Showing papers on "Aircraft noise published in 1987"

Helicopter Model Rotor-Blade Vortex Interaction Impulsive Noise: Scalability and Parametric Variations

Abstract: Acoustic data taken in the anechoic Deutsch-Niederlaendischer Windkanal (DNW) have documented the blade vortex interaction (BVI) impulsive noise radiated from a 1/7-scale model main rotor of the AH-1 series helicopter Averaged model scale data were compared with averaged full scale, inflight acoustic data under similar nondimensional test conditions At low advance ratios (mu = 0164 to 0194), the data scale remarkable well in level and waveform shape, and also duplicate the directivity pattern of BVI impulsive noise At moderate advance ratios (mu = 0224 to 0270), the scaling deteriorates, suggesting that the model scale rotor is not adequately simulating the full scale BVI noise; presently, no proved explanation of this discrepancy exists Carefully performed parametric variations over a complete matrix of testing conditions have shown that all of the four governing nondimensional parameters - tip Mach number at hover, advance ratio, local inflow ratio, and thrust coefficient - are highly sensitive to BVI noise radiation

Noise of a model high speed counterrotation propeller at simulated takeoff/approach conditions (F7/A7)

Abstract: A high speed advanced counterrotation propeller, was tested in the NASA-Lewis 9 x 15 foot Anechoic Wind Tunnel at simulated takeoff/approach conditions of 0.2 Mach number. Acoustic measurements were taken with fixed floor microphones, an axially translating microphone probe, and with a polar microphone probe which was fixed to the propeller nacelle and could take both sideline and circumferential acoustic surveys. Aerodynamic measurements were also made to establish the propeller operating conditions. The propeller was run over a range of blade setting angles from 36.4/36.5 to 41.1/39.4 deg, tip speeds from 165 to 259 m/sec, rotor spacings from 1.56 to 3.63 based on forward rotor tip chord to aerodynamic separation, and angles of attack to + or - 16 deg. First order rotor alone tones showed highest directivity levels near the propeller plane, while interaction tone showed high levels throughout sideline directivity, especially toward the propeller rotation axis. Interaction tone levels were sensitive to propeller row spacing while rotor alone tones showed little spacing effect. There is a decreased noise level associated with higher propeller blade numbers for the same overall propeller thrust.

The role of helicopter noise‐induced vibration and rattle in human response

Abstract: Our understanding of community reaction to helicopter noise is incomplete and inadequate. While A‐weighting appears to work outdoors and at modest noise levels, and the community response in terms of percentage of population highly annoyed can be correlated with respect to the day/night average sound level (DNL) descriptor, questions remain as to the role of perceived building vibrations and rattle in human response to helicopter noise. Does hearing windows, ceiling tiles, or objects in the room rattle or does the general perception of building vibration increase the public’s adverse response to helicopter noise? To answer these questions, this study examined the role of vibration and rattle in human response to helicopter noise. Results showed that the A‐frequency‐weighting is generally adequate to assess community response to helicopter noise when no vibration or rattle is induced by the noise. When rattle or vibrations are induced by the helicopter noise, however, A‐weighting does not assess the commun...

System for reducing aircraft noise and hush kit

Abstract: A system is provided for existing three-engine jet aircraft to reduce the noise levels at take-off and landing in order to meet governmental noise regulations. The system is particularly suitable for 727-200 aircraft having three JT8D engines. A hush kit is formed of the modified components for installation in existing airplanes.

Identification and proposed control of helicopter transmission noise at the source

Abstract: Helicopter cabin interiors require noise treatment which is expensive and adds weight. The gears inside the main power transmission are major sources of cabin noise. Work conducted by the NASA Lewis Research Center in measuring cabin interior noise and in relating the noise spectrum to the gear vibration of the Army OH-58 helicopter is described. Flight test data indicate that the planetary gear train is a major source of cabin noise and that other low frequency sources are present that could dominate the cabin noise. Companion vibration measurements were made in a transmission test stand, revealing that the single largest contributor to the transmission vibration was the spiral bevel gear mesh. The current understanding of the nature and causes of gear and transmission noise is discussed. It is believed that the kinematical errors of the gear mesh have a strong influence on that noise. The completed NASA/Army sponsored research that applies to transmission noise reduction is summarized. The continuing research program is also reviewed.

Cruise noise of the 2/9 scale model of the Large-scale Advanced Propfan (LAP) propeller, SR-7A

Abstract: Noise data on the Large-scale Advanced Propfan (LAP) propeller model SR-7A were taken in the NASA Lewis Research Center 8 x 6 foot Wind Tunnel. The maximum blade passing tone noise first rises with increasing helical tip Mach number to a peak level, then remains the same or decreases from its peak level when going to higher helical tip Mach numbers. This trend was observed for operation at both constant advance ratio and approximately equal thrust. This noise reduction or, leveling out at high helical tip Mach numbers, points to the use of higher propeller tip speeds as a possible method to limit airplane cabin noise while maintaining high flight speed and efficiency. Projections of the tunnel model data are made to the full scale LAP propeller mounted on the test bed aircraft and compared with predictions. The prediction method is found to be somewhat conservative in that it slightly overpredicts the projected model data at the peak.

Combustion noise from gas turbine aircraft engines measurement of far-field levels

Abstract: Combustion noise can be a significant contributor to total aircraft noise. Measurement of combustion noise is made difficult by the fact that both jet noise and combustion noise exhibit broadband spectra and peak in the same frequency range. Since in-flight reduction of jet noise is greater than that of combustion noise, the latter can be a major contributor to the in-flight noise of an aircraft but will be less evident, and more difficult to measure, under static conditions. Several methods for measuring the far-field combustion noise of aircraft engines are discussed in this paper. These methods make it possible to measure combustion noise levels even in situations where other noise sources, such as jet noise, dominate. Measured far-field combustion noise levels for several turbofan engines are presented. These levels were obtained using a method referred to as three-signal coherence, requiring that fluctuating pressures be measured at two locations within the engine core in addition to the far-field noise measurement. Cross-spectra are used to separate the far-field combustion noise from far-field noise due to other sources. Spectra and directivities are presented. Comparisons with existing combustion noise predictions are made.

Laboratory study of sidewall noise transmission and treatment for a light aircraft fuselage

Abstract: The noise transmission through a fuselage from a twin-engine propeller aircraft was measured using a pneumatically driven horn sound source to simulate a propeller noise distribution. Sidewall noise treatments included a double-wall, production-type treatment and several combinations of fiberglass and lead-vinyl. The noise reduction through the fuselage sidewall without treatment is shown to agree in magnitude and trend with the reduction of propeller noise through an aircraft fuselage in flight, but differs substantially at low frequency from the transmission loss measured with a diffuse noise source. The effects of the horn source position and angle of incidence are shown. The treatments were evaluated using their insertion loss, defined as the reduction of the interior sound level that occurred when the treatment was installed. The fiberglass is shown to weigh less and to have insertion loss values that are often higher than those of the double-wall and lead-vinyl treatments. Variations of temperature in the test chamber are shown to result in changes of the noise spectrum within the fuselage. The changes observed may explain part of the variability of treatment insertion loss measured in the flight of a propeller aircraft.

Aircraft noise synthesis system

Abstract: A second-generation Aircraft Noise Synthesis System has been developed to provide test stimuli for studies of community annoyance to aircraft flyover noise. The computer-based system generates realistic, time-varying, audio simulations of aircraft flyover noise at a specified observer location on the ground. The synthesis takes into account the time-varying aircraft position relative to the observer; specified reference spectra consisting of broadband, narrowband, and pure-tone components; directivity patterns; Doppler shift; atmospheric effects; and ground effects. These parameters can be specified and controlled in such a way as to generate stimuli in which certain noise characteristics, such as duration or tonal content, are independently varied, while the remaining characteristics, such as broadband content, are held constant. The system can also generate simulations of the predicted noise characteristics of future aircraft. A description of the synthesis system and a discussion of the algorithms and methods used to generate the simulations are provided. An appendix describing the input data and providing user instructions is also included.

Community reactions to helicopter noise: Results from an experimental study

Abstract: Reactions to low numbers of helicopter noise events (less than 50 per day) have been studied in a community setting utilizing a new type of study design. Community residents were repeatedly interviewed about daily noise annoyance reactions on days when helicopter noise exposures had, without the residents’s knowledge, been controlled for study design purposes. The effects of maximum noise level and number of noise events on helicopter noise annoyance were found to be consistent with the principles contained in Leq‐based noise indices. Although the best estimate of the effect of number of noise events is very nearly the same as that represented by the energy summation principle contained in Leq‐based indices, the possibility that the number of noise events has only a small effect on annoyance cannot be rejected at the conventional p<0.05 level. The effect of the duration of noise events was also found to be consistent with Leq‐based indices. After removing the effect of differences in duration and noise le...

External noise evaluation of the XV-15 TiltRotor aircraft

Abstract: The noise emission of the XV-15 in both helicopter and airplane operation and in level and constant-slope descending flight is measured (as sound exposure levels) in a series of test flights over a ground microphone array. The aircraft characteristics, test facility, and procedures are described, and the results are presented in extensive graphs and characterized in detail. Consideration is given to engine-rpm and nacelle-tilt effects, noise duration, sideline radiation, altitude effects, acoustic waveforms, and variations with glideslope. The XV-15 is found to produce less noise in its airplane mode than in the helicopter mode, where the noise levels are similar to those of other helicopters.

Results of the 1986 NASA/FAA/DFVLR main rotor test entry in the German-Dutch wind tunnel (DNW)

Abstract: An acoustics test of a 40%-scale MBB BO-105 helicopter main rotor was conducted in the Deutsch-Niederlandischer Windkanal (DNW) The research, directed by NASA Langley Research Center, concentrated on the generation and radiation of broadband noise and impulsive blade-vortex interaction (BVI) noise over ranges of pertinent rotor operational envelopes Both the broadband and BVI experimental phases are reviewed, along with highlights of major technical results For the broadband portion, significant advancement is the demonstration of the accuracy of prediction methods being developed for broadband self noise, due to boundary layer turbulence Another key result is the discovery of rotor blade-wake interaction (BWI) as an important contributor to mid frequency noise Also the DNW data are used to determine for full scale helicopters the relative importance of the different discrete and broadband noise sources For the BVI test portion, a comprehensive data base documents the BVI impulsive noise character and directionality as functions of rotor flight conditions The directional mapping of BVI noise emitted from the advancing side as well as the retreating side of the rotor constitutes a major advancement in the understanding of this dominant discrete mechanism

Structural influence of the cabin floor on sound transmission into aircraft - Analytical investigations

Abstract: The structural influence of the cabin floor on the transmission of low-frequency propeller noise into aircraft interiors has been examined using a simplified analytical model. The response amplitudes and distributions of shell displacement and internal acoustic pressure are examined for various frequencies and floor configurations. In general, at lower frequencies, the floor exerts little structural influence on the transmission of acoustic energy to the interior. However, as the frequency nears half the cylinder ring frequency, the floor can be seen to alter significantly the internal pressure distributions and response.

Control of airport- and aircraft-related noise in the united states

Abstract: Regulatory control of aircraft noise in the airport community environment is becoming increasingly common. Such controls are being applied to both civilian and military activities. These controls apply to two aspects of the noise problem: operational control of aircraft, both fixed and rotary wing, and land use controls around airports enacted by communities potentially affected by aircraft. A summary is given of the status of airport noise regulations enacted by municipalities (i.e., cities, counties) within the United States. To date, 2,000 municipalities have been inventoried in terms of existing noise controls that have land use implications. In addition, more than 200 airports have been evaluated regarding applicable operational and land use controls that are now in effect. The results of this analysis will be a discussion of 28 categories of noise control measures, categorized primarily in terms of operational and land use controls. Emphasis will be on describing these noise-control-related techniques and where they are being applied (i.e., case-study method), but not on their legal or political standing.

Annoyance response to simulated advanced turboprop aircraft interior noise containing tonal beats

Abstract: A study is done to investigate the effects on subjective annoyance of simulated advanced turboprop (ATP) interior noise environments containing tonal beats. The simulated environments consisted of low-frequency tones superimposed on a turbulent-boundary-layer noise spectrum. The variables used in the study included propeller tone frequency (100 to 250 Hz), propeller tone levels (84 to 105 dB), and tonal beat frequency (0 to 1.0 Hz). Results indicated that propeller tones within the simulated ATP environment resulted in increased annoyance response that was fully predictable in terms of the increase in overall sound pressure level due to the tones. Implications for ATP aircraft include the following: (1) the interior noise environment with propeller tones is more annoying than an environment without tones if the tone is present at a level sufficient to increase the overall sound pressure level; (2) the increased annoyance due to the fundamental propeller tone frequency without harmonics is predictable from the overall sound pressure level; and (3) no additional noise penalty due to the perception of single discrete-frequency tones and/or beats was observed.

Structureborne noise control in advanced turboprop aircraft

Abstract: Structureborne noise is discussed as a contributor to propeller aircraft interior noise levels that are nonresponsive to the application of a generous amount of cabin sidewall acoustic treatment. High structureborne noise levels may jeopardize passenger acceptance of the fuel-efficient high-speed propeller transport aircraft designed for cruise at Mach 0.65 to 0.85. These single-rotation tractor and counter-rotation tractor and pusher propulsion systems will consume 15 to 30 percent less fuel than advanced turbofan systems. Structureborne noise detection methodologies and the importance of development of a structureborne noise sensor are discussed. A structureborne noise generation mechanism is described in which the periodic components or propeller swirl produce periodic torques and forces on downstream wings and airfoils that are propagated to the cabin interior as noise. Three concepts for controlling structureborne noise are presented: (1) a stator row swirl remover, (2) selection of a proper combination of blade numbers in the rotor/stator system of a single-rotation propeller, and the rotor/rotor system of a counter-rotation propeller, and (3) a tuned mechanical absorber.

Detection of In-Flight Propeller-Induced Structure-Borne Noise

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. It can be safely assumed that this potential source of interior noise may well adversely affect achievable interior noise levels unless noise control measures are conscientiously incorporated into the aircraft design. Through the use of a laboratory-based test apparatus, techniques were developed to estimate the level of in-flight structure-borne noise transmission from combined frequency response function ground testing and in-flight structural response measurements. All phases of the procedure were simulated in the laboratory and the expected level of accuracy of the procedure is addressed.

Environmental Noise Assessment for Military Aircraft Training Routes. Volume 2. Recommended Noise Metric.

Abstract: : Operations on low level Military Aircraft Training Routes (MTRs) generate a unique noise environments. A review of available information on the nature of, and potential subjective response to, this environment has been carried out. The noise exposure from MTR operations is well below threshold limits for hearing damage or other physiological effects. However, based on this review, an interim noise metric is recommended for evaluation of the potential annoyance response of communities to MTR noise environments. Keywords: Aircraft noise.

Test procedures for detection of in-flight propeller-induced structure-borne noise

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. It can only be safely assumed that this potential source of interior noise may well hold up achievable interior noise levels unless noise control measures are conscientiously incorporated into the aircraft design. Through the use of a laboratory based test apparatus, techniques were developed to estimate the level of in-flight structure-borne noise transmission from combined frequency response function ground testing and in-flight structural response measurements. All phases of the test procedure were simulated in the laboratory and the expected level of accuracy of the procedure is addressed.

The importance of the measurement of annoyance in prediction of effects of aircraft noise on the health and well-being of noise-exposed communities. environmental annoyance: characterization, measurement, and control -- proceedings of the international symposium on environmental annoyance, conference centre woudschoten, the netherlands, september 15-18, 1986

Abstract: This article reviews the many different ways in which people become annoyed, and describes both community and laboratory studies undertaken to quantify human response to noise, with particular reference to aircraft noise. The stress-reduction model of individual response to aircraft noise is then described in more detail. The model is based on the premise that individuals will attempt to reduce, avoid, or eliminate stress in their lives. The model suggests that aircraft noise is perceived within two general sets of factors: situational factors and human factors. That is, qualities of the individual's physical, social and psychological environments are important to his perception of the noise, only when the perception is filtered through the various meanings associated with noise, through the interaction of activities and/or through evaluations of the adverse nature of the noise pe se, is stress produced. The individual will make every attempt to reduce this stress, and two methods of doing so are proposed: overt behaviour and internal adjustment. Research related to the stress reduction model is reviewed, followed by a discussion on the effects upon health of noise annoyance. For the covering abstract of the conference see IRRD 820771. (TRRL)

NASTRAN application for the prediction of aircraft interior noise

Abstract: The application of a structural-acoustic analogy within the NASTRAN finite element program for the prediction of aircraft interior noise is presented. Some refinements of the method, which reduce the amount of computation required for large, complex structures, are discussed. Also, further improvements are proposed and preliminary comparisons with structural and acoustic modal data obtained for a large, composite cylinder are presented.