Showing papers on "Aircraft noise published in 1978"

Helicopter noise: state-of-the-art

Abstract: This paper reviews helicopter external noise with particular emphasis on the noise due to helicopter main and tail rotors. The bases for annoyance and audibility are discussed. Sources of rotor noise include steady, periodic, and random loads on the rotor blades, as well as volume displacement and nonlinear aerodynamic effects at high blade Mach numbers. Either main or tail rotors can be dominant noise sources at various frequencies and observer positions.

High Velocity Jet Noise Source Location and Reduction. Task 2. Theoretical Developments and Basic Experiments.

Abstract: : This report presents the results of Task 2 conducted under the subject program over a period of 45 months. Task 2 was formulated as a fundamental theoretical and experimental study aimed at an understanding of the noise generation and suppression mechanisms of high velocity jets. The mechanisms investigated included changes in turbulence structure, fluid shielding, and alteration of convective amplification of jet noise sources. Several other areas such as physical shielding, shock associated noise, lip noise, effect of fluid/particle additives on jet noise, orderly structure in jets, ejector aeroacoustics, and flight effects on jet noise were also investigated. The most significant achievements of Task 2 were as follows. A unified aeroacoustic theory composed of three ingredients were developed: (1) the prediction of mean properties of the jet plume, (2) deduction of turbulence properties relevant to jet noise by similarity arguments, and (3) the prediction of far-field noise including the effects of fluid shielding. A semiempirical shock noise prediction procedure was also developed. Exhaustive theory-data comparisons for a wide range of nozzle configurations and velocity/temperature combinations were conducted and have confirmed the essential validity of this model as a prediction tool. A comprehensive series of experiments with simple suppressor elements (such as a single rectangular tube, twin jets, linear arrays of jets, circular arrays of jets) was carried out and revealed the importance of acoustic shielding by adjacent jets. A fundamental series of experiments, specifically tailored to reveal fluid shielding as a jet noise suppression mechanism, was successfully conducted.

Investigation of Interior Noise in a Twin-Engine Light Aircraft

Abstract: This paper describes experimental studies of interior noise in a twin-engine, propeller-driven, light aircraft. An analytical model for this type of aircraft is also discussed. Results indicate that interior noise levels in this aircraft due to propeller noise can be reduced by reducing engine rpm at constant airspeed (about 3 dB), and by synchrophasing the twin engines/propellers (perhaps up to 12 dB). Ground tests show that the exterior noise pressure imposed on the fuselage consists of a complex combination of narrow-band harmonics due to propeller and engine exhaust sources. This noise is reduced by about 20-40 dB (depending on the frequency) by transmission through the sidewall to the cabin interior. The analytical model described uses modal methods and incorporates the flat-side geometrical and skin-stringer structural features of this light aircraft.

Jet noise source modification due to forward flight

Abstract: XTENSIVE use of noise-absorbi ng materials in the inlet and exhaust ducting of modern high bypass ratio turbo fan engines has reduced the noise generated by fans, compressors, turbines, and burners to the level that the jet noise (noise generated in the exhaust plume by mixing the high velocity jet with the ambient air) is an important part of the total noise signature of current aircraft. The characteristics of jet noise from turbojet and turbofan engines have been well documented under static conditions. However, aircraft noise certification limits must be satisfied under actual aircraft flyover conditions during takeoff and approach operations. Thus, it is important that methods be developed to more accurately predict the jet noise under flight conditions. The effect of flight on jet noise of a circular jet exhaust has been simulated by testing in wind tunnels.1'2 Noise measurements obtained in wind tunnel tests show that the jet exhaust noise of nozzles operating subsonically is reduced in flight from the static levels at all measurement angles. However, noise measurements obtained from some flyover tests3 have shown less noise reduction than the results of the wind tunnei tests. In order to help resolve the differences in wind tunnel and flyover results, it is necessary to understand the effects of flight on the fundamental mechanisms of jet noise generation. Measuring the changes caused by flight in the aerodynamic parameters responsible for noise generation and relating these changes to the measured noise reductions caused by flight will provide a more basic understanding of the effect of flight on jet noise than is currently available. This understanding will allow eventual improvements in our ability to develop more accurate in-flight predictions. The noise reduction in flight is generally attributed to changes in the strength of the acoustic sources distributed throughout the jet shear layer. The acoustic source strength at a point in the jet shear layer is determined from the local aerodynamic flow properties. The mean and turbulence flow quantitites needed to determine the acoustic source strength are mean velocity, mixing layer growth, turbulence intensity, integral length scale, eddy convection velocity, and integral

Long‐distance focusing of Concorde sonic boom

Abstract: Infra‐acoustic signals from supersonic flights of Concorde are regularly recorded in northern Sweden at distances up to 5000 km from the aircraft. Relatively high signal amplitudes (up to 0.1 N/m2) are explained by a kind of long‐distance focusing effect. Principle and consequences of the focusing effect are discussed.

Prediction of ground effects on aircraft noise

Abstract: A unified method is recommended for predicting ground effects on noise. This method may be used in flyover noise predictions and in correcting static test-stand data to free-field conditions. The recommendation is based on a review of recent progress in the theory of ground effects and of the experimental evidence which supports this theory. It is shown that a surface wave must be included sometimes in the prediction method. Prediction equations are collected conveniently in a single section of the paper. Methods of measuring ground impedance and the resulting ground-impedance data are also reviewed because the recommended method is based on a locally reactive impedance boundary model. Current practice of estimating ground effects are reviewed and consideration is given to practical problems in applying the recommended method. These problems include finite frequency-band filters, finite source dimension, wind and temperature gradients, and signal incoherence.

The Generation, Radiation and Prediction of Supersonic Jet Noise. Volume 1

Abstract: : The work presented in this report forms a continuation of the fundamental studies on the generation and radiation of supersonic jet noise, reported in technical reports AFAPL-TR-72-53 (six volumes), AFAPL-TR-74-24, and AFAPL-TR-76-65 (four volumes) The total noise from a supersonic jet is taken to consist of contributions from three independent noise sources: (1) small-scale turbulent mixing noise, (2) noise from large-scale turbulence structure, and (3) shock-associated noise The generation, radiation and prediction of each of these noise components is described in this report In addition, the mean and turbulent flow characteristics of heated and unheated, subsonic and supersonic jets, measured by a laser velocimeter system, are also presented Finally, a computer program for the prediction of jet noise, based on fundamental principles as far as possible at the present time, is presented and described in the form of a user's guide A complete listing of this computer program is given in the Appendix volume (Volume 11) of this report

Passenger ride quality in transport aircraft

Abstract: Quantitative relationships are presented which can be used to account for passenger ride quality in transport aircraft. These relations can be used to predict passenger comfort and satisfaction under a variety of flight conditions. Several applications are detailed, including evaluation of use of spoilers to attenuate trailing vortices, identifying key elements in a complex maneuver which leads to discomfort, determining noise/motion tradeoffs, evaluating changes in wing loading, and others. Variables included in the models presented are motion, noise, temperature, pressure, and seating.

Noise Control Handbook of Principles and Practices

Abstract: NOISE CONTROL HANDBOOK OF PRINCIPLES AND PRACTICES Engineering, law, health care, architecture and design, transportation, government administration—each in its own way is concerned with the control and elimination of noise pollution from the environment. This handbook provides a basic guide for preparing, enforcing, and meeting noise control requirements. General enough to contain all the essentials of noise control, it is sufficiently detailed to meet specialized needs in areas such as aircraft noise reduction and housing insulation. You need no prior knowledge of the noise control field for a full understanding of this material. Each chapter is written by a recognized expert and concentrates on a particular aspect of noise abatment. The first sections focus on a basic explanation of sound and noise. Here you will discover the theoretical basis for the commonly used measuring techniques, as well as how to judge which instrumentation is best suited to your particular needs and situation. Practical applications of the important and widely used Equivalent Sound Level system of measuring noise are thoroughly explained. Using common industrial and construction situations, the book demonstrates the basic principles of noise control and acoustical treatment in the conservation of human hearing. This coverage includes necessary technical formulas, advance design methods, and explanations of sound test codes and legislation. There is a full discussion of both sides of the airport'aircraft noise controversy. The authors examine the measurement of noise levels, legislative and regulatory efforts to keep these levels with:n safe limits, and the impact of lonj terrfi exposure to noise on the human population. The Handbook deals in realistic teims with the problems of vehicular engine and tire noise levels. And vou will find a complete section on noise leveis In tn« home—how they can adversely effect people and how to design and insulate against them. In addition, the authors fully explain how to go about writing an acceptable Environmental Impact Statement. You learn how to write, word and properly define the limits in noise control ordinances. To round out the comprehensive coverage, you also find a thorough evaluation of the noise levels of recreational activities and vehicles, such as power boats, off-road motorcycles and snowmobiles. Whether you are a layman concerned with the ecological impact of noise or an engineer concerned with machine design, Noise Control will serve as your basic reference to today's critical problems in noise reduction and abatmer. l.

Evaluation of aero commander propeller acoustic data: Taxi operations

Abstract: Acoustic data are analyzed from a series of ground tests performed on an Aero Commander propeller-driven aircraft with an array of microphones flush-mounted on one side of the fuselage. The analyses were concerned with the propeller blade passage noise during static operation at several different engine speeds and included calculations of the magnitude and phase of the blade passage tones, the amplitude stability of the tones, and the spatial phase and coherence of the tones. The results indicate that the pressure field impinging on the fuselage represents primarily aerodynamic (near field) effects in the plane of the propeller at all frequencies. Forward and aft of the propeller plane aerodynamic effects still dominate the pressure field at frequencies below 200 Hz; but at higher frequencies, the pressure field is due to acoustic propagation from an equivalent center located about 0.15 to 0.30 blade diameters inboard from the propeller hub.

Noise due to rotor-turbulence interaction

Abstract: A procedure for calculating the noise due to turbulent inflow to a propeller or helicopter rotor in hover is summarized. The method is based on a calculation of noise produced by an airfoil moving in rectilinear motion through turbulence. At high frequency the predicted spectrum is broadband, while at low frequency the spectrum is peaked around multiples of blade passage frequency. The results of a parametric study of the variation of the noise with rotor tip speed, blade number, chord, turbulence scale, and directivity angle are given. A comparison of the theory with preliminary experimental measurements shows good agreement.

A method for calculating externally blown flap noise

Abstract: Several basic noise components were described. These components are: (1) compact lift dipoles associated with the wing and flaps; (2) trailing edge noise associated with the last trailing edge; and (3) quadrupole noise associated with the undeflected exhaust jet and the free jet located downstream of the trailing edge. These noise components were combined to allow prediction of directivity and spectra for under the wing (UTW) slotted flaps with conventional or mixer nozzles, UTW slotless flaps, upper surface blowing (USB) slotless flaps, and engine in front of the wing slotted flaps. A digital computer program listing was given for this calculation method. Directivities and spectra calculated by this method were compared with free field data for UTW and USB configurations. The UTRC method best predicted the details of the measured noise emission, but the ANOP method best estimated the noise levels directly below these configurations.

Aircraft Noise Identification System By Correlation Technique

Abstract: The growth of environmental noise by traffic, construction, or manufacturing bring increasing interference in listening to television and radio, as well as in other human activities. The Japanese government's national -projects for noise abatement are making progress in the revision of land-use compatibility and sound-proofing of dwellings. Through the Broadcasting Act, the Japan Broadcasting Corporation (NHK) has made a discount of the TV receiver's fee for compensation of interence in listening to TV by aircraft noise within restricted areas around airports and airbases.

Noise prediction technology for CTOL aircraft

Abstract: The application of a new aircraft noise prediction program to CTOL noise prediction is outlined. Noise prediction is based on semiempirical methods for each of the propulsive system noise sources, such as the fan, the combustor, the turbine, and jet mixing, with noise-critical parameter values derived from the thermodynamic cycle of the engine. Comparisons of measured and predicted noise levels for existing CTOL aircraft indicate an acceptable level of accuracy.

Status of Some Current Research in Jet Noise

Abstract: A review is presented of the status of jet noise technology and suppression concepts being pursued. The current ability of predicting noise from jets is discussed, and some aspects of jet noise suppression are dealt with. Particular attention is given to the problem of inflight effects on jet noise suppressors which still remains a critical issue.

Psychological Assessment of Aircraft Noise Index

Abstract: The aim of this paper is to describe the nature and development of a new index called Psychological Assessment of Aircraft Noise Index (PAANI). An intervening variable UFR (Unfavorableness Ratio) is used without setting directly the relation between psychological measurements and psychological reaction of the inhabitants to aircraft noise as is done for Weighted Equivalent Continuous Perceived Noise Level and Noise and Number Index. UFR is made for an aircraft by using the Theory of Quantification, which evaluates the physical measurements (maximum A‐weighted noise level, duration of noise above 70 dB, and frequency spectra of the noise). The characteristic I (r) of the noise at the rth location is defined as the number of aircraft over fixed value of UFR on the distribution of UFR calculated from the aircraft passing over the location. While the psychological reaction to noise of the inhabitants in the rth location is estimated from data obtained by a social survey, the PAANI is determined from the...

Airframe noise of the dc-9-31

Abstract: Airframe noise measurements are reported for the DC-9-31 aircraft flown at several speeds and with a number of flap, landing gear, and slat extension configurations. The data are corrected for wind effects, atmospheric attenuation, and spherical divergence, and are normalized to a 1 meter acoustic range. The sound pressure levels are found to vary approximately as the fifth power of flight velocity. Both lift and drag dipoles exist as a significant part of the airframe noise. The sideline data imply that a significant side-force dipole exists only for the flap- and gear-down configurations; for others, the data imply the existence of only the lift and drag dipoles. The data are compared with airframe noise predictions using the drag element and the data analysis methods. Although some of the predictions are good, further work is needed to refine the methods, particularly for the flap- and gear-down configurations.

Reduction of Cabin Noise and Vibration by Intrinsic Structural Tuning

Abstract: To control low-frequency interior noise in commercial and general-aviation aircraft, a concept based on intrinsic tuning and damping of the fuselage structural elements has been under development at Boeing for some time. This paper describes the results of some laboratory and field tests that were conducted for evaluation of the concept.

The generation, radiation and prediction of supersonic jet noise. volume ii. appendix. computer program listing

Abstract: : This appendix volume presents a complete listing of the unified jet noise prediction computer program (UNIJET), developed to predict the total noise from a subsonic or supersonic jet under static conditions In addition, a listing of the computer program (called INTEG) to predict absolute turbulent mixing noise levels at 90 deg to the jet axis, using laser velocimeter turbulence measurement, is also given A detailed description of these two programs in the form of a user's guide is given in the main volume of this report (Author)

Potential acoustic benefits of circulation control rotors

Abstract: The fundamental aeroacoustic mechanisms responsible for noise generation on a rotating blade are theoretically examined. Their contribution to the overall rotor sound pressure level is predicted. Results from a theory for airfoil trailing edge noise are presented. Modifications and extensions to other source theories are described where it is necessary to account for unique aspects of circulation control (CC) aerodynamics. The circulation control rotor (CCR), as embodied on an X-wing vertical takeoff and landing (VTOL) aircraft, is used as an example for computational purposes, although many of the theoretical results presented are generally applicable to other CC applications (such as low speed rotors, propellers, compressors, and fixed wing aircraft). Using the analytical models, it is shown that the utilization CC aerodynamics theoretically makes possible unprecedented advances in rotor noise reduction. For the X-wing VTOL these reductions appear to be feasible without incurring significant attendant performance and weight penalties.

What's all the noise about?

Abstract: Light aircraft noise is a growing problem, so much so that internationally proposed noise limits are expected to be enforced in many countries before long. This article highlights the main themes of a conference on light aircraft noise.

Helicopter cabin noise: Methods of source and path identification and characterization

Abstract: Internal noise sources in a helicopter are considered These include propulsion machinery, comprising engine and transmission, and turbulent boundary layer effects It is shown that by using relatively simple concepts together with careful experimental work it is possible to generate reliable data on which to base the design of high performance noise control treatments

The influence of background noise level and signal duration on the judged annoyance of aircraft noise

Abstract: A series of 72 flyover events were assessed by a jury of 35 observers, during 12 separate listening sessions conducted in a controlled test area designed to simulate typical indoor listening conditions Each aircraft signal was superimposed on a controlled random traffic background signal having a duration exceeding that of the aircraft event The presence of a steady mean traffic background noise can reduce the perceived noisiness of aircraft flyover events, provided that the judgment time available is sufficiently greater than the event time (time in excess of background) For a given peak event level, a reduction in associated background noise of 21 dBA is shown to be equivalent subjectively to an increase of 55 dBA in peak event level, with fixed background conditions Although regressions obtained with the noise pollution index, L sub NP, for single event judgments generally showed a lower correlation than the L sub o and L sub p - L sub o regression variables, the score data did show a number of significant trends which are also associated with the LNP index variations computed for single noise events

Insulating buildings from aircraft noise

Abstract: Field measurements of insulation against aircraft noise were made for a wide range of building components. Single‐figure ratings of noise insulation were derived, and a design procedure for use of these ratings is presented. The dependence of the effective sound insulation on the spectral balance of aircraft noise is examined.

Wind tunnel investigations of model rotor noise at low tip speeds

Abstract: Experimental and related analytical results on model rotor rotational and broadband noise obtained in the anechoic wind tunnel and rotor facility are summarized Factors studied include various noise sources, effects of helicopter performance parameters on noise generated by a model main rotor, appropriate scaling laws for the various types of main rotor noise, and the effects of intensity and size scales of injected turbulence on the intensity and spectra of broadband noise