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Helicopters have been playing an increasingly important role in military and civilian activities. The review of helicopter noise reduction is needed to be updated, because new technologies are constantly emerging. Firstly, the mechanism and propagation properties of the rotor (tail rotor) noise are described, and the co‑axial rigid rotor high‑speed helicopter and the tilting rotor are introduced. There are complicated aerodynamic or acoustic interaction in two new configuration helicopters. Secondly, the progress of helicopter flight noise measurement, as the verification approach for low noise design technologies, is demonstrated. Then, the design technologies for silent helicopters are summarized. The passive noise reduction strategies have been intensively implemented, and reduced helicopter radiated noise level greatly. The strategies of variable rotor speed and low noise trajectory are gradually maturing. However, the active control technologies are in the laboratory stage, and need further research on driving devices and control parameters et al. The final part summarizes the state of art for low noise control strategies in helicopters, and outlooks the development trend. [ABSTRACT FROM AUTHOR]

The tranquillity in national parks worldwide is currently under threat from intrusion of anthropogenic noise of a growing tourism industry and activity related to park management. This was addressed by creating informative tranquillity maps, where perceived tranquillity can be considered a key indicator of soundscape quality in natural areas. Tranquillity of an area can be assessed using TRAPT (Tranquillity Rating Prediction Tool), that has been developed and refined for assessing urban green spaces, national parks and wilderness areas in the United Kingdom. The subjective response to helicopter noise levels of a sample group of 35 people representing the general New Zealand population was obtained, based on visual and audio stimuli that were collected in Aoraki/Mt Cook National Park. These results were used to produce a revised TRAPT equation. It was discovered that levels under 32 dBA correspond to an excellent level of tranquillity. This threshold was used to produce a noise level exposure calculation for two national parks using noise prediction model AEDT (Aviation Environmental Development Tool). Contours representing tranquillity duration were then calculated and plotted, to serve as a planning tool for use by the Department of Conservation. A similar approach could be used for other national parks worldwide.

The study explored the perception and impact of helicopter noise, using Mgbuoshimini community in Nigeria as case study. A social survey was conducted in the community to find out if there is any significant relationship between helicopter noise on the one hand, and health, annoyance, day-time nap, night sleep, speech interference, and students’ concentration on the other hand. The sample size was calculated using the Taro Yamane's formula allowing for a 98% accuracy and a total of 2500 questionnaires was distributed using the stratified random sampling method. However, only 1862 questionnaires (representing a 74.5% response rate) were returned. The findings of the study showed that helicopter flyover noise has multiple effects on the residents of Mgbuoshimini Community and these includes adverse health effects, annoyance, day sleep disturbance, speech interference, and students’ concentration. This also implies that there may be other health issues associated with the airport noise which may not be discernible by respondents. The study therefore recommends that the Nigeria Orientation Agency or other relevant government agencies must enlighten and educate the residents of Mgbuoshimini Community on the effects of helicopter noise on their health.

The article presents the results of comparative experimental and computational studies of the acoustic characteristics of the model of a helicopter rotor performed in the modernized test section of the T-1K wind tunnel (WT). In order to identify and minimize the influence of spurious reflections on the studied acoustic characteristics using the Maximum Length Sequence method (M-sequence method) in the T-1K WT, we analyzed the microphone arrangement, which showed that the largest spurious contribution to the measured signal comes from reflections from the WT T-1K wall located behind the microphones. Comparison of the experimentally measured noise levels with the wall raised and lowered confirmed that the wall influences the measured acoustic noise characteristics of the model rotor. Comparison of experimental data with numerical simulation results allowed the conclusion that the WT T-1K modernized wind tunnel is fundamentally suitable to validate numerical methods of calculating the aeroacoustic characteristics of model rotors. [ABSTRACT FROM AUTHOR]

In an attempt to reduce acoustic pollution due to terminal flight phases (lift-offs and landings) in the surroundings of heliport, project MANOEUVRES developed a device capable of estimating the acoustic footprint of helicopters on the ground. This device requires knowledge of certain quantities that cannot be directly measured through physical sensors: the tip-path plane angle of attack and the main rotor thrust coefficient. Previous research has demonstrated that these quantities can be accurately estimated using observers that are properly fed with directly measurable flight mechanics and rotor state variables. However, these observers, which are based on linear mathematical models identified offline and employed through interpolation with respect to nominal airspeed, have shown poor robustness when the number of identification input cases is reduced, as required for a realistic design of observers in the field. This issue has particularly emerged when non-trimmed manoeuvres were considered during observation testing. To address this issue, this paper introduces a new baseline for the observation model, which includes dynamic pressure as an additional input. Moreover, a different model structure is considered depending on the observed variable. Specifically, for the tip-path plane angle of attack, a single model covers the entire airspeed range, while observation models for the rotor thrust coefficient are interpolated based on flight altitude. This new approach demonstrates results of comparable or superior quality to previous observation models. Furthermore, it exhibits increased robustness when the pool of identification cases used for observer synthesis is significantly reduced. Such improved performance and ease of synthesis pave the way for the setup and adoption of the proposed observers in the field. [ABSTRACT FROM AUTHOR]

In this paper, high-speed impulsive noise radiated from helicopter rotors in hover and forward flight is computed by using a convected FW-H equation in the frequency domain with a rotating permeable data surface. Compared to the time-domain formulations, the frequency-domain formula avoids retarded time computation and flow data interpolation with respect to the retarded time. In addition, the complicated emission surface construction is obviated, while a supersonic rotating data surface is chosen to account for quadrupole sources outside the sonic cylinder. The selection of a rotating data surface close to the rotor blades improves noise source resolution and thus ensures noise prediction accuracy at a low computational cost, compared to a stationary data surface approach. Besides, the use of a rotating data surface for hovering helicopter rotors enables noise source identification by a steady flow simulation. Application of the methodology to hovering and forward-flight helicopter rotors has shown good agreement with experimental data, thus indicating that the approach is an accurate and efficient computational tool for noise prediction of helicopter rotors operating at high tip Mach number. Since the formula is derived in a moving medium frame, the effect of forward-flight speed on the radiated noise can be explicitly taken into account. [ABSTRACT FROM AUTHOR]

Aeroacoustics has received great attention in the past decade, owing to the ever stricter noise regulations. Despite the stochastic nature of most aeroacoustic systems, non-deterministic investigations in regards to computational aeroacoustics (CAA) are limited. In this paper, uncertainty quantification has been achieved for the noise propagation stage of hybrid CAA, and also on the noise prediction of a non-lifting helicopter rotor in hover. Analytical and computational fluid dynamics test cases have been analyzed, with uncertainties propagated through these systems using non-intrusive polynomial chaos methods. It is shown here that the source of the uncertainty in the noise is dominated by the major characteristic properties of the simulations, such as the mean flow Mach number and blade tip Mach number. Only at a low tip Mach number uncertainties in the blade thickness may contribute significantly to the noise uncertainty. Apart from this, it is seen to be unlikely that small uncertainties in the geometry, ambient conditions and observer properties will contribute significantly to the noise uncertainty. A peak pressure uncertainty of up to 20% is seen in the hovering helicopter test case, from small, realistic uncertainties. This highlights the importance of considering uncertainties in CAA investigations. [ABSTRACT FROM AUTHOR]

Helicopter gearboxes significantly differ from other transmission types and exhibit unique behaviours that reduce the effectiveness of traditional fault diagnostics methods. In addition, due to lack of redundancy, helicopter transmission failure can lead to catastrophic accidents. Bearing faults in helicopter gearboxes are difficult to discriminate due to the low signal-to-noise ratio in the presence of gear vibration. In addition, the vibration response from the planet gear bearings must be transmitted via a time-varying path through the ring gear to externally mounted accelerometers, which cause yet further bearing vibration signal suppression. This research programme has resulted in the successful proof of concept of a broadband wireless transmission sensor that incorporates power scavenging while operating within a helicopter gearbox. In addition, this article investigates the application of signal separation techniques in detection of bearing faults within the epicyclic module of a large helicopter (CS-29) main gearbox using vibration and acoustic emissions. It compares their effectiveness for various operating conditions. Three signal processing techniques, including an adaptive filter, spectral kurtosis and envelope analysis, were combined for this investigation. In addition, this research discusses the feasibility of using acoustic emission for helicopter gearbox monitoring. [ABSTRACT FROM AUTHOR]

In this work, the mechanical power transferred from the main gearbox to the cabin of a helicopter is obtained experimentally and used as an input to the vibroacoustic model of the fuselage for interior noise prediction. The estimation of the transmitted mechanical power is based on the experimental evaluation of the coupled mechanical mobility and in-flight accelerations measured at the points connecting the main gearbox to the fuselage, i.e. the antitorque plate and struts. The predicted values, together with the airborne noise sources (upper deck cavity noise and aerodynamic excitation due to the turbulent boundary layer on the fuselage skin) are then fed into the vibroacoustic model of the fuselage/cabin, which is obtained using a Statistical Energy Analysis (SEA) approach. The reliability of the approach is finally demonstrated by comparing the SEA simulation results and in-flight noise measurements in terms of sound pressure level in the passenger cabin compartment. [ABSTRACT FROM AUTHOR]

A suite of computational tools capable of predicting in-plane low-frequency rotorcraft noise and its control using active flaps operating in closed-loop mode is developed. A combined code, consisting of Active VIbration and NOise Reduction (AVINOR), a comprehensive code for active control of vibration and noise, and an acoustic code called HELIcopter NOIse Reduction (HELINOIR), is validated against wind tunnel tests. The in-plane noise reduction obtained with a single 20% chord plain trailing edge flap for a rotor configuration resembling the MBB BO-105 during level flight at a moderate advance ratio is studied. Different configurations of far-field and near-field feedback microphone locations were examined. A near-field microphone located on a hypothetical left boom position was found to be at the best location and produced an in-plane noise reduction of up to 6 dB. However, this noise reduction was accompanied by an out-of-plane noise increase of 18 dB and a 60% increase in the vertical hub shear. These penalties can be partially alleviated by using a dual-plain-flap configuration. [ABSTRACT FROM AUTHOR]

This paper presents the development and application of a time-dependent adjoint-based method for aeroacoustic design optimization of flexible helicopter rotors in forward flight. The rotor noise signature at a far-field observer is computed with the hybrid aeroacoustic approach. A near-body coupled computational fluid dynamics/computational structural dynamics solver provides flow and geometry data to a Ffowcs Williams--Hawkings acoustic module that propagates the acoustic waves to a far-field observer. Forward and adjoint sensitivity formulations are derived that correspond to analogues of the multidisciplinary flexible aeroacoustic analysis problem. The newly implemented aeroacoustic capability is first verified to effectively perform forward flight noise prediction by comparison with the PSU-WOPWOP acoustic tool. Upon successful validation, a gradient-based optimization method is used to minimize the required torque of the flexible HART-II rotor in trimmed forward flight while reducing the acoustic signature at a far-field observer by changing the shape of the blades. The adjoint formulation developed in this work is used to efficiently compute the sensitivity required by the optimization algorithm. [ABSTRACT FROM AUTHOR]

The frequency bandwidth of the sound from a light helicopter, such as a Robinson R44, extends from about 13 Hz to 2.5 kHz. As such, the R44 has potential as a low-frequency sound source in underwater acoustics applications. To explore this idea, an experiment was conducted in shallow water off the coast of southern California in which a horizontal line of hydrophones detected the sound of an R44 hovering in an end-fire position relative to the array. Some of the helicopter sound interacted with seabed to excite the head wave in the water column. A theoretical analysis of the sound field in the water column generated by a stationary airborne source leads to an expression for the two-point horizontal coherence function of the head wave, which, apart from frequency, depends only on the sensor separation and the sediment sound speed. By matching the zero crossings of the measured and theoretical horizontal coherence functions, the sound speed in the sediment was recovered and found to take a value of 1682.42 ± 616.20 m/s. This is consistent with the sediment type at the experiment site, which is known from a previous survey to be a fine to veryfine sand. [ABSTRACT FROM AUTHOR]

This paper presents the recent advances in reduction of multifrequency noise inside helicopter cabin using an active structural acoustic control system, which is based on active gearbox struts technical approach. To attenuate the multifrequency gearbox vibrations and resulting noise, a new scheme of discrete model predictive sliding mode control has been proposed based on controlled auto-regressive moving average model. Its implementation only needs input/output data, hence a broader frequency range of controlled system is modelled and the burden on the state observer design is released. Furthermore, a new iteration form of the algorithm is designed, improving the developing efficiency and run speed. To verify the algorithm's effectiveness and self-adaptability, experiments of real-time active control are performed on a newly developed helicopter model system. The helicopter model can generate gear meshing vibration/noise similar to a real helicopter with specially designed gearbox and active struts. The algorithm's control abilities are sufficiently checked by single-input single-output and multiple-input multiple-output experiments via different feedback strategies progressively: (1) control gear meshing noise through attenuating vibrations at the key points on the transmission path, (2) directly control the gear meshing noise in the cabin using the actuators. Results confirm that the active control system is practical for cancelling multifrequency helicopter interior noise, which also weakens the frequency-modulation of the tones. For many cases, the attenuations of the measured noise exceed the level of 15 dB, with maximum reduction reaching 31 dB. Also, the control process is demonstrated to be smoother and faster. [ABSTRACT FROM AUTHOR]

A framework for using continuous wavelet transforms to isolate and extract blade–vortex interaction noise from helicopter acoustic signals is described. The extraction method allows for the investigation of blade–vortex interactions independent of other sound sources. Experimentally acquired acoustic data from full-scale helicopter flyover tests are first transformed into time-frequency space through the wavelet transformation, with blade–vortex interactions identified and filtered by their high-amplitude, high-frequency impulsive content. The filtered wavelet coefficients are then used to create a pressure signal solely related to blade–vortex interactions. Analysis of a synthetic data set is conducted and shows that blade–vortex interactions can be accurately extracted so long as the blade–vortex interaction wavelet energy is comparable to the wavelet energy in the main rotor harmonic. [ABSTRACT FROM AUTHOR]

This paper investigates different methodologies for the evaluation of the acoustic disturbance emitted by helicopter's main rotors during unsteady maneuvers. Nowadays, the simulation of noise emitted by helicopters is of great interest to designers, both for the assessment of the acoustic impact of helicopter flight on communities and for the identification of optimal-noise trajectories. Typically, the numerical predictions consist of the atmospheric propagation of a nearfield noise model, extracted from an appropriate database determined through steady-state flight simulations/measurements (quasi-steady approach). In this work, three techniques for maneuvering helicopter noise predictions are compared: one considers a fully unsteady solution process, whereas the others are based on quasi-steady approaches. These methods are based on a three-step solution procedure: first, the main rotor aeroelastic response is evaluated by a nonlinear beam-like rotor blade model coupled with a boundary element method for potential flow aerodynamics; then, the aeroacoustic near field is evaluated through the 1A Farassat formulation; finally, the noise is propagated to the ground by a ray tracing model. Only the main rotor component is examined, although tail rotor contribution might be included as well. The numerical investigation examines the differences among the noise predictions provided by the three techniques, focusing on the assessment of the reliability of the results obtained through the two quasisteady approaches as compared with those from the fully unsteady aeroacoustic solver. [ABSTRACT FROM AUTHOR]

The paper presents the results of experimental studies on developing a technique to determine the acoustic properties of models of main helicopter rotors on an open test bench. The method of maximum length sequences is used to choose the optimum arrangement of microphones for an open test bench that would minimize the influence of parasitic echo. The results of processing the data of an acoustic experiment with a model rotor are detailed. [ABSTRACT FROM AUTHOR]

The Council of European Aerospace Societies (CEAS) Aeroacoustics Specialists Committee (ASC) supports and promotes the interests of the scientific and industrial aeroacoustics community on the European scale, and European aeronautics activities internationally. Each year, the committee highlights several of the research and development projects in Europe. This paper is the 2015 issue of this collection of Aeroacoustic Highlights, compiled from contributions submitted to the CEAS-ASC. The contributions are classified in different topics; the first categories being related to specific aeroacoustic challenges (airframe noise, fan and jet noise, helicopter noise, aircraft interior noise), while the two last sections are devoted respectively to recent improvements and emerging techniques and to general advances in aeroacoustics. Furthermore, a concise summary of the CEAS-ASC workshop “Broadband noise of rotors and airframes” held in La Rochelle, France, in September 2015 is included in this report. [ABSTRACT FROM AUTHOR]

Among the several sources of acoustic annoyance produced by rotorcraft in operating conditions, blade-vortex interactions (BVIs) capture the interest of much of the current research. This paper deals with the reduction of BVI noise from helicopter main rotors by application of the active twist rotor concept (ATR), exploiting smart materials for twisting blades through higher-harmonic torque loads. An optimal, multi-cyclic, control approach is applied to identify the control law driving the ATR actuation during the occurrence of severe BVI events. Numerical predictions are obtained through a computational tool that is able to predict the aeroelastic response of the rotor blades and the emitted noise in arbitrary steady flight conditions. The approach for the control law identification is described and numerical results concerning aeroelastic and aeroacoustic performance of the controlled rotor are presented to assess the proposed methodology. [ABSTRACT FROM AUTHOR]

The Council of European Aerospace Societies (CEAS) Aeroacoustics Specialists Committee (ASC) supports and promotes the interests of the scientific and industrial aeroacoustics community on an European scale and European aeronautics activities internationally. Each year the committee highlights some of the research and development projects in Europe. This paper is the 2014 issue of this collection of Aeroacoustic Highlights, compiled from informations submitted to the CEAS-ASC. The contributions are classified in different topics; the first categories being related to specific aeroacoustic challenges (airframe noise, fan and jet noise, helicopter noise, aircraft interior noise) and two last sections are respectively devoted to recent improvements and emerging techniques and to general advances in aeroacoustics. For each section, the present paper focus on accomplished projects, providing the state of the art in each research category in 2014. A number of research programmes involving aeroacoustics were funded by the European Commission. Some of the highlights from these programmes are summarised in this paper, as well as highlights funded by national programmes or by industry. [ABSTRACT FROM AUTHOR]

To measure the noise footprint of a helicopter approach procedure, microphones are usually distributed over a large area. In this paper, however, a method is utilized where the measurement of the noise footprint is conducted in parts by distributing the microphones over a limited area and executing the same approach procedure, while the position of the landing point is shifted. The flight tests are conducted with the EC135-ACT/FHS helicopter. Guidance to the pilots is provided by use of a "tunnel-in-the-sky" head down pilot display. Since the same flight procedure is executed multiple times, it is possible to conduct a statistical analysis on the accuracy and reproducibility of the approach procedure in terms of position, velocity, and generated noise. This statistical analysis shows that the geometric accuracy is on the order of 6 m in the horizontal plane and 12 m in the vertical plane. The variations in the measured sound exposure level from one approach to another are on the order of 3-7.5 dB(A). The approach procedure is also executed with excessive cyclic pilot control inputs to identify an upper limit of the extra noise generated by unsteady flight. However, these approaches are indistinguishable, in terms of the sound exposure level or sound pressure level, from the approacheswith normal pilot control inputs. [ABSTRACT FROM AUTHOR]

To evaluate the relationship between aircraft noise exposure and the quality of national park visitor experience, more than 4600 visitor surveys were collected at seven backcountry sites in four U.S. national parks simultaneously with calibrated sound level measurements. Multilevel logistic regression was used to estimate parameters describing the relationship among visitor responses, aircraft noise dose metrics, and mediator variables. For the regression models, survey responses were converted to three dichotomous variables, representing visitors who did or did not experience slightly or more, moderately or more, or very or more annoyance or interference with natural quiet from aircraft noise. Models with the most predictive power included noise dose metrics of sound exposure level, percent time aircraft were audible, and percentage energy due to helicopters and fixed-wing propeller aircraft. These models also included mediator variables: visitor ratings of the "importance of calmness, peace and tranquility," visitor group composition (adults or both adults and children), first visit to the site, previously taken an air tour, and participation in bird-watching or interpretive talks. The results complement and extend previous research conducted in frontcountry areas and will inform evaluations of air tour noise effects on visitors to national parks and remote wilderness sites. [ABSTRACT FROM AUTHOR]

• EPNL methodology allows to assess helicopter noise. • EPNL index takes into account subjective perception of noise generated by a helicopter. • Stabilization of speed during the approach has impact on the results of noise measurements. One of the elements of type certificate of aircraft is noise certificate. The article presents the method and results of certification tests for a helicopter with take-off mass exceeding 3175 kg. The performed tests involved 38 landing approaches, 51 overflights and 28 take-offs. For each test, the recorded data was analyzed in terms of acoustic signal (time history of 1/3-octave spectra), helicopter's flight trajectory, operating parameters (velocity relative to ground and relative to air, rotational speed of the main rotor) and meteorological parameters (wind speed, pressure, temperature, relative air humidity). [ABSTRACT FROM AUTHOR]

Abstract: A stethoscope is the standard clinical tool used for auscultation. It amplifies chest sounds and enables diagnosis of both circulatory and respiratory systems. However, in many emergency transportation environments, ambient noise levels in excess of 90dB prevent successful auscultation. In this paper, data is collected in a U.S. Army Sikorsky UH-60 helicopter to analyze the inherent noise present during routine aeromedical transport. Data is gathered with a commercially available stethoscope retrofitted with an array of accelerometers and microphones. Peak acoustic noise levels are recorded in excess of 125dB and ambient vibration levels are measured to be near 0.5g RMS. The signal-to-noise ratio is much below 1 for the entire frequency range of interest in auscultation. It is shown using a combination of simulations and experimental data that existing active noise cancellation techniques using a reference microphone would be ineffective in this scenario. However, similar adaptive noise cancellation techniques using a reference accelerometer within the device would be significantly more effective at enabling auscultation. [Copyright &y& Elsevier]

The article offers noise news briefs as of April 1, 2014. More than 1,000 high-tech sensors linked to smartphone applications would be installed in Dublin, Ireland to monitor noise levels, pollution and flood. New York Governor Andrew Cuomo has ordered the New York Port Authority to conduct a review of the impacts of airplane noise on communities. The U.S. Federal Aviation Administration (FAA) has issued its final rule on stage three helicopter noise certification standards on March 4.

Results froma jointDARPA/Boeing/NASA/Armywind tunnel test demonstrated the ability to reduce in-plane, low-frequency noise of the full-scaleBoeing-SMART(Smart Material ActuatedRotorTechnology) rotor with active flaps. Test data reported in this paper illustrated that near-field acoustic energy in the first six blade-passing harmonics could be reduced by up to 6 dB at a moderate-airspeed, level flight condition at an advance ratio of 0.30. Reduced noise levels were attributed to selective active flap schedules that modified in-plane blade airloads on the advancing side of the rotor, generating counteracting acoustic pulses that partially offset the negative pressure peaks associated with in-plane, steady thickness noise. These favorable reduced-noise operating states are a strong function of the active flap actuation amplitude, frequency, and phase. The reduced noise levels resulted in reduction of predicted aural detection distance, but incurred vibratory load penalties due to increased hub shear forces. [ABSTRACT FROM AUTHOR]

A new method of characterizing the external noise radiation of rotorcraft is presented, making use of ground-based acoustic measurements. The method employs time-domain de-Dopplerization to transform the acoustic pressure time-history data collected from a fixed array of ground-based microphones to the equivalent time-history signals observed by an array of virtual in-flight microphones traveling with the helicopter. The now-stationary signals observed by the virtual microphones are then periodically averaged with the main and tail rotor blade passages, which are inferred from the acoustic signals using wavelet analysis. The averaging process suppresses noise that is not periodic with the respective rotor, allowing for the separation of main and tail rotor pressure time histories. The averaged measurements are then interpolated across the range of directivity angles captured by the microphone array in order to generate separate acoustic hemispheres for the main and tail rotor noise sources. This method facilitates a more direct comparison of ground-based noise measurements of rotorcraft with theoretical predictions and wind tunnel measurements of isolated rotors. The new method is successfully applied to ground-based microphone measurements of a Bell 206B3 helicopter and demonstrates the strong directivity characteristics of harmonic noise radiation from both the main and tail rotors of that helicopter. [ABSTRACT FROM AUTHOR]

A phenomenological noise-modeling method called Fundamental Rotorcraft Acoustic Modeling from Experiments is used to demonstrate the changes in rotor harmonic noise generation of a helicopter operating at different ambient conditions. The method is based upon a nondimensionai representation of the governing acoustic and performance equations of a single-rotor helicopter. Measured external noise is used together with parameter-identification techniques to develop a model of helicopter external noise that is a hybrid between theory and experiment. The method is used to evaluate the main rotor harmonic noise of a Bell 206B3 helicopter operating at different altitudes. The variation with altitude of blade-vortex interaction noise, known to be a strong function of the helicopter's advance ratio, depends upon which definition of airspeed is used. If normal flight procedures are followed and indicated airspeed is held constant, the true airspeed of the helicopter increases with altitude. This causes an increase in advance ratio and a decrease in the speed of sound, which results in large changes to blade-vortex interaction noise levels. Additionally, thickness noise on this helicopter becomes more intense with increasing altitude. The noise magnitude and directivity variations differ when flight conditions are defined by true airspeed. These results suggest that existing empirical helicopter rotor noise-source models may give incorrect noise estimates when they are used at conditions where data were not measured and may need to be corrected for mission land-use planning purposes. [ABSTRACT FROM AUTHOR]

The aim of this work is to study the influence of the blade shape and curvature changing of characteristics of the helicopter's rotational noise. To this end, we consider 4-digits NASA profiles whose rotational noise is compared with one of the parabolic blade. Calculations results show that 4-digits blade generated rotational noise of higher level and its distribution on the blade's shape are smooth in comparison with noise distribution of parabolic blade. However, the parabolic blade has more expressive maximum of noise level than the 4-digits blade. It can be explained by the difference of low-frequency spectrum generated by parabolic blade. We determine that the maximum of the rotational noise L (with respect to 2 · 10-5 Pa) was at the distance of 0,7-0,75 blade radius. This agrees with Gutin's model assumptions. As the distance from the blade increases, the sound wave for both parabolic and 4-digital blades turns into the plane wave. Nevertheless, the pressure level of the 4-digits blade is higher 6 Db than the one for parabolic blade. The calculation data can be used for modeling of the cross-section of the helicopter's blade shape to reduce the rotational noise. [ABSTRACT FROM AUTHOR]

semiempirical method for calculating the far acoustic field of the lift rotor of a helicopter operating in the regime of oblique flow over it is described. The basic parametric relations for the acoustic radiation power of rotor noise components have been obtained on the basis of the Lamb idea that vortex-free motion arises under the action of a periodic force on an infinitely small volume of the medium. All sources of lift rotor noise are subdivided into two groups pertaining, respectively, to the inductive and profile parts of the total power supplied to the rotor. A comparison has been made between the results of calculation of the harmonic components of lift rotor noise made on the basis of the power method and the experimental data for the Mi-28 helicopter. [ABSTRACT FROM AUTHOR]

Abstract: The number of helicopter operations has rapidly increased during the last 20years in Japan. Helicopter noise sounds different from other aircraft; the waveform of the sound pressure is impulsive and the signal duration is relatively long. The Environmental Agency of Japan implemented new guidelines for evaluating noise exposure around small airports, including heliports, in 1990. This study was executed in connection with the development of provisional guidelines. Psychoacoustic experiments were carried out to identify an evaluation index for helicopter noise. In order to examine the effect of duration independently, we not only used original sound recordings, but also synthesized sound signals. The durations of these sound signals were time compressed or expanded without degrading the quality of the original sound recording. The test results show that the effect of duration is significant, and that the A-weighted sound exposure level is a better index than the maximum A-weighted sound pressure level for the evaluation of helicopter noise. [Copyright &y& Elsevier]

Abstract: The connection between vortex particle methods and aeroacoustics is considered within the framework of Lighthill’s acoustic analogy which allows to decouple the flow from noise propagation. For the flow, techniques such as tree-algorithms and the particle-mesh method are brought together with the aim to achieve the best possible performance in view of analyzing complex problems. The flow results are then input to the acoustic wave equation which is solved in integral form. It will involve monopole, dipole and quadrupole terms which can be successively integrated. The significance of such an approach is first demonstrated in two problems, both related to vortex–solid interactions. The first is a generic one and considers the interaction of a vortex filament interacting with a sphere while the second considers helicopter noise as an example of a complex engineering set-up. [Copyright &y& Elsevier]

The paper presents a method to increase the computational accuracy by preserving additional constraints on the numerical solution. The new technique can noticeably increase the precision of a standard finite-volume flow solver by a modification to the flux computation procedure without changing its essential features. The efficiency of this method is demonstrated by application to the prediction of sound from high-speed helicopter blades. Several open domain boundary conditions for this application are also developed and compared for a model problem of a two-dimensional transonic aerofoil in an unsteady free stream. [ABSTRACT FROM AUTHOR]

Aircraft overflight noise from helicopter tours is frequently encountered in such national parks as Grand Canyon, Hawaii Volcanoes, Haleakala, and Bryce Canyon. Noise is an environmental stressor and is associated with a variety of physiological and psychological effects, some of which are long-lasting. Psychologically, attributing a stressor to a nonhostile origin (e.g., a helicopter rescue mission) could mitigate stress effects. In this study, 200 undergraduates rated National Park scenes while exposed to either natural sounds (birds, brooks, wind), helicopter noise attributed to tourist overflights, helicopter noise attributed to backcountry maintenance operations, or helicopter noise attributed to the rescue of a back country hiker. Regardless of the source, 60 decibel (dB(A)) helicopter noise resulted in lower ratings of scenic beauty, solitude, tranquility, freedom, naturalness, and preference, and higher ratings of annoyance. These effects occurred across all types of scenery. Results suggest that park management-related overflight noise is just as disturbing as tourist aircraft noise, and that the impact of this noise is substantial across demographic variables and across types of vistas. [ABSTRACT FROM AUTHOR]