Showing papers on "Aircraft noise published in 1986"

Prediction of helicopter rotor discrete frequency noise: A computer program incorporating realistic blade motions and advanced acoustic formulation

Abstract: A computer program has been developed at the Langley Research Center to predict the discrete frequency noise of conventional and advanced helicopter rotors. The program, called WOPWOP, uses the most advanced subsonic formulation of Farassat that is less sensitive to errors and is valid for nearly all helicopter rotor geometries and flight conditions. A brief derivation of the acoustic formulation is presented along with a discussion of the numerical implementation of the formulation. The computer program uses realistic helicopter blade motion and aerodynamic loadings, input by the user, for noise calculation in the time domain. A detailed definition of all the input variables, default values, and output data is included. A comparison with experimental data shows good agreement between prediction and experiment; however, accurate aerodynamic loading is needed.

Aircraft noise exposure, noise sensitivity, and everyday errors

Abstract: The present study compared self-reports of everyday errors given by subjects who lived in an area with a high level of aircraft noise with those of a similar group who lived in an area with a low l...

Aircraft Noise Prediction Program theoretical manual: Propeller aerodynamics and noise

Abstract: The prediction sequence used in the aircraft noise prediction program (ANOPP) is described. The elements of the sequence are called program modules. The first group of modules analyzes the propeller geometry, the aerodynamics, including both potential and boundary-layer flow, the propeller performance, and the surface loading distribution. This group of modules is based entirely on aerodynamic strip theory. The next group of modules deals with the first group. Predictions of periodic thickness and loading noise are determined with time-domain methods. Broadband noise is predicted by a semiempirical method. Near-field predictions of fuselage surface pressrues include the effects of boundary layer refraction and scattering. Far-field predictions include atmospheric and ground effects.

Nonlinear Acoustic Propagation of Broadband Noise

Abstract: Broadband jet noise is invariably assumed to propagate according to linear acoustic laws, although there are clear indications from both a limited set of experimental data taken in unusual, though appropriate, circumstances, and from simple weakly nonlinear acoustic theory that nonlinear propagation effects may significantly distort the noise spectrum over ranges of importance in full-scale aircraft flyovers. This paper starts with a review of the experimental situation and discusses the implications for aircraft noise prediction and control that might follow recognition of the importance of nonlinear propagation effects. Three rational theoretical approaches to the problem are then examined. The first relies on a simple expansion in amplitude (or, equivalently, frequency or range), and despite its obvious limitations is useful in giving a quantitative indication of the frequency, range and OASPL at which nonlinear spectral distortion can be expected to be significant near the peak frequency (and, by implication, even more significant at the higher frequencies which are so important in the determination of Perceived Noise Levels). This method can also incorporate attenuation effects, along with those of spherical spreading, directional nonuniformity, and weak nonlinearity. From it are also derived scaling laws for the variation with jet speed of the nonlinearly induced contribution to the spectrum and its dependence on range.

Recent advances in aeroacoustics

Abstract: The state of the art in theoretical and numerical models and experimentation on broadband noise propagation from high speed aircraft components such as jets and helicopter blades is detailed. Attention is given to noise produced by jets, shear layers and turbulence and to the acoustic properties of rotors. The generation and propagation of acoustic noise are discussed in terms of the evolution of concepts for flow-tone generation to include hydrodynamic instabilities. Consideration is also given to techniques for experimental studies and theoretical models for nonlinear features of acoustic propagation of broadband noise and acoustic propagation in partially-choked or soft-walled ducts.

A Comparative Study of Tail Rotor Noise Mechanisms

Abstract: A study was made of helicopter tail rotor noise, particularly that due to interactions with the wakes of the main rotor, hub, and fuselage, and with the engine exhaust. Both harmonic and broadband noise were analyzed. The disturbed flow into the tail rotor was modeled using combinations of aerodynamic and acoustic flow codes along with some necessary estimates of turbulence properties. Representative calculations show that the main rotor wake is the strongest contributor to both harmonic and broadband tail rotor noise. The fuselage separation wake and the engine exhaust flow are also very important to both harmonic and broadband noise. The hub and hub-shaft wakes are important contributors to the broadband noise only. The tip vortices do not seem to be important to broadband noise, but their effects on harmonic noise were not modeled accurately enough to draw any conclusions from this study.

Application of the Baseline Rotonet system to the prediction of helicopter tone noise

Abstract: The capabilities of the baseline Rotonet system designed to predict helicopter noise are analyzed. The modules of the system utilized for main and tail rotor geometry and blade section aerodynamic characteristics, for analyses, and for source-to-observer geometry, and atmospheric and ground effects calculations are described; a diagram of the system is provided. The Rotonet system produces axial force, tone noise, and sound pressure level information and a one third octave spectrum related to rotor tone noise and broadband noise sources. Main rotor noise predictions are compared with flight data. It is observed that both sets of data reveal increase loading on the advancing side and decrease loading on the retreating side. The tone noise and sound pressure levels for the first and second harmonics correlate well with the flight data; however, there is only fair agreement for the third harmonics of the sound pressure level. Analysis of the spectra display lower noise levels for higher altitudes and lower speeds. It is noted that the baseline Rotonet system is applicable for predicting performance and noise signatures for the lower harmonics. A phase II Rotonet system for evaluating higher harmonics is being developed.

An experimental investigation of the interior noise control effects of propeller synchrophasing

Abstract: A simplified cylindrical model of an aircraft fuselage is used to investigate the mechanisms of interior noise suppression using synchrophasing techniques This investigation allows isolation of important parameters to define the characteristics of synchrophasing The optimum synchrophase angle for maximum noise reduction is found for several interior microphone positions with pure tone source excitation Noise reductions of up to 30 dB are shown for some microphone positions, however, overall reductions are less A computer algorithm is developed to decompose the cylinder vibration into modal components over a wide range of synchrophase angles The circumferential modal response of the shell vibration is shown to govern the transmission of sound into the cylinder rather than localized transmission As well as investigating synchrophasing, the interior sound field due to sources typical of propellers has been measured and discussed

DFVLR/FAA (Deutsche Forschungs-und Versuchsanstalt fuer Luft und Raumfahrt/Federal Aviation Administration) Propeller Noise Tests in the German- Dutch Wind Tunnel DNW

Abstract: : A joint effort to develop high quality propeller acoustics data, which could be used by manufacturers for acoustic design purposes, and by researchers to validate established or newly developed theoretical noise prediction methods. Specifically, the program addressed propeller Mach number and disc plane attitude effects as related to noise certification test and evaluation procedures. Changes in Mach number, as they affect acoustic data adjustments, were explored through independent variation of tunnel flow velocity, propeller rotational speed and ambient air temperature. The tests on the effect of in flow angle on propeller noise also incorporated the influence of a typical engine nacelle on the flow field and, hence, on the propeller noise. In this report, acoustic and operational data acquisition and reduction procedures are described. Only a selection of typical test results is presented, in order to illustrate the acoustic analysis techniques.

Cruise noise of counterrotation propeller at angle of attack in wind tunnel

Abstract: The noise of a counterrotation propeller at angle of attack was measured in the NASA Lewis 8- by 6-Foot Supersonic Wind Tunnel at cruise conditions Noise increases of as much as 4 dB were measured at positive angles of attack on the tunnel side wall, which represented an airplane fuselage These noise increases could be minimized or eliminated by operating the counterrotation propeller with the front propeller turning up-inboard This would require oppositely rotating propellers on opposite sides of the airplane Noise analyses at different bandwidths enabled the separate front- and rear-propeller tones, as well as the total noise, at each harmonic to be determined A simplified noise model was explored to show how the observed circumferential noise patterns of the separate propeller tones might have occurred The total noise pattern, which represented the sum of the front- and rear-propeller tones at a particular harmonic, showed trends that would be hard to interpret without the separate-tone results Therefore it is important that counterrotation angle-of-attack noise data be taken in such a manner that the front- and rear-propeller tones can be separated

Acoustic treatment of the NASA Langley 4- by 7-meter tunnel: A feasibility study

Abstract: A feasibility study for upgrading the NASA Langley 4- by 7-Meter Tunnel so that it may be used for aeroacoustic research related to helicopters is described. The requirements for noise research leading to the design of the next generation of helicopters impose a set of acoustic test criteria that no existing wind tunnel in the United States can presently meet. Included in this feasibility study are the following considerations: (1) an evaluation of general wind-tunnel requirements and desired tunnel background noise levels for helicopter aeroacoustic research; (2) an assessment of the present acoustic environment for testing model rotors; (3) a diagnostic investigation of tunnel background noise sources and paths; (4) acoustic treatment options for tunnel background noise reduction and a trade-off study between these options; (5) an engineering feasibility assessment of the selected option; and (6) an integrated analysis of study components and recommendations of treatment for an approach to meet the tunnel background noise reduction goal. It is concluded that the Langley 4- by 7-Meter Tunnel is a fundamentally suitable facility for helicopter aeroacoustic research. It is also concluded that acoustic treatment of this facility for meeting the required tunnel background noise goal can be accomplished technically at reasonable risk and cost.

Noise pollution: Effects and control

Abstract: FUNDAMENTALS OF NOISE AND HEARING: Introduction: Noise Pollution, Man and Sound Physics of Noise Review of Noise Propagation in the Atmosphere Aural Reception Recent Advances in Understanding Hearing Mechanisms and Hearing Impairment Psychophysics of Hearing EFFECTS OF NOISE ON MAN: Auditory After-effects of Noise Non-auditory Effects of Noise: Physiological and Psychological Effects Noise Interference with Oral Communication Noise Pollution during the Night: A Possible Risk Factor for Health Hearing Conservation SOURCES OF NOISE AND ITS CONTROL: Road Traffic: Generation, Propagation and Control Aircraft Noise Generation and Control: Noise Around Airports Acoustic Shielding: Noise Reduction by Thin and Wide Barriers Solid-borne Noise Control in Buildings and Machinery Impact Machinery Noise: Prediction and Control Building Noise Control: The Main Problems, Available Technology and Future Trends Noise Pollution Control: Present Possibilities of Controlling Noise Inside Buildings Juridicial and Legal Aspects of Noise Control Epilogue.

Effects of an internal floor on low frequency sound transmission into aircraft cabins - An experimental investigation

Abstract: A simplified cylindrical model of an aircraft fuselage is used to experimentally study the effects of an internal floor on low frequency sound transmission into aircraft cabins. A scaled-down lattice floor support and floor skin are designed based upon selected characteristics of a business aircraft. Thus, the model provides a simplified procedure for studying the effects of various structural modifications as well as other important effects. Modal decomposition of the shell response and the corresponding pressure response at various interior locations are presented. Results indicate that the main effect of the floor on interior pressure levels is due to modification of the interior acoustic mode shape and not due to the structural modification of the fuselage caused by the lattice floor support.

Wing loads induced by a propeller wake

Abstract: A wing located in the wake of a propeller sustains unsteady loading. This loading can generate components of structureborne noise which are transmitted to the interior of the aircraft. The vibration of the wing can also contribute to reradiation of propeller noise. An analysis system for the evaluation of wing unsteady loads is presented. The propeller wake is modeled as a potential helical tip vortex and unsteady airfoil theory is used to determine the wing response. The result is the forcing function for determining the structureborne noise. The wing response is evaluated as a function of propeller thrust coefficient, advance ratio, and propeller installation geometry. The system capabilities are demonstrated by application to three significantly different aircraft.

Laboratory study of the effects of sidewall treatment, source directivity and temperature on the interior noise of a light aircraft fuselage

Abstract: This paper describes a laboratory study of add-on {coustic treatments for a twin-engine, propeller-driven aircraft fuselage. The sound source was a pneumatic-driver, with attached horn to simulate propeller noise distribution, powered by a white noise signal. Treatments included a double-wall, production-line treatment and various fiberglass and lead-vinyl treatments. Insertion losses, space-averaged across six interior microphone positions, were used to evaluate the treatments. In addition, the effects of sound source angle and ambient temperature on interior sound pressure level are presented. The sound source angle is shown to have a significant effect on one-third octave band localized sound pressure level. While changes in ambient temperature are shown to have little effect on one-third octave band localized sound pressure level, the change in narrowband localized sound pressure level may be dramatic.

Propeller-induced structure-borne noise - Laboratory-based test apparatus

Abstract: A potentially important source of structure-borne interior noise transmission in advanced turboprop aircraft is the impingement of the propeller wake/vortex on downstream aerodynamic surfaces. The expected levels of propeller wake/vortex-induced structure-borne noise transmission are not known nor can they be determined with present-day technology. A test apparatus has been designed, built and calibrated for the purposes of studying propeller-induced, structure-borne noise transmission in prototypical aircraft structures. The principal approach to the test apparatus design was to provide a physical means of separating the airborne and structure-borne noise components so that the structure-borne noise transmission response could be studied directly without airborne noise contamination. This was accomplished by housing the receiving fuselage structure in an acoustic shield fitted with a wing-to-fuselage acoustic seal. Initial evaluation of the wing-to-fuselage acoustic seal indicates adequate airborne noise isolation to allow direct study of structure-borne noise transmission.

Airport Noise Control Strategies

Abstract: : The Federal Aviation Administration's (FAA) Airport Noise Control Strategies Data File contains a comprehensive listing of airport noise control actions. The information contained in this data file is an updated version of the June 1983 Airport Noise Control Strategies report. Originally, the data were collected from the Environmental Data Bank of March 1979, the National Business Aircraft Association and the Aircraft Operators Council International. Additional data were garnered from the FAA Airport Facility Directory. For this version, the FAA regional offices reviewed the latest information and made appropriate changes. Thirty-seven categories of noise control actions have been identified and are in use, singly or in combination, by approximately 400 airports.

Structureborne noise in aircraft: Modal tests

Abstract: As part of an investigation to develop measurement techniques for structureborne noise, three modal surveys have been conducted on an OV-10A aircraft and the results have been presented The purpose of the modal surveys was to identify suitable locations for mounting accelerometer and strain gages in subsequent tests in which transfer functions relating wing vibration to interior noise were to be determined These surveys are as follows:(1) wing/fuselage modal survey utilizing one shaker under the right wing; (2) complete wing modal survey utilizing two shakers, one under each wing; and (3) fuselage side panel modal survey utilizing a small instrumented hammer The predominant frequencies and damping ratios for each analysis were listed in tables The primary mode shapes at the lower frequencies and at frequencies near the expected engine driving frequencies have been shown for each survey

A comparison of the structureborne and airborne paths for propfan interior noise

Abstract: In the present comparative study of aircraft interior noise levels due to airborne and structureborne paths for a given propeller source, a structure model is used that treats the fuselage interior as a rectangular cavity with five rigid walls; the sixth, or fuselage sidewall, is a stiffened panel. The wing is modeled as a simple beam carried into the fuselage by a large, discrete stiffener representing the carry-through structure. Comparisons are made on the basis of interior noise over a range of propeller rotational frequencies at a fixed thrust. It is found that the relative importance of the structural and airborne paths is highly dependent on the structural parameters.