Leung Choi Chow

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.

Design and Testing of Low Noise Landing Gears

In the approach phase of large commercial aircraft, airframe noise – and in particular that from landing gears – is one of the dominant aircraft noise components. Within a European co-financed research project entitled “Significantly Lower Community Exposure to Aircraft Noise” (codenamed SILENCER) a study in “advanced low noise landing gear design” was performed to develop operational landing gears which take into account aeroacoustic constraints early in the design stage. Airbus aircraft typical configurations of low wing with underslung engines and A340 type gears were selected as reference. RANS flow field calculations were performed and used to identify and thus avoid the impingement of high speed flow onto critical gear structure elements. The evaluation of CFD results with respect to the effects on aerodynamic noise was performed on the basis of related experimental experience and a semi-empirical landing gear noise model. Both low noise advanced nose and main landing gears were designed and manufactured at full scale for noise testing in the 8 m by 6 m open test section of the German-Dutch Wind Tunnel (DNW-LLF).

Validation of a prediction model for aerodynamic noise from aircraft landing gear

The continued development of quiet engines is now giving rise to situations where airframe noise is comparable with engine noise at approach. The landing gear is a major contributor to airframe noise, and this paper outlines development and testing of a semi-empirical noise model for predicting the benefit of noise control fairings. The model is based on nondimensional source spectra derived from a data base of full-scale tests on A320 landing gear installed in the DNW wind tunnel and is used to predict the noise reduction potential of a variety of fairings installed on the main and noise gears from an A340 aircraft . The model is found to give good agreement with data and this gives confidence that it may be used as an engineering tool to optimise fairing design. The paper also discusses the way in which the model can provide a framework for incorporating CFD data in the design process and also number of factors which complicate the comparison of noise data from wind tunnel and flyover tests.

Experimental Assessment of Low Noise Landing Gear Component Design

Landing gear related airframe noise is one of the dominant aircraft noise components at approach, so continued research efforts to reduce landing gear noise are essential. This paper describes further development of an advanced low noise main landing gear that was previously designed and tested in the European SILENCER project. The work was carried out under the current European co-financed TIMPAN project (Technologies to IMProve Airframe Noise) using a ¼ scaled landing gear model that was tested in the German-Dutch Wind Tunnel. A variety of gear configurations were tested including a new side-stay design and various modifications to the bogie inclination, wheel spacing, bogie fairings with different flow transparency, leg-door configurations and brake fairings. The farfield noise data from the tests are compared with results from a landing gear noise prediction model, transposed to full scale flight conditions and compared with the full scale test data obtained for the original SILENCER advanced A340 style 4-wheel main landing gear. An optimal combination of tested gear modifications led to a further noise reduction of up to 8 dB(A) in terms of overall A-weighted noise levels relative to the original advanced gear configuration.

Bluff body noise control using perforated fairings

Landing gears of commercial aircraft make an important contribution to total aircraft noise in the approach configuration. Using fairings to shield components from high-speed impingement reduces noise. Furthermore, perforating these fairings has been confirmed by flight tests to enable a further reduction. A fundamental study has been performed to investigate and optimize the benefits of bleeding air through the fairing by application of perforations. Experiments have been performed with a simplified fairing–strut combination to clarify the influence of perforations on flow behavior and acoustics. The fairing self-noise is reduced significantly by breakdown of the vortex shedding process, resulting in a reduction of the associated broadband noise level. A redistribution of the velocities is achieved depending on the applied porosity. However, increasing the porosity can result in adverse noise effects due to the bled mass flow washing the strut. Self-noise of the perforations manifests itself at higher frequencies, although scaling of this phenomenon with orifice diameter opens up the possibility to shift it above the upper limit of the audible range.

Control of landing gear noise using meshes

In previous investigations of aerodynamic noise from landing gears, solid fairings have been used to shield components from the flow so as to reduce the farfield noise. Larger noise reductions have been achieved by designing advanced low noise gears, and in recent studies further reductions of advanced landing gear noise have been achieved by using flowporous fairings. In this study, the application of porous fairings to conventional landing gears is considered to investigate whether their noise reduction potential can be achieved without the potential penalties of re-designing the landing gear with noise as an additional design constraint, as was done for the advanced gears. The results show that porous fairings can provide significantly greater noise reductions than solid fairings, and that porous material wraps can provide useful attenuation of noise with minimal weight penalty.

The use of a fairing and split plate for bluff body noise control

Landing gears have been identified as major noise source contributors during the aircraft’s approach to landing phase. Techniques such as using fairings to alter the flow around the landing gear components have shown to reduce noise . This study investigates methods to further improve the fairing performance. A fundamental study has been performed to understand the influence of using a fairing on a simple cylindrical strut when a split plate is placed in the cavity between the fairing and the strut. Aerodynamic and acoustic tests are performed. Wind tunnel tests equipped with a phased microphone array as well as far field measurements have been conducted out on the fairing-cylindrical strut configuration. The results show that the splitter plate either reduces or eliminates the vortex shedding which in turn reduces noise. This is due to the dividing plate blocking the interaction between the two opposing shear layers aft of the fairing’s trailing edge and reducing their interaction with the downstream strut. Lower velocities around the shell and the strut are observed with the use of the splitter plate as well as reduction of recirculating flow within the shell strut cavity. This alteration in the flow characteristics results in a broadband noise reduction and hence reducing the self-noise generated by the fairing.

Modelling landing gear noise with installation effects

This paper describes continued development and validation of a semi-empirical model of the aerodynamic noise generated by aircraft landing gears at approach. The model has been developed with Airbus funding over a number of years1,2 , but recent studies carried out as part of the EC funded SILENCER project have led to the introduction of a number of new effects that occur on aircraft in flight. The core of the noise model is based on nondimensional source spectra derived from a database of full-scale tests on landing gears installed in the DNW wind tunnel. The new installation effects have been derived using the results of small-scale model tests3 and by analysis of flight test data4. The model is used to examine differences between the performance of noise control fairings installed on a landing gear in a wind tunnel and similar fairings installed on a real aircraft in flight.

The influence of aerodynamic interactions between components on landing gear noise

The design of aircraft landing gears requires careful optimisation for each specific design of new aircraft, to include factors such as the overall size, strength and weight of the gear, the number of wheels, the method of transferring loads to the wing, the drag and, last but not least, the noise of the gear. The aim of the study described here was to generate an experimental database to assist in the design of future low noise landing gears by investigating the influence of a number of design parameters on the noise of the gear. Firstly, factors affecting the noise of the main landing gear bogie are considered, including the number of wheels, the bogie angle, etc. Secondly, the effect of a double sidestay design on the overall noise of a main landing gear has been investigated. Finally, a number of other landing gear design parameters are considered: the effect of a small bogie fairing, an optimised routing for dressings, the effect of yaw angle in the flow. The paper focuses particularly on the effect of various aerodynamic interactions between components which in some cases were found to have a strong influence on the overall noise of the landing gear.