[american institute of aeronautics and astronautics 11th aiaa/ceas aeroacoustics conference -...

American Institute of Aeronautics and Astronautics

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Design and Testing of Low Noise Landing Gears

Werner M. Dobrzynski* and Britta Schöning† Deutsches Zentrum für Luft- und Raumfahrt, Braunschweig, 38108, Germany

Leung Choi Chow‡ Airbus UK, Filton, Bristol, BS99 7 AR, Great Britain

Chris Wood§ BAESYSTEMS Regional Aircraft, Woodford, Cheshire SK7 1QR, Great Britain

Malcolm Smith**

ISVR, Southampton, Highfield Hampshire, Great Britain

Chistelle Seror†† Messier-Dowty SA, Vélizy Villacoublay, 78140, France

In the approach phase of large commercial aircraft, airframe noise - and in particular that from landing gears - is one of the dominating 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 de-sign” 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 dedicated to identify and thus avoid the im-pingement of high speed flow onto critical gear structure elements. The evaluation of CFD results with respect to the effects on aerodynamic noise were performed on the basis of re-lated 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). Relative to the conventional reference gears a reduction of broadband landing gear noise in the order of -5 to -6 dB (i.e. on source level) was achieved.

I. Introduction ow noise levels of modern high bypass ratio aero-engines caused airframe noise to be one of the dominating aircraft noise components during approach and landing. Therefore efforts are needed towards a significant re-

duction of airframe noise to cope with the challenging aircraft noise reduction visions of minus 10 dB for the year 2020. For large commercial aircraft landing gears represent the dominating sources of airframe noise. Since add-on noise reduction devices have shown a limited potential in the order of -3 dB when attached to conventional landing gears, a more drastic approach is needed in the design of future low noise landing gears.

* Research Engineer, Institute of Aerodynamics and Flow Technology, Lilienthalplatz 7, 38108 Braunschweig, Germany. † Mathematician, Institute of Aerodynamics and Flow Technology, Lilienthalplatz 7, 38108 Braunschweig, Ger-many. ‡ Research Engineer, Aerodynamics Department, Building 09B, Filton, Bristol, England BS99 7AR, Great Britain. § Design Engineer, Technology Group, Woodford Aerodrome, Chester Road, Woodford, Stockport, Cheshire SK7 1QR, Great Britain. ** Research Engineer, University of Southampton, Highfield Hamphire, Great Britain. †† Research Engineer, R&T Department, Zone Aéronautique Louis Breguet, 78140 Vélizy-Villacoublay, France.

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11th AIAA/CEAS Aeroacoustics Conference (26th AIAA Aeroacoustics Conference)23 - 25 May 2005, Monterey, California

AIAA 2005-3008

Copyright © 2005 by DLR. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.


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Within a European co-financed research project entitled “Significantly Lower Community Exposure to Aircraft Noise” (SILENCER) therefore a study in advanced low noise landing gear design was performed with partners from European aircraft industries, research establishments and academia. The objective of this study was to develop op-erational landing gears for future aircraft which take into account aeroacoustic aspects early in the design stage. Ac-cordingly the design was based on the following constraints in the order of priority:

• Functionality and safety • Low noise • Weight • Maintainability.

II. Overall Design Philosophy One of the main reasons for today’s landing gear noise problems is the ever increasing length of gears. This de-

velopment is driven by the continuous increase in high bypass ratio engine/nacelle diameter, while maintaining en-gine to ground clearances with a conventional under the wing engine installation.

Since a change in aircraft configuration was considered beyond the scope of this project it was decided to retain the under wing mounted main and fuselage mounted nose landing gear installations. For the same arguments alterna-tive installations utilizing much shorter main landing gears retracting into modified fuselage belly fairings were dis-missed. After thorough discussions amongst the project partners on the overall design guidelines for the develop-ment of advanced low noise landings gears the following constraints were defined prior to the detailed design proc-ess:

• Gears of same size as A340 gears, • Main landing gears to connect to the wing structure at 3 points, • Main landing gears to be stowed inwards in the wing/under belly fairing and have 4 wheels per gear, • Nose landing gear to be stowed forwards into the fuselage and have 2 wheels. As a consequence the advanced gear overall architecture was fixed to a conventional design and thus does not

account for future advanced aircraft configurations. While this approach can be considered a severe limitation in the objective to develop optimum low noise gears it enabled a straight forward assessment of the achieved noise benefit through a direct comparison of noise data as acquired in the earlier EU co-financed project RAIN (Reduction of Air-frame and Installation Noise) for the baseline A340 gears1.

III. Approach for Low Noise Design The strategy for the development of advanced low noise landing gears was based on the knowledge gained

throughout extensive noise testing of A340 gears in terms of noise contributions from different gear components and the respective noise reduction potential through add-on fairings. This process was facilitated by the use of a semi-empirical noise model derived from previous tests on an A320 gear2. The primary focus therefore was directed to-wards the modification of the noisiest components.

CFD calculations were performed to assess the potential benefit of design changes. Flow field calculations were dedicated to identify and thus avoid local flow separations and the impingement of high speed flow onto critical gear structure elements. The evaluation of CFD results with respect to the expected effects on aerodynamic noise was performed on the basis of common understanding of flow noise generation and related experimental experience.

The design process therefore was performed according to the following iterative working method: 1. Initial redesign of reference A340 gears based on available noise test data and engineering judgment.

This work aimed at the removal of structural details and the redistribution of hydraulic and electrical services from and a smoothing of gear components which are exposed to the flow.

2. CFD calculations and assessment of flow field data with respect to the expected effects on noise relative to the reference gear.

3. Development of improved component design based on CFD data and engineering judgment. 4. Repetition of steps 2. and 3., until all critical areas of high speed turbulent flow interaction with gear

components were eliminated (in practice only up to 3 CFD runs could be performed for each gear due to budgetary limitations).

In the following, the finally achieved low noise designs are presented and compared to the reference gear designs along with the corresponding underlying rationale for selected configuration changes. After that the CFD tools will be described and some intermediate results presented to illustrate the fine tuning of the design process.


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A. Low Noise Component Design 1. Nose landing gear (NLG)

From noise testing and modeling of the A340 reference NLG (Fig. 1) the upper leg area, the steering system and the tow-bar were identified as the most important sources of aerodynamic noise. The low noise NLG design (Fig. 2) therefore accounts for these findings through the following major design modi-fications:

• Installation of a deployable ramp type spoiler to protect the upper gear leg area from high speed inflow.

• Inverted steering mechanism (i.e. the complex steering structure now is located in the bay).

• Tow-bar rotated to the rear (positioned in the low speed wake of the leg).

The spoiler was designed to alternatively operate as a bay door or as a spoiler. Though this complex dual function structure results in some additional weight compared to the reference bay doors this solution was considered, since no other design options could be identified to eliminate or protect the complex drag-stay folding mechanism from the inflow. The inverted steering system requires a 3-tube leg design, which also adds extra weight and complexity. Increased maintenance effort is expected from the rear tow-bar installation.

In addition to the above described major design changes the following de-tail modifications were included:

• The torque link is installed forward/upstream of the leg and is pro-tected by a laterally extended streamline fairing to avoid link-wake/leg interaction noise.

• All lights are removed from the gear (to be flush installed in the fuse-lage instead).

• Both inner and outer hub-caps are installed and rubber in-fills are ap-plied between wheel rim and tire to smooth the outer wheel/tire con-tour.

2. Main landing gear (MLG)

Fig. 3 shows the A340 reference MLG design. The corresponding noise test and modeling results indicated high noise levels to originate from the flow around the leg/door and side-stay arrangement, the articulation link and the brake/axle systems. Accordingly the following drastic design changes were considered in the low noise MLG design (Fig. 4):

• Telescopic side-stay to replace (i) the original folding side-stay (in-cluding the down lock mechanism), (ii) the retraction actuator and (iii) to realize a much smaller and aerodynamically shaped leg door design and avoid the necessity for an additional hinged door.

• The articulation link is eliminated, since the bogie is aligned with the inflow direction, and the torque links are installed forward/upstream of the main leg and are protected by a laterally extended streamline fairing to avoid link-wake/leg interaction noise.

• Fairings are attached to protect the forward parts of all brakes from the inflow.

Compared to the reference design, significant extra weight is caused by the new telescopic side-stay. Nevertheless this design feature was considered since it allows for an elegant design of the leg/door arrangement. The leading edge of the leg door is hinged so the leg door serves as a conventional wing-bay closure door when the gear is stowed and is articulated to form an aerodynamically shaped fairing once the gear is deployed. The inboard portion of the gear between the drag stay and the main fitting is also covered to complete the aerodynamic fairing of the whole upper leg area when the gear is deployed. The junction between leg door fairing and side-stay is also treated by a “fillet type” aerodynamic fairing.

Figure 2. Advanced low noise nose landing gear (NLG)

Figure 1. Reference A340 nose landing gear (NLG)

Figure 3. Reference A340 main landing gear (MLG)


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In addition to the above described major design changes the following detail modifications were incorporated:

• The bogie beam front end is covered by a streamline fairing.

• Outer hub-caps (perforated for brake cooling air flow) are installed and rubber in-fills are applied be-tween wheel rim and tire to smooth the outer wheel/tire contour.

After a thorough discussion on the potential benefits and drawbacks related to the elimination of conventional brake rods, the latter finally were retained. Alternative design solu-tions would lead to a massive increase in weight.

B. CFD Tools 3. CAD modeling and preparation

Original CAD models of the gears’ structures contained up to about 10000 panels. In order to enable the generation of surface grids the number of panels had to be reduced to about 1300. Very small and sharp angular panels were adapted to avoid extreme small cells to be produced in the grid generation process. Short curves and lines in the sur-face contours were concatenated to avoid agglomerations of points in the grid. The upper part of the gear was cut by a solid plane, which corresponds to the supporting wall in the wind tunnel set-up for later noise testing of landing gears in the DNW-LLF. 4. Grid generation

The commercial software package Centaur (by CentaurSoft) was applied for hybrid grid generation (tetrahedral, hexahedral, prismatic, pyramid elements). The system consists of two parts: In the first step an interactive program is used to read CAD data in IGES format to perform some CAD cleaning if necessary and to define the boundary conditions. In a second step the complete grid is computed automatically. For more details see Ref. 3.

In the hybrid Navier-Stokes grids a number of 20 prism lay-ers is assigned to simulate the boundary layer. The first spacing is 0.1. For the NLG a total of 270 000 and for the MLG a total of 390 000 surface grid points was used. The corresponding numbers of volume grid points was about 4 to 6108 ⋅ , respec-tively. An example of the surface grid for the final advanced low noise MLG configuration is shown in Fig 5. 5. CFD calculation

For CFD calculations DLR’s Tau code was used, which is a finite-volume Euler / Navier-Stokes solver working with hy-brid, unstructured or structured grids. The code is composed of three independent modules: a pre-processing module, the solver and a grid adaptation module.

Preprocessing module: The governing equations are solved on the dual grid of control volumes which has to be determined from the initial grid. The code employs an edge-based data structure, which makes the flow solver independent from the types of the initial grid cells. Thus different kinds of cells must only be considered in the pre-processing, where all the metric data are provided. For multigrid computa-tions coarse grids are constructed by agglomerating the control volumes of the secondary grid in order to create a new grid of coarser control volumes. The coarse grid control volumes can be merged again in order to achieve an even coarser grid. The pre-processing module also divides the initial grid in several domains so that the flow solver can compute on several processors in parallel using MPI (message passing interface) library.

Flow solver: The flow solver is a three-dimensional parallel hybrid multigrid code. It is a finite volume scheme for solving the Reynolds-averaged Navier-Stokes equations. The flow variables are stored in the vertices of the ini-tial grid. The temporal gradients are discretized using a multistep Runge-Kutta scheme. For accelerating the conver-gence to steady state a local time-stepping concept is employed. The calculation of the inviscid fluxes is performed using an AUSM or a Roe type 2nd-order upwind scheme. Alternatively, a central method with either scalar or ma-trix dissipation is employed. The viscous fluxes are discretized using central differences. The turbulence model im-

Figure 5. Example of MLG surface grid

Figure 4. Advanced low noise main landing gear (MLG)


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plemented in the solver is the one-equation transport model according to Spalart and Allmaras (SA). The model uses only local quantities for calculating turbulent transport, which makes it suitable for unstructured methods. For time-accurate solutions a global as well as a dual-time-stepping scheme is implemented.

Grid adaptation module: The primary grid can be adapted to the flow solution by cell division if a better resolution of the flow field is required in certain regions of the computational domain.

Full multigrid CFD calculations were performed on Hitachi SR 8000 (56 processors) and NEC SX5 (4 processors). For a free stream Mach number of 0.2 the Reynolds number, when based on wheel diameter, is about 1.25 million for both the NLG and the MLG. The reference temperature was defined as T = 300 K.

According to the scope of the project only the steady mean flow field was calculated although some unsteadiness can be expected in the flow through the gear structure. However, in most areas of interest the flow field was found to be almost steady, since the aerodynamic force coefficients converged. When force coefficients have reached a steady condition, this can be taken as indication that the flow is also steady (allowing for some local unsteadiness).

A typical example of the calculated surface pressure distri-bution is depicted in Fig. 6.

C. Assessment of CFD Results and Selection of Final Designs

The computed flow fields allow for a quali-tative comparison of the effects of different fairing designs on the mean flow distribution and also enabled the noise model to be updated using local flow data. Based on this information the corresponding benefits of certain fairing shapes or modifications thereof can be deter-mined. However, due to the limited number of grid points a quantitative data evaluation is not feasible. Therefore no direct quantitative com-parison between CFD results and experimental data is possible. The noise model indicated that reductions of -6.5 dB for the NLG and -5.7 dB for the MLG would be achieved for the opti-mized designs.

For the assessment of flow field characteris-tics from CFD calculations for the advanced nose and main landing gears the respective re-sults were evaluated in numerous cut planes for either constant vertical y- or horizontal z-coordinates. Figs. 7 and 8 depict the positions of selected cut planes for the assessment of flow characteristics for the preliminary low noise gear designs.

An example of calculated flow characteris-tics is presented in Fig. 9 for a horizontal cut through the NLG axle/torque link area. The vectors of the velocity, as shown in this figure, are projections in the plane. The “red” colour describes the free stream Mach number of M = 0.2. “Pink” colours are velocities corre-sponding to M > 0.2. Fig. 9a displays the flow characteristics for the “preliminary” design with the torque link in its

Figure 6. Surface pressure (cp) distribution for the final NLG design (flow from left to right)

FlowFlow

Figure 7. Cut planes for NLG CFD data evaluation

FlowFlow

Figure 8. Cut planes for MLG CFD data evaluation


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original rearward position and shows that the leg wake interacts with the downstream located link (encircled area). Since this is considered detri-mental in terms of noise the torque link was moved in a forward/upstream position and protected by a laterally extended streamline fairing to protect both the link and the downstream leg from the flow. The correspondingly calculated flow field for this “final design” is depicted in Fig. 9b and shows no further interaction phenomena.

A similar example is provided in Fig. 10 for a horizontal cut through the MLG bogie area. Fig. 10a shows a strong flow interaction with the main fitting pivot for the preliminary MLG design. This effect was elimi-nated by extending the torque link fairing downwards as can be seen for the final design and is displayed in Fig. 10b.

IV. Full Scale Advanced Landing Gear Mock-up Noise Test

D. Wind Tunnel Test Set-up The DNW-LLF can be operated in a free-jet

configuration with a nozzle cross section of 6 m by 8 m. This is still sufficiently large to install a full-scale A340 Airbus main landing gear in the core flow. The maximum wind speed for this tunnel con-figuration is 78 m/s (152 kt), which is close to the real landing approach speed for this aircraft. The anechoic test-hall (the lower limiting frequency is 80 Hz for broadband noise and 200 Hz for tones) allows farfield noise measurements outside the flow field at a lateral distance of about 18 m from the very landing gear (which corresponds to more than 4 times the overall gear dimension).

The existing DNW test set up is capable to ac-commodate A340 type landing gears. The identical test set-up was used as was previously applied for noise testing of A340 reference gear configurations (in the RAIN project) in order to allow for a later noise data comparison. The test set-up is shown in Fig. 11.

Figure 11. Test set-up in DNW-LLF open test section

Flow

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Figure 9. Mach number distribu-tions in a horizontal plane (z = -3300 mm) for the MLG in it’s a) preliminary design and b) final design Flow

Preliminary Design Final Design

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beam joint

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Figure 10. Mach number distributions in a horizontal plane (z = -1400 mm) for the NLG in it’s a) preliminary design and b) final design


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The side-wall arrangement is used to simulate the “in-flight” geometric/acoustic environment (reflection geome-try from the wing surface) and to reduce flow noise radiation from the support structure. One side wall of the wind tunnel nozzle is extended by means of an additional (vertical) side-wall of 7 m length. This dimension roughly cor-responds to the wing chord of an A340 aircraft at the spanwise position of the main gear. In order to simulate the actual in-flight lower wing surface boundary layer thickness the wind tunnel boundary layer is “peeled off” by means of a scoop which is installed along the side-wall‘s leading edge.

The nose landing gear was installed horizontally in the side-wall, corresponding to a vertical orientation in the aircraft. In contrast, the main landing gear-leg is (laterally) inclined by 12.8° (with respect to the lower A340 wing surface). Therefore in the test set-up a corresponding angle was realized between the gear-leg and the horizontal plane.

During landing approach the A340 aircraft typically operates at a 6.5° angle-of-attack with respect to the inflow direction. Since in the test set-up the flow direction has to be parallel to the surface of the side-wall this difference between inflow direction and aircraft axis must be accounted for. Based on the respective gear installation angles in the aircraft (and accounting for deviations of local flow- from flight-directions) the following gear-leg orientations were decided upon for the wind tunnel set-up:

• nose gear leg inclination 3.5° forward and • main gear leg inclination 2.0° backward.

E. Measurement Techniques and Data Analysis Farfield noise microphones and two planar microphone arrays (for source localization) were applied simultane-

ously. One array was used to determine source distributions for a flyover "sideline" radiation (horizontal array with 96 microphones) while the other array (vertical array with 128 microphones) could locate sources which would radi-ate directly towards the "ground" (see Fig. 11 for array positions). Details of the array design and the data analysis is documented in Ref. 4.

A total of 18 microphones were installed close to the wall of the test hall, i.e. 9 microphones each at angular increments of ∆ϕ = 10° in streamwise direc-tion (polar angle ϕ = 0° against the flow direction), in two rows at different heights above the test hall floor, corresponding to azimuthal radiation angles of ψ =0° (in the horizontal plane) and ψ = 12.8° (Fig. 12). All farfield measurement positions were equipped with 1/2“-diameter B&K 4191 type condenser micro-phones. As is indicated in Fig. 12b the uppermost up-stream wall microphone position corresponds to a po-lar radiation angle of about ϕ = 50°. In order to in-crease the angular measurement range in the forward direction, one additional inflow microphone was in-stalled in the very nozzle area (see Fig. 12b).

Since the inflow microphone support structure was suspected to generate excess flow noise, it was re-moved (by means of a remote controlled mechanism) when "conventional" farfield noise data were taken. In order to avoid excessive (flow induced) pseudo sound at the microphones, the arrays were positioned out-of-flow. In order to avoid distortions of the sound field in case of farfield noise measurements by means of the wall mounted microphones, the vertical array and its supporting structure was moved upstream behind the wind tunnel nozzle.

The analysis and reduction of farfield noise data aimed at the determination of noise level spectra and radiation directivities for different landing gear con-figurations at different flow velocities. If this basic information is at hand, the measured data may ulti-

ϕ

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Figure 12. Farfield microphone positions; a) view against flow direction, b) top view


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mately be extrapolated towards the operational conditions as specified for approach noise certification. To obtain the true source characteristics from wind tunnel out-of-flow acquired acoustic data, sound pressure levels have to be corrected for insufficient signal-to-noise ratios, for the effects of shear-layer refraction5 (including wave convec-tion), microphone directivity, atmospheric absorption6 and for the effect of convective amplification. All farfield noise data were normalized towards a constant propagation radius and will be presented in terms of 1/3-oct. band levels.

To visualize local flow conditions at selected gear components tufting tests were performed. Pictures from two different view angles were recorded by means of two video cameras.

F. Gear Configurations The baseline configurations of both low noise advanced nose and main landing gears were tested. However, aim-

ing at the acquisition of data which would help to judge on the effectiveness of different design specifics, both gears were prepared to allow for some changes.

For the nose landing gear this was: • remove spoiler (baseline inclination angle 30.6° corresponding to 20° on aircraft), • reduce spoiler inclination angle to 20.6° (corresponds to 10° on aircraft), • cover spoiler side walls with absorptive material, • remove rear bay doors, • remove tow-bar, • remove torque-link fairing, • remove tire in-fills.

For the main landing gear this was: • exchange the new telescopic side-stay against the conventional A340 side-stay, • change the bogie inclination angle to -10° (toe down), • change the bogie inclination angle to +22° (toe up), • remove torque-link and torque-link fairing, • remove brake-rods, • install serrations at the leg door fairing trailing-edges (this modification aimed at the quantification of

potential trailing edge noise contributions). With minor exceptions all these configuration changes were tested separately. It should be emphasized that the

interpretation of acoustic changes from testing of different configurations is difficult. In particular if large gear com-ponents are removed the flow field around adjacent gear structures is changed and causes corresponding changes in noise generation at these structures. As a result such tests do not provide accurate information on the potential noise contribution from the removed component by simply evaluating measured level differences. Therefore tests without wheels and tires, as performed in Ref. 7, have not been conducted in this study.

Tests were performed for 3 different wind speeds, i.e. 50, 62.5 and 78 m/s, and the different gear modifications, respectively.

G. Test Results In the following selected examples of main test results from both the advanced NLG and MLG are provided to

illustrate the effect of flow speed on farfield noise spectra and directivities and the noise reduction potential of ad-vanced gear configurations relative to both the reference A340 gears and these gears with add-on fairings also tested in the earlier RAIN project (see Ref. 1). During the tests it turned out that certain minor gear modifications (relative to the baseline design) provided some additional noise reduction or have no measurable effect on noise at all. Since not all these details can be documented in this paper data will be presented for the respectively quietest gear configu-rations only.

NLG-8 (Fig. 13) is the quietest NLG configuration tested and deviates from the baseline configuration by the following modifications:

• The tow bar is removed since it was still found to contribute significantly to the farfield noise signature (though it was installed behind the leg). The rationale behind this decision is that future NLGs will no longer need tow-bars because the aircraft will be jacked up for towing.

• The spoiler side walls are covered with a 20 mm thick layer of absorbing foam. This modification was tested to check on the potential noise generation from spoiler edge vortex/ side wall interaction and in-deed provided a measurable noise reduction.


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• The wheel/tire rim in-fills are removed since no measurable noise reduction was detected with these de-vices installed.

MLG-4 (Fig. 14) is the quietest MLG configuration tested and deviates from the baseline configuration by the following modifications:

• The bogie is inclined -10° toe down which resulted in a slight noise reduction relative to the zero degree bogie orientation.

• The leg-door fairing aft edge serrations are removed since no measurable effect on noise was detected with these devices installed.

Since it was realized that the azimuthal noise directivity of landing gear noise is omnidirectional within the azi-

muthal angular range of about °≤≤° 120 ψ (covered by the two vertically spaced rows of microphones), the data from microphones at similar streamwise (ϕ) positions were averaged.

6. Effect of Speed on Farfield Noise Spectra

In order to check on the relevant scaling laws for the effect of flow speed on broadband landing gear noise, spec-tra are presented in a non-dimensional form, i.e. levels are normalized on the basis of a velocity to the 6th power law (relative to an arbitrary reference velocity vref) versus a Strouhal number. Since no uniform source dimension is ap-plicable for aerodynamic noise from flow through a complex gear structure this Strouhal number is based on an arbi-trary reference length dimension s.

In Figs. 15 and 16 noise spectra are shown for the NLG-8 and the MLG-4 configurations, respectively, as obtained for 3 different wind speeds for a polar radiation angle of ϕ = 90° (aircraft overhead position). As is obvious from these figures the anticipated noise data reduction procedure in the average pro-vides reasonable results. Some data scat-ter is observed to originate from level humps which do not scale with flow speed. This also was observed from the RAIN A340 baseline test results but no final explanation for these effects can yet be provided. Similar results are ob-tained for all other configurations and polar radiation directions.

Figure 13. Advanced NLG-8 configuration installed in the DNW-LLF test set-up

Figure 14. Advanced MLG-4 configuration installed in the DNW-LLF test set-up

1 10 100Strouhal Number fm·s/v

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Figure 15. Non-dimensional representation of 1/3-oct band spectra for the NLG in its "low noise" configuration, i.e. without tow-bar and absorbing foam covers on the spoiler side walls (NLG-8) for a ϕ = 90° radiation direction


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Due to insufficient signal-to-noise ratio at low Strouhal numbers and for the lowest wind speed, in some of these spectra data were cancelled resulting in unphysical “infinite” level dips in Fig. 15.

The noise reduction potential to be obtained for the advanced low noise landing gear designs is depicted in Figs. 17 and 18 through a comparison with the results as obtained for the RAIN A340 reference gear configura-tions and with RAIN add-on fairings attached, respectively, for the maximum test wind speed and a ϕ = 90° radiation direction. From Fig. 17 it is observed that the advanced low noise NLG-8 de-sign is superior over the RAIN add-on

fairing configuration. Compared to the A340 reference NLG, a broadband noise reduction potential of up to -7 dB is ob-tained. For the advanced low noise MLG-4 configuration, however, when compared to the RAIN add-on fairing configuration an additional noise reduction is obtained only at high Strouhal numbers (Fig. 18). Com-pared to the A340 reference MLG, a noise reduction potential of up to -6 dB is ob-tained at high Strouhal numbers. The reduc-tion potential degrades continuously to-wards lower Strouhal numbers.

This result is not surprising since for the advanced NLG-8 configuration almost all complex gear structures could be removed, while the advanced MLG-4 configuration still suffers from interaction noise sources in the bogie/axle structure, which limits the noise reduction potential in the mid to low frequency range.

7. Farfield Noise Level Directivity Noise level directivities are presented for three different Strouhal number ranges. For that reason sound energies are respectively integrated in the Strouhal number ranges: 52 ≤< St ,

205 ≤< St and 6320 ≤< St . Comparisons of corresponding noise

level directivities are presented in Figs. 19 and 20 for the advanced low noise landing gear designs with those data as obtained for the RAIN A340 reference configuration and with RAIN add-on fairings attached, for the maximum test wind speed. For the advanced NLG-8 configuration (Fig. 19) an almost


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Figure 16. Non-dimensional representation of 1/3-oct band spectra for the MLG in its "low noise" configuration, i.e. with the bogie at a -10° toe down inclination (MLG-4) for a ϕ = 90° radiation direc-tion


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Figure 17. Comparison of non-dimensional 1/3-oct. band spec-tra for the advanced NLG-8 configuration with the RAIN A340 reference configuration and with RAIN add-on fairings for the maximum test wind speed and a ϕ = 90° radiation direction


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Reference MLGRAIN add-on fairingsAdvanced Low Noise MLG


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70

80

L m -

60·lo

g(v/

v ref)

(dB

) ϕ = 90°



Figure 18. Comparison of non-dimensional 1/3-oct. band spec-tra for the advanced MLG-4 configuration with the RAIN A340 reference configuration and with RAIN add-on fairings for the maximum test wind speed and a ϕ = 90° radiation direction


11

constant noise reduction potential in the order of -6 dB is achieved for the test range of polar radiation angles and for different Strouhal numbers, respectively. It is interesting to note that the general shape of the different NLG noise directivities is almost the same for all configurations. A different result is obtained for the MLG configurations (Fig. 20). In contrast to both A320 (tested previously8 in 1995 but results not included in Fig. 20) and A340 refer-ence MLG noise directivities, which exhibit level maxima at high Strouhal numbers both in forward and rear arc radiation directions, the advanced MLG-4 noise directivities do not show these marked noise peaks. The correspond-ing noise directivity is almost perfectly omnidirectional. The highest noise reduction therefore is achieved for both forward and rear arc radiation directions. This potential is increasing continuously towards higher Strouhal numbers.

8. Effects on Noise of Selected Configuration Changes and Measured Noise Source Distributions

One example of a comparison between noise spectra as obtained for the advanced low noise NLG-4 configura-tion and for a configuration with reduced spoiler deflection angle is provided in Fig. 21 in terms of 1/3-oct. band level differences for all test speeds and different radiation angles, respectively. Due to the reduced spoiler inclination a 1 to 2 dB level increase is observed in different frequency ranges which corresponds to an average increase in overall A-weighted noise levels in the order of 1.5 dB. This result shows that the original designed deflection angle was properly chosen to avoid flow impingement on the rear bay doors and the upper leg area.

This is also obvious from the corresponding source localization results which are provided in the following fig-ures in terms of relative 1/3-oct band levels (see dB color contours in figures). In Fig. 22 the noise source distribu-tion as obtained with the horizontally orientated array for the advanced NLG-8 configuration is depicted, corre-sponding to a “side view” for an aircraft flyover. In this figure the inserted schematic contours of the gear are dis-

50 70 90 110 130Polar Radiation Angle ϕ (°)

70

80

L Ban

d 6

0lo

g(v/

v ref)

(dB

2 < St ≤ 5


70

80

L Ban

d 6

0lo

g(v/

v ref)

(dB

5 < St ≤ 20


70

80

L Ban

d 6

0lo

g(v/

v ref)

(dB

20 < St ≤ 63


70

80

L Ban

d 6

0lo

g(v/

v ref)

(dB

2 < St ≤ 5


70

80

L Ban

d 6

0lo

g(v/

v ref)

(dB

5 < St ≤ 20


70

80

L Ban

d 6

0lo

g(v/

v ref)

(dB

20 < St ≤ 63

L m, b

and

–60

log

(v/v

ref)



70

80

L Ban

d 6

0lo

g(v/

v ref)

(dB

2 < St ≤ 5


70

80

L Ban

d 6

0lo

g(v/

v ref)

(dB

5 < St ≤ 20


70

80

L Ban

d 6

0lo

g(v/

v ref)

(dB

20 < St ≤ 63


70

80

L Ban

d 6

0lo

g(v/

v ref)

(dB

2 < St ≤ 5


70

80

L Ban

d 6

0lo

g(v/

v ref)

(dB

5 < St ≤ 20


70

80

L Ban

d 6

0lo

g(v/

v ref)

(dB

20 < St ≤ 63

L m, b

and

–60

log

(v/v

ref)



Figure 19. Comparison of polar noise directivities of normalized 1/3-oct. band levels in different bands of Strouhal number for the advanced NLG-8 configuration with the RAIN A340 baseline con-figuration and with RAIN add-on fairings for the maximum test wind speed and different radiation directions, respectively


70

80

90

L Ban

d - 6

0·lo

g(v/

v ref

) (d

B

2 < St ≤ 5


70

80

90

L Ban

d - 6

0·lo

g(v/

v ref

) (d

B

5 < St ≤ 20


70

80

90L B

and -

60·

log(

v/v r

ef)

(dB

20 < St ≤ 63


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80

90

L Ban

d - 6

0·lo

g(v/

v ref

) (d

B

2 < St ≤ 5


70

80

90

L Ban

d - 6

0·lo

g(v/

v ref

) (d

B

5 < St ≤ 20


70

80

90L B

and -

60·

log(

v/v r

ef)

(dB

20 < St ≤ 63L m

, ban

d–

60 lo

g (v

/vre

f)



70

80

90

L Ban

d - 6

0·lo

g(v/

v ref

) (d

B

2 < St ≤ 5


70

80

90

L Ban

d - 6

0·lo

g(v/

v ref

) (d

B

5 < St ≤ 20


70

80

90L B

and -

60·

log(

v/v r

ef)

(dB

20 < St ≤ 63


70

80

90

L Ban

d - 6

0·lo

g(v/

v ref

) (d

B

2 < St ≤ 5


70

80

90

L Ban

d - 6

0·lo

g(v/

v ref

) (d

B

5 < St ≤ 20


70

80

90L B

and -

60·

log(

v/v r

ef)

(dB

20 < St ≤ 63L m

, ban

d–

60 lo

g (v

/vre

f)



Figure 20. Comparison of polar noise directivities of normalized 1/3-oct. band levels in different bands of Strouhal number for the advanced MLG-4 configuration with the RAIN A340 baseline con-figuration and with RAIN add-on fairings for the maximum test wind speed and different radiation directions, respectively


12

torted because the x- and y-scales of the diagram are different. For the interpretation of these plots it must be noted that the position of the support-ing wall surface is located at about y = - 4 m. Please also note that due to the applied auto-scaling procedure each individual plot may be scaled differently. Fig. 22 shows the main source area around the torque link fairing main leg junc-tion, while no noise contribution is detected to originate from both the spoiler and the bay doors. At high frequencies a second (slightly weaker) source appears “behind“ the supporting wall, representing the mirror source due to sound re-flection at the wall surface.

In Fig. 23 the noise source distributions as ob-tained with the vertically orientated array for the advanced baseline MLG configuration are pre-sented, corresponding to a “ground view” in a flyover situation. According to this plot the major noise sources are located in the bogie area. No individual component contribution can be sepa-rated except for high frequencies, where hot spots appear at the axles and close to the main leg bo-gie pivot. It is interesting to note that in the most important mid frequency range the rear bogie area represents the major noise source.

In Fig. 24 the noise source distribution for the identical MLG configuration is shown in a “side

view” for an aircraft flyover (ob-tained with the horizontally orien-tated array). In this figure again the inserted schematic contours of the gear are distorted because the x- and y-scales of the diagram are different.

According to Fig. 24 major noise sources are essentially lo-cated in the bogie area at low fre-quencies. In the mid frequency range both the junction between leg and telescopic side stay and the rear link (supporting hydraulic services) represent dominating noise sources. These results show that the upstream torque link fair-ing does not completely shield the rear located link against the im-pingement of turbulent wake flow from the leg. The wheel caps can clearly be identified as sources of high frequency noise. In contrast to the advanced NLG (where such

effects are not observed) the advanced MLG wheel caps are perforated to allow for brake cooling. Noise radiation from the caps, however, is not considered critical, since corresponding sources are not clearly visible from a “ground

ϕ ≈ 128°

-6

-3

0

3

6

-6

-3

0

3

6

ϕ ≈ 88°

-6

-3

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6

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-3

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6

Frequency (kHz) v (m/s)

1/3-

Oct

ave

Ban

d L

evel

Diff

eren

ce ∆

L (

dB)

Ove

rall

A-w

eigh

ted

SPL

Diff

eren

ce ∆

OA

SPL,

(d

B)

V = 50.0 m/s62.578.0

ϕ ≈ 128°

-6

-3

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-6

-3

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-3

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3

6

Frequency (kHz) v (m/s)

1/3-

Oct

ave

Ban

d L

evel

Diff

eren

ce ∆

L (

dB)

Ove

rall

A-w

eigh

ted

SPL

Diff

eren

ce ∆

OA

SPL,

(d

B)

V = 50.0 m/s62.578.0

Figure 21. Comparison of noise data from the nose land-ing gear in its “low noise” configuration (NLG-8) and with reduced spoiler inclination (positive values indicate a noise increase for reduced spoiler deflection)

Figure 22 Noise source distribution from a “side view” for the NLG-8 configuration and different 1/3-oct. band frequencies, respectively (wind speed 50 m/s from left to right)


13

view” (see Fig. 23). It is important to note that no significant noise contribution can be detected to originate from the leg/door fairing arrangement.

Compared to the original side stay the increase in weight from the new MLG telescopic side stay remains critical with respect to a future application, unless its beneficial effect on noise can be demonstrated. Unfortunately no direct comparison of farfield noise data with either the original or the telescopic side stay in-stalled can provide its full noise benefit, because the telescopic device allows for a streamline and thus low noise leg/door arrange-ment which could not be changed within this test campaign. Never-theless it is interesting to compare noise source plots as obtained for the baseline advanced MLG con-figuration (see Fig. 24) with those as obtained for the advanced MLG but equipped with the original side stay (Fig. 25). From this figure excess broadband noise sources can be detected at that part of the original side stay, where the complex down-lock mechanism is located.

V. Summary Efforts are needed towards a

significant reduction of all air-frame noise contributors among which landing gears represent the dominating noise sources for large commercial aircraft. Since add-on noise reduction devices have shown a limited potential in the order of -3 dB when attached to conventional landing gears, the development of future landing gears must account for aeroacous-tics requirements at design stage.

Such advanced low noise main landing gears (MLG) and nose landing gears (NLG) for future aircraft application were developed in the EU co-financed project SILENCER taking pro-duction costs, maintenance as-pects and acoustic benefits into account. A340 landing gears were taken as reference. The basic design philosophy was to remove small scale com-ponents from the current complex gear structures and streamline the remaining large components as far as possible

500 Hz 630 Hz 800 Hz500 Hz 630 Hz 800 Hz500 Hz 630 Hz 800 Hz500 Hz 630 Hz 800 Hz

Figure 23. Noise source distribution from a “ground view” for the ad-vanced baseline MLG configuration and different 1/3-oct. band frequen-cies, respectively (wind speed 50 m/s from left to right)

Figure 24. Noise source distribution from a “side view” for the advanced baseline MLG configuration and different 1/3-oct. band frequencies, re-spectively (wind speed 50 m/s from left to right)


14

and redesign the gear structure in order to avoid interaction of lo-cally high speed turbulent wake flows with gear components as far as possible. The strategy for the development of advanced low noise landing gears was based on the knowledge gained throughout extensive noise testing of A340 gears in the earlier EU co-financed project RAIN, the use of a semi-empirical noise model, and was supported by CFD calcula-tions of the 3D flow around the entire gear structure.

Full scale mock-ups of such low noise landing gears were built and noise tested in the DNW-LLF in its free-jet configuration with a nozzle cross section of 6 m by 8 m. Both farfield noise data were taken and noise source localiza-tion techniques (microphone ar-rays) were applied simultane-ously.

Farfield noise spectra for both the advanced NLG and the advanced MLG turned out to scale on a Strouhal number basis, with sound intensities increasing with wind speed to the power of 6. No major tonal noise components were observed. The MLG turns out to be about 5 dB noisier than the NLG when compared for the identical wind tunnel flow speed. In a real aircraft environment, however, under wing MLG installation effects (reduced local speed due to wing circulation) are likely to compensate some of this difference.

In terms of overall A-weighted noise levels the advanced low noise NLG is up to -7 dB quieter compared to the

A340 reference gear for forward arc radiation directions. For other radiation directions the noise reduction potential is close -6 dB. The measured attenuation spectrum for the NLG agrees well with predictions made prior to the tests using the noise model (Fig. 26a). For the advanced MLG a reduction of overall A-weighted noise levels up to about -5 dB is achieved when compared to the A340 reference gear for both forward and rear arc radiation directions. For the overhead radiation direction, however, a noise reduction potential of only -3.7 dB is obtained. The initial noise model predictions for the MLG were satisfactory, although an update of the model using information about the flow derived from the tests did improve the correlation with the test data (Fig. 26b).

For both the advanced NLG and advanced MLG a similar noise reduction potential is achieved at high Strouhal numbers (frequencies). However, the advanced MLG design provides much less noise reduction in the mid- and low Strouhal number (frequency) range when compared to the noise reduction as achieved for the advanced NLG design. From source localization measurements the advanced MLG bogie area is identified as the remaining major noise contributor.

From additional testing of respective different gear configurations and the inspection of corresponding noise source distributions from array measurements the following major observations were made regarding the effective-ness in noise reduction of selected gear design features:

• For the NLG the rear installed tow-bar still represents one important source of high frequency noise and the spoiler is an essential feature for noise reduction.

• For the MLG the telescopic side-stay produces less high frequency noise compared to that from the original side-stay. In addition the telescopic side-stay allows for a low noise design of the leg door ar-rangement. As a consequence no “measurable” noise contributions originate from the leg door fairing area. For the MLG design tested a slight toe-down bogie inclination seems to be beneficial in reducing flow interaction with the rear axle.

Figure 25. Noise source distribution from a “side view” for the advanced MLG configuration but with the original side stay installed and different 1/3-oct. band frequencies, respectively (wind speed 50 m/s from left to right)


15

VI. Conclusion In the “European Visions 2020”, and in a similar manner spelled out in the NASA QAT program, a noise reduc-

tion target of -10 dB in 20 years is defined based on 1997 aircraft technology. In order to cope with this challenging goal all aircraft noise sources need to be treated simultaneously and this includes landing gear noise during the ap-proach flight phase. Based on the results of this project one may speculate that a -10 dB landing gear noise reduction cannot be achieved for conventional aircraft with under the wing installed engines and correspondingly large landing gears. Therefore further efforts in landing gear noise reduction must be directed towards the development of fuse-lage mounted short landing gears, which of course necessitates corresponding changes of the airframe structure.

Acknowledgments The authors thank Messier-Dowty, Airbus-UK, BAE-Systems and Airbus–France for their contributions in the

gear design process, the manufacturing of major main landing gear and nose landing gear components and their sup-port during the test conduct. Brakes and tires related low noise modifications were performed by Messier-Bugatti. The acoustic data acquisition was prepared and conducted by DNW. Finally EC co-funding of this research project is also acknowledged.

References 1Dobrzynski, W., Chow, L. C., Guion, P., Shiells, D., “Research into Landing Gear Airframe Noise Reduction”, AIAA/CEAS

Meeting Paper 2002-2409, Breckenridge/CO, 2002. 2Smith, M.G., Chow, L.C., “Validation of a Prediction Model for Aerodynamic Noise from Aircraft Landing Gears”,

AIAA/CEAS Meeting Paper 2002-2581, Breckenridge/CO, 2002. 3http://www.centaursoft.com 4Sijtsma, P., Holthusen, H., “Source location by phased array measurements in closed wind tunnel test sections”, AIAA/CEAS

Meeting Paper 99-1814, 5th AIAA/CEAS Aeroacoustics Conference, Seattle/ USA, May 1999. 5Amiet, R. K., "Correction of Open Jet Wind Tunnel Measurements for Shear Layer Refraction", AIAA Meeting Paper 75-

532, Hampton, VA./USA, March 24-26, 1975. 6Bass, H. E., Sutherland, L. C., Zuckerwar, A. J., "Atmospheric Absorption of Sound: Update", J. Acoust. Soc. Am. 88(4), pp.

2019-2021, Oct. 1990. 7Guo, Y. P., Yamamoto, K. J., Stocker, R. W., “An Empirical Model for Landing Gear Noise Prediction”, AIAA/CEAS Meet-

ing Paper 2004-2888, Manchester/UK, 2004. 8Dobrzynski, W., Buchholz, H., “Full-Scale Noise Testing on Airbus Landing Gears in the German Dutch Wind Tunnel”,

AIAA/CEAS Meeting Paper 97-1597, 3rd AIAA/CEAS Aeroacoustics Conference, Atlanta/USA, May 12-14, 1997.

102

103-10

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8

10Noise reduction relative to Baseline A340 GEAR

Frequency (Hz)

∆ dB

predictedmeasured

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103-10

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∆ dB

predictedmeasureda b

Figure 26. Comparison of predicted and measured noise reduction between the baseline A340 gears and the advanced gears: a) original NLG prediction and b) updated MLG prediction using flow information from the tests

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