center. The United StatesNavy has also provided funding for improvements tothe propagation algorithms, namely propagation overvarying terrain 2.Presented at Heli Japan 2002, Tochigi, Japan,November 11-13, 2002. This paper is declared awork of the U.S. Government and is not subject tocopyright protection in the United States.The Rotorcraft Noise Model (RNM) is acomputer program that simulates sound propagationthrough the atmosphere. As a noise source, rotorcraftand tiltrotors are more complex than fixed-wingaircraft. Rotorcraft sources are three dimensional innature and the directivity and spectral content varywith flight condition, namely flight speed and flightpath angle. A single engine operating state parameter(a generalization not applicable to rotorcraft) typicallycharacterizes fixed wing noise emissions. At its core,RNM utilizes single or multiple sound hemispheres(broadband and pure tone with phase) for a givenflight condition to define the three-dimensionalvehicle spectral source characteristics.RNM calculates the noise levels in a variety ofmetrics at receiver positions on the ground either atpoints of interest or on a uniform grid. RotorcraftT213-6-1