Henri Siller

Flight Test Investigation of Add-On Treatments to Reduce Aircraft Airframe Noise

A major task within the European Community funded Project SILENCE(R), was to flight-test high-lift devices low-noise modifications and landing gear noise reduction fairings. This work was part of the airframe noise reduction investigation that was initiated in 2001, with a design based on both computational and experimental work, aiming at modifications that fit the actual Airbus A340-300 test aircraft. Landing-gear fairings were designed and manufactured by both Airbus and Messier-Dowty. They are add-on elements that can be mounted on the existing gears without affecting the operation of the landing gear. The add-on treatment modifications (designed and manufactured by Airbus) on the high-lift devices consist of covering the numerous cavities in the slat-retraction system and flap-side edge space, that have been identified to be responsible for a significant part of the noise. The tests were conducted in September 2003 at Tarbes airport. To investigate the noise perceived on the ground, Airbus’ noise system of microphones was used. To help the result interpretation and detection of possible spurious effects, a large array of microphones was operated by ONERA and DLR. On board measurements (pressure and acceleration sensors, strain gages, etc.) were also implemented to assess local effect of devices. Back-to-back tests were achieved in 11 flights, by successively removing all modifications in small groups. Examples of the effect of SILENCE(R) devices is assessed for typical landing configuration and also for specific aerodynamic noise configurations, that were designed to separate the effects of landing gear and high-lift device noise reduction.

Airbus A319 Database from Dedicated Flyover Measurements to Investigate Noise Abatement Procedures

A series of flyover noise tests on the Airbus A319 performed in the framework of the German project ”Noise Optimized Approach and Departure Procedures (LAnAb)” took place at Parchim airport (Germany) in June 2004. A noise database was created that will be a support for the validation of aircraft noise prediction models dedicated to investigate noise abatement procedures. In all, 37 take-off and 82 approach conditions were simulated. Depending on the simulated flight phase, different values of engine power, airspeed, position of the high-lift devices, and also of the landing gears were tested. The aim of this paper is to show the different possibilities of using the signals recorded by a phased-array of microphones installed on the ground to analyse aircraft noise and confront some prediction models to the results.

BUZZ-SAW NOISE SPECTRA AND DIRECTIVITY FROM FLYOVER TESTS

A fly-over noise test with an Airbus A319 was conducted jointly by DLR and Lufthansa with support from Airbus and Snecma. The signals of a large array of 238 electret microphones were recorded. In a first test, the noise emission of each engine was studied by running the other engine with flight-idle power. De-dopplerised narrow-band frequency spectra averaged over a large number of microphones were used. Alternatively, the noise emission from the inlet of each engine was evaluated with both engines at take-off power with the aid of a phased microphone array focused on each moving engine inlet. The results show as expected that buzz-saw noise is mainly radiated in the forward arc. The peaks at multiples of the shaft frequency can easily be identified in the de-dopplerised narrow-band spectra. The directivity of a number of frequency components is evaluated. The characteristics of buzz-saw noise are seen to vary between individual engines and even, for the same engine, between different fly-overs. The resulting directivities also indicate that mode scattering has a large influence on the radiated buzz-saw noise field.

A Hybrid Time-Frequency Approach for the Noise Localization Analysis of Aircraft Fly-overs

A hybrid time-frequency approach based on acoustic beamforming has been successfully developed in order to determine the absolute contribution of the aircraft noise components measured during fly-overs. The method, derived from DAMAS, 1 accounts for the fact that the sources move relative to the microphones. Indeed, the motion is responsible for a frequency shift of the sidelobes and a strong modification of the point-spread functions compared to the static case. The method developed is hybrid in the sense that the beamforming algorithm is applied in the time domain while the point-spread functions are approximated in the frequency domain. When the sound sources are characterized by broadband spectra, it becomes possible to discard the frequency coupling between the sources and the sidelobes. This enables to reduce the computing time considerably. In the present publication, the method is applied to AIRBUS A340 fly-overs with the high-lift devices deployed, landing gear up, and the engines running at idle. The study is focused on the influence of some processing parameters (grid spacing, number of iterations, cut of the focusing grid, and frequency bandwidth) on the source breakdown. It turns out that the noise spectra of the aircraft components are almost independent of reasonable variations of these parameters. This indicates that the method is relatively robust. A detailed investigation of the noise sources (flaps, slats, and engines) with respect to the variations of the airspeed, the ECAM position, the engine rating, etc. is not part of this work.

Acoustic measurements of a contra-rotating open rotor in an open jet wind-tunnel

Acoustic measurements of a scaled model of a contra-rotating open rotor engine were performed in an open wind tunnel facility. The motivation for the work presented here was to analyse the possibilities for quantitative acoustic measurements in an environment that was not designed for acoustic measurements and where reverberations, shear layer refraction and turbulence distort the sound field. Extensive acoustic measurements were performed using a linear microphone array with 104 microphones on the floor of the hall. Room acoustics effects were investigated with a reference sound source and the sound field of the rotors was measured for different configurations. The results show that reflections from the ceiling and side walls interfere with the direct sound field and influence the measured directivity due to strong local variations of the sound pressure level from one microphone position to the next. General trends and differences between the configurations can be shown from spatially averaged frequency spectra.

Investigating landing gear noise using fly-over data: The case of a Boeing 747-400

Investigation of the landing gear noise of the Boeing 747-400 in flight. Comparison of different fly-over velocities in order to obtain a scaling law for the Sound pressure Level based on microphone Array measurements.

Investigation of engine tones in flight

Acoustic fly-over measurements with Boeing 747-400 aircraft have been performed in order to extend an existing noise data base for Airbus A319/20 and MD-11 aircraft with another widely used wide-body aircraft. The data were recorded with a large multi-arm spiral microphone array with 238 microphones which extended over an area of 42 by 35 m. The flight trajectories were determined using a combination of onboard flight recorder and GPS data and an array of laser distance meters on the ground. A linearised trajectory of the aircraft was determined for every fly-over in order to calculate frequency spectra for different emission angles at emission time by compensating the Doppler effect. Sound source maps were calculated using the standard beamforming algorithm and a deconvo- lution method for moving sources. Fly-over measurements were performed with different configurations in order to analyse airframe and engine noise sources separately. In this paper, the main focus is on the analysis of fly-overs with different engine speeds, but also, some results for landing gear noise are presented.

Reduction of Approach Noise of the MD-11

The noise emission of the Boeing MD-11 with CF6-80C2 engines can be reduced substantially during the landing approach. This is the result of a flight-test campaign, which was performed by Lufthansa and DLR in 2002. The landing noise is dominated by a strong tonal radiation at the blade-passing frequency. Operating the aircraft with the flaps set to 35 degrees rather than to 50 degrees reduces the tone level substantially. The tone level becomes insignificant, when the wing mounted engines are operated with a lower speed and the tail mounted engine with a higher speed while keeping the flap setting 35 degrees. Implementation of this differential thrust operation in daily routine would require a modification of the thrust schedule and its certification. Strong tones were also found for frequencies that indicate an interaction between the fan stage and the first rotor in the compressor. A further strong tone is emitted from a position close to the main landing gear of the aircraft. Its precise origin is still unclear, but it is assumed that the tone is generated by a cavity in the landing gear and can be eliminated with a small modification. The broad-band noise at frequencies above 2 kHz in the rear arc is dominated by the noise emission from the primary nozzles of the engines. Installation of a hot-stream liner should yield a considerable noise reduction, which would also reduce the take-off noise. The study demonstrates the value of flyover noise testing with phased arrays of microphones, a method that makes it possible to localize the source positions of the dominating sources.

Localisation of Sound Sources on Aircraft in Flight

This paper presents beamforming techniques for source localization on aicraft in flight with a focus on the development at DLR in Germany. Fly-over tests with phased arrays are the only way to localize and analyze the different aerodynamic and engine sources of aircraft in flight. Many of these sources cannot be simulated numerically or in wind-tunnel tests because they they are either unknown or they cannot be resolved properly in model scale. The localization of sound sources on aircraft in flight is performed using large microphone arrays. For the data analysis, the source signals at emission time are reconstructed from the Doppler-shifted microphone data using the measured flight trajectory. Standard beamforming techniques in the frequency domain cannot be applied due transitory nature of the signals, so the data is usually analyzed using a classical beamforming algorithm in the time domain. The spatial resolution and the dynamic range of the source maps can be improved by calculating a deconvolution of the sound source maps with the point spread function of the microphone array. This compensates the imaging properties of the microphone array by eliminating side lobes and aliases. While classical beamfoming yields results that are more qualitative by nature, the deconvolution results can be used to integrate the acoustic power over the different source regions in order to obtain the powers of each source. ranking of the sources. These results can be used to rank the sources, for acoustic trouble shooting, and to assess the potential of noise abatement methods.Copyright

Flow and noise modification by suction and blowing on a Rod-Airfoil Configuration

Turbulence and noise generated by a wing section embedded in the wake of a cylindrical rod are modified by applying suction or blowing near the leading edge of the airfoil. The dominant noise source for a rod-airfoil configuration is the impingement of the Karman vortices onto the airfoil leading edge. Therefore, a modification of the leading edge aerodynamics is expected to modify both the sound emission and the turbulent flow of the wake-airfoil configuration. The flow field near the leading edge of the airfoil is dominated by quasi-periodic vortices shed by the rod. Because the Reynolds number of the cylinder is in the range between 10,000 and 30,000, a significant broadband noise contribution is also found that broadens the tonal peaks around the vortex shedding frequencies. The aerodynamics of the leading edge are modified by boundary layer suction or blowing over perforated panels near the leading edge on both sides of the airfoil. When air is blown out through the leading-edge panels, the level of the main Strouhal peak in the far field could be significantly reduced while suction may even increase the peak. The aerodynamics of the airfoil are changed drastically by the manipulation: Blowing destabilizes the boundary layer and may even induce separation while suction stabilizes the boundary layer.