Towards a Lower Helicopter Noise Interference in Human Life

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2 Towards a Lower Helicopter Noise Interference in Human Life Fausto Cenedese Acoustics and Vibration Department AGUSTA, Via G. Agusta 520, Cascina Costa (VA), Italy Noise Regulation Workshop September 17th 2003, Friedrichshafen,, Germany

3 Contents Introduction Experimental Activity Prediction Methodology Description Preliminary Validation Activity Results and Comments Conclusions

4 Introduction

5 Introduction Continuously Rising Concern for Environmental Issues Noise Regulations More Restrictive Acoustic Impact is a Critical Aspect of Today s s Helicopter Design Reducing Noise Emission for Increased Community Acceptance is becoming a Major Challenge for Helicopter Manufacturers

6 Introduction Develop Noise Reduction Technology Develop and Validate Noise Prediction Methods Design and Operations Human Factors Reduced Noise Impact On Human Life Identify Quantification Units Develop Criteria

7 Introduction EPNdB Year

8 Introduction Experimental tests are: too expensive Availability of the helicopter Require time for preparation,, test, collect all data, analysis etc. Specific environment Necessity of a tool for land use prediction which must be: Reliable Accepted by the authorities

9 Introduction AGUSTA Operational Aeroacoustic Simulator Integrated Simulation Procedure coupling: AEROELASTICITY AERODYNAMICS AEROACOUSTICS Prediction of Noise Footprints Emitted by Rotors: in Wind Tunnel Testing Prediction of Noise Footprints Emitted by Helicopters: in Real Flight Operations, under assumption of Steady Operating Conditions

10 Introduction Applications: Development phase Wind Tunnel Data for Validation Activity Maturity phase Direct Comparison with Flight Test Results from a Real Flying Helicopter Acoustic Campaign

11 Introduction Flying Helicopter Acoustic Characterisation through Prediction is an Extremely Challenging Task Theoretical and Experimental Aeroacoustics Research Efforts Deeper Understanding of Helicopter Noise Generation Phenomena and Propagation Mechanisms Most of Available Computational Techniques are limited at Interpolating Experimentally Acquired Database rather than Directly Simulating Noise Emission at Sound Sources Application of these Tools is Fairly Limited at Reproducing Wind Tunnel Conditions, while the Simulation of Real Flight Operations still Poses an Extreme Challenge

12 Experimental Activity: Main Aspects A short dedicated Flight Test Program was carried out during an Acoustic Certification Test at Cameri Airbase, Northern Italy, in June 2001 To Provide a Useful Database for Validation of the AGUSTA Operational Aeroacoustic Simulator OBJECTIVES To Verify the Reliability of the Tool when applied to a real Flying Helicopter rather than to the Wind Tunnel Model

13 Experimental Layout: Microphones & GPS SECONDARY RUNWAY Y HOVER PAD X MAIN RUNWAY FLIGHT TRACK GPS GROUND STATION 200 [m] 200 [m] Microphone X [m] Y [m] Z [m] The helicopter was flown over a linear array of microphones deployed perpendicular to the flight path

14 Test Program Wide series of Steady Flight Operations with Variations of some of the Test Conditions: helicopter trajectory; flight speed; altitude. Five valid runs performed for each test condition, in order to assess a statistical repeatability of the obtained results.

15 Aircraft Tracking!DGPS for precise Helicopter Guidance during Acoustic Flight Testing Negligible navigational errors GPS UHF GPS ERROR CORRECTED POSITION Very high 3D precision RADIO MODEM PC Accuracy of ± 0.3 [m] for both horizontal and vertical displacement over distances of 150 [m]! quick-look graphic output of aircraft trajectory for real-time evaluation of test data validity.

16 Weather Conditions & Acoustic Levels METEOROLOGICAL INSTRUMENTATION 10 [m] Tower Mounted Weather Station, to measure:! relative humidity and temperature;!barometric pressure;!wind speed and direction. ACOUSTIC MEASUREMENTS ON GROUND!Array of 11 microphones mounted on tripods perpendicular to the runway.!microphones connected to 16 Bit resolution Digital Tape Recorders s and acquired signals digitized at a rate of [Hz].!Ground Based Microphones Signals simultaneously acquired by a Central Acoustic Data Acquisition System.!Digitized Data of all Channels recorded and stored by a powerful W/S.

17 Acoustic Data Processing Effective noise footprints on the ground had been reproduced by relating the acquired acoustic measurements to the aircraft trajectory data. With the helicopter flying over the linear microphone array, noise signals reaching the observers were connected to corresponding aircraft positions The helicopter was then frozen at a point in space, so that noise directivity data could be projected onto the ground, resulting in the effectively released noise footprints Measured acoustic emission during a flyover Transformed single source location for determination of effective noise footprint over a plane

18 Acoustic Data Processing (cont.) GPS trajectory data Helicopter Positions with respect to the Experimental Microphone Array and Correlated Instants in Time Extrapolation of a series of Sampling Windows centered around the predetermined instants from each acoustic signal Translation of digital acoustic time domain data into frequency domain through 4096 point FFT transformations after application of a Hanning window Re-sampling of each signal at a rate of 4096 [Hz] in order to have a useful frequency range of about 2000 [Hz], in which all the mid frequency components are included Computation of narrowband spectra, which were finally integrated to obtain Overall Sound Pressure Levels at each microphone

19 Y X Y Prediction Methodology: Logical Scheme Experimental database Simulation based on Simulation experimental based on experimental pressure BNPUTIL data? pressure data? BENP input files no no CATIA Aeroacoustic database from simulation BENPTRASF input file BENPFILTER geometry BENP PREEPNL FEPNL Experimental database BENP output files yes ROSITA Wake induced velocity PREEPNL output distribution files NUVOLA FEPNL output files BNPUTIL Noise footprint at each time instant grid GEROS Sound pressure time histories Aeroelastic-Aerodynamic and frequency contents 5 Simulation Blade motion and -5 elastic deformations; wake induced velocities Noise footprint at each time instant Aerodynamic coefficients on each blade section Sound pressure frequency contents X Cp r/r = x/c;chord x/l Certification noise indices BENP input files Pressure 10 Convergence? yes Time Aeroacoustic database from simulation Aeroacoustic database from simulation 0 Noise footprints animation during maneuver Helicopter operating conditions Pressure Freq Noise footprints animation during maneuver CAMRAD/JA GYROX no TECPLOT FILES BENPFILTER BENPTRASF

20 Results Comparison of normalised simulated (a) and experimental (b) noise e footprint radiated by the helicopter in level flight at the maximum horizontal speed Simulation Flight Direction Experiment M/R & T/R Y [m] Y [m] Reference Flight TC: V = [m/s] RPM = 100% Height over Ground = 150 [m] X [m] X [m] (a) (b)

21 Remarks Noise intensities and directivities are in good agreement with those t of the flying helicopter. Differences less than 2 [db] in sound pressure levels absolute values v are shown almost throughout the entire simulated region. Further deviations from experimental noise footprints could be explained e as follows: " the helicopter has been trimmed by matching aircraft equilibrium,, thus leading to rotors controls values rather different from the experimental ones, probably due to the real wind conditions; " interactional effects of main and tail rotors have not been considered here; " the simulations limited to rigid motion of the blades; " the engine noise not included at all in the simulations;

22 0.92 Results Comparison of normalised simulated noise footprint radiated by the t main rotor in level flight at the maximum horizontal speed, including (a) or not (b) fuselage scattering effectse Scattering Flight Direction No Scattering Scattering Effects: M/R only Y [m] Y [m] Reference Flight TC: V = [m/s] RPM = 100% Height over Ground = 150 [m] X [m] X [m] 0.87 (a) (b)

23 Future Developments of the Simulator Flight testing and simulation improvements: Flight Testing Techniques need to be refined to cover Footprint Acquisition for the whole Flight Envelope; Improve data base; from a Simulation point of view, many peculiar problems need to be assessed: " blade- vortex interaction; " advancing blade shock delocalisation; " main and tail rotor interaction; " engine noise; Data Reduction Tools have to be further validated and improved to account for: " propagation distance attenuation; " atmospheric absorption; " reflection effects on ground and solid obstacles. Optimum Operational Flight Envelopes from the noise emission point of view.

24 Future Developments Now each European country uses its own approach; Five working groups are active for the European Commission to formulate a basis for a new European policy on noise prevention and protection; Harmonized European model?