Review of landing gear acoustic research at Messier ...

AIAA 2016-2770 Aeroacoustics Conferences 30 May - 1 June, 2016, Lyon, France 22nd AIAA/CEAS Aeroacoustics Conference

Review of landing gear acoustic research at Messier-Bugatti-Dowty Quentin Bouvy1* Messier-Dowty Ltd, Gloucester, GL2 9, UK

Downloaded by Quentin Bouvy on June 29, 2016 | http://arc.aiaa.org | DOI: 10.2514/6.2016-2770

Thierry Rougier2, Amine Ghouali3, Antoine Boillot4, Bertrand Petot5 Messier-Bugatti-Dowty, Vélizy-Villacoublay, France, 78140

The present paper summarises the main findings of both experimental and numerical acoustic applied research on landing gears carried out by Messier-Bugatti-Dowty and partners since the first collaborative project initiated fifteen years ago. Experimental tests performed in wind-tunnel have highlighted many conclusions about landing gear noise sources and associated low noise solutions. Recent CFD studies on detailed landing gear geometries extend the experimental results and even more; allow optimization of low noise solutions identified during experimental campaigns and confirmed by simulations.

I. Introduction

T

HE noise signature of aircraft during take-off and landing phases becomes a major concern with the increase of air traffic and since the introduction of jet and fan powered aircraft, almost 50 years ago. For decades, the engines efficiency drove the aircraft noise reduction. Nowadays, the gain in fuel consumption achieved with high bypass ratio induces a lower noise signature of the engines. At landing phase, the engine noise has been reduced to the level of airframe noise so that new sources are emerging1. Landing gear noise is considered as a major contributor to the airframe noise, during the landing phase of short, medium and long range aircraft. For more than fifteen years, Messier-Bugatti-Dowty (MBD), as system integrator has been investigating innovative solutions to mitigate the landing gear noise by initiating dedicated applied acoustic research programs. This paper describes the main achievements of MBD but also collaborative projects MBD was part of and that aimed to evaluate and decrease the landing gear noise of medium and long range aircraft. Both experimental and numerical research are described and different types of landing gears are considered.

II. Research objectives The regulation, the airliners but also the airports are pushing the aerospace industry to lean forward quieter aircraft. From Chapter 2 of the regulation, instituted in 1972 for turbojet airplanes, to the Chapter 14 that will be applied for aircraft entering into service after December 2017, a reduction1 of more than 10 dB in terms of Effective Perceived Noise Levels (EPNL) is imposed, for each of the three certification points. Even so, the new generation of long range aircraft already shows sufficient and impressive margin with respect to the future requirements. Despite this, the introduction of local restrictions in major European airports draws the attention of airline operators. Indeed, most of those large airports are used as airport hubs for flag carrier airlines and are mindful of the local community noise arising from the growing airport activity. To face the requirements and tackle the acoustic challenge, reducing noise at dominant sources remains one of the main objectives. As mentioned, during approach phase, the landing gear noise shows an important contribution 1

R&T Project Leader. R&T Project Leader. 3 R&T Project Leader. 4 R & T Program Manager. 5 Landing Gears and Integration R&T Programs Director. 2

*

Corresponding author: [email protected]

1 American Institute of Aeronautics and Astronautics

Copyright © 2016 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.


to the airframe noise signature. The important findings, conclusions and noise mechanism understandings acquired for several years will build a strong basis for the future integration of acoustic requirements into a design optimization process, as presented in Figure 1. Today MBD complemented years of participation in collaborative projects in which experimental aspect was mainly treated with detailed numerical studies, helping the understanding of the dominant mechanisms of the landing gear noise. The numerical simulations extend the understanding at very detailed scales, about integration issues but also allow potential design optimisation taking aero-acoustic parameters into account.

Figure 1. Main landing gear noise findings will help to build future design optimization processes that consider acoustic requirements.

III. Collaborative Projects A. RAIN (Reduction of Airframe and Installation Noise, 1998-2002) First of all, the RAIN project was covering both experimental (DNW wind tunnel and flight tests) and numerical work on 2-wheel nose, 4 and 6-wheel main A340 landing gears2. Thanks to the notable amount of experimental results, several comparisons and noise investigation at aircraft level were available. One of the main objective of RAIN was to identify the most dominant sources in order to rank them according to their contribution to the overall airframe noise. A topic that is still an important area of research today. The dominating noise sources identified on the main gear was the lower part of the gear including the brake system. However the precise location of the sources is sensitive to the flow direction and thus to the bogie angle. It was shown that the number of wheels (4 or 6) for main landing gear is not a key parameter, especially at higher frequencies3. Therefore, the 4-wheel noise level signature is similar to the 6-wheel one. However, a correlation exists between the size of the gear and the noise levels. The noise levels associated to the 2-wheel main landing gear noise (A320) is lower than the 4 and 6-wheel main gear of the A340. This correlation is less evident on frequencies. In addition to the source identification study, fairings were used to protect the noisiest part of the gears. A combined use of fairings led to an overall decrease of 3 dB(A) for nose gear and around 2.5 dB(A) for the main gear4 in terms of A-weighted noise levels - noted dB(A). On the nose landing gear, the most effective fairings were the axle, the towing bar and the steering system fairings while extra reduction were provided by wheels and upper leg fairings. For the main landing gears, the brake fairings were identified as efficient on the 4 and 6-wheel configurations. However, filling the gap between the leg and the door leads to an increase of the noise on the 4wheel configuration. An additional 2 dB(A) nose reduction is achieved when aligning the bogie with the flow direction. Wheel caps also bring a decrease of the lower frequency noise. During the tests, a tone was found to occur with the low-noise configuration. The importance of local optimization of the flow was thus emphasized. This is especially true as some efficient fairings where not considered as realistic and operational devices4. In terms of EPNL, the total aircraft noise reduction is modelled and evaluated at 1.1 EPNdB5. On the basis of flight tests, it was possible to conclude that landing gears showed the main contribution to the noise during approach phase. First, landing gear system is responsible for tonal noise which is induced by open cavity and must be avoided in the aircraft design. In terms of broadband noise, the forward arc angles are affected by the main landing gear while the nose landing gear mainly impacts the rear arcs3. 2 American Institute of Aeronautics and Astronautics


B. SILENCE(R) (Significantly Lower Community Exposure to Aircraft Noise, 2001-2006) Landing gear noise reduction through operational solutions was an objective of the SILENCE(R) project for which wind tunnel (DNW) large scale test of CFD optimized fairings and novel landing gear designs were carried out as well as flight tests on the same A340 used for RAIN airframe noise tests6. A 1.8 EPNdB reduction at landing gear level was transposed to a 0.4 EPNdB at the level of the aircraft with nose and main landing gear fairings. An intermediate low-noise configuration, associated to a 0.7 EPNdB landing gear noise reduction, is obtained when the cavities and apertures are filled. The main novelty of the project was the use of a ramp-type spoiler to shield the upper part of the nose landing gear. Also, the steering system was moved forward the leg. Other low noise solutions (brake fairings, telescopic side-stay, and bogie alignment with flow direction) together allowed a new decrease of about 3 dB on a large frequency range, in comparison to RAIN results7. The decrease of upstream components wake impingement is emphasized as an efficient way the decrease the noise. The bogie angle and its direction to the flow was also an investigated parameter. Indeed, the modification of the bogie angle could be a potential area of noise decrease without any add-on fairing. The 4-wheel configuration showed an increase of almost 5dB reached with toe up conditions at 35° angle8, 9. This is the same trend as in the RAIN project, where a noise reduction of more than 1.5dB was achieved by decreasing the bogie angle from 33° toe up to a 0°, but on a 6-wheel configuration. On the toe up configuration, the dominant noise sources in the bogie area are located at all brakes/axles positions. When the bogie is aligned with the flow, only the front axle area is identified as a strong source. The proposed explanation is that the alignment of the bogie with the flow direction allows a flow deflection that shields the bogie beam components and reduce their noise emissions4. Based on the results of SILENCE(R), it is considered that the targeted 10dB landing gear noise reduction in “European Visions 2020” would likely1 to be achieved with breakthrough technologies rather than modification on conventional aircraft with wing installed engines and large landing gears 8. C. TIMPAN (Technology to IMProve Airframe Noise, 2006-2009) As the full name of the project suggests, the main objective of TIMPAN was to improve the low noise gear design. This was partly done through flow control technologies and local optimizations with the aim to develop operational low noise solutions without weight penalties whenever possible. Several configuration of a ¼ scaled 4wheel landing gear model were tested in wind tunnel facilities and compared to the results of the flight test results of SILENCE(R)12. Advanced passive and active flow control solutions as meshes 10 and air curtain11 have been studied, both showing noise reduction potential. The quietest configurations was identified as being a combination of a narrow wheel spacing, a negative angle between the flow and the bogie, porous fairings, brake fairings and a modified side-stay design. The combination of the modifications leads to a decrease of 8dB(A). The reduction affects the whole frequency range and all directions with a significant impact for middle-to-high frequencies and forward angles directivity12. As in the RAIN projects, the dominant source of the 4-wheel landing gear is the lower part of the gear, i.e. the bogie area. On the quietest configuration, the sources are located around the main fitting and near the side-stay. Indeed, most of the advanced low-noise design modifications are focusing on the bogie area. It is also found that the noise from the bogie area is directional while the noise originating from the upper part of the gear is rather omnidirectional. This highlights the importance to reduce the noise sources of the lower part of the gear that has the main impact on ground. Transposed to the flight conditions, a decrease of about 7 EPNdB is achieved thanks to the advanced low noise main landing gear design, which means a decrease of 1.5 EPNdB at aircraft level 12. As in RAIN and SILENCE(R) the angle of the bogie with respect to the flow is an important parameter influencing the noise of the bogie area. Despite the fact that the optimal angle is associated to the whole gear architecture and configuration, it was found that an angle of 15° toe down shows the lowest noise signature on this 4-wheel configuration. This time, the front wheels are below the rear ones with respect to the aircraft flight direction axis in the quietest configuration. This is one step further in comparison with the two previous projects in which it was found and confirmed that aligning the wheels with the flow direction provides noise reduction with respect to a toe up configuration (front wheels above).

D. OPENAIR (OPtimisation for low Environmental Noise impact AIRcraft, 2009-2014) A multi-disciplinary design approach was integrated in the OPENAIR project that went over SILENCE(R) and TIMPAN. Several axes were treated including airframe noise and especially the main landing gear noise signature. Low noise configurations were investigated numerically and in wind tunnel facilities, at small and full scale. This 3 American Institute of Aeronautics and Astronautics


combined a deceleration plate that reduced the upstream wake velocities, without adverse flow displacement effects, hub caps, wheel and torque link fairings, but also dressing re-routing, side-stay cover and leg cavity treatments13. E. Summary and lessons learned 1. Sources ranking and impact of landing gear size The identification of the noise sources of the landing gears was extensively treated in RAIN. The noisiest part of the landing gears appears to be the lower part, including the wheels, the axle(s) and brake system. The torque link area also shows a dominant contribution. The main fitting and door area comes after. This is true for 2, 4 and 6wheel configurations. Besides this, the noise levels scales with the landing gear size. The noisiest being the 6-wheel configuration, than the 4-wheel, with noise signature close to the 6-wheel configuration, and finally 2-wheel main and nose landing gear. The main landing gears mainly affect the forward angles. 2. Potential improvements with fairings Different solutions are proposed to decrease the noise of the different parts that show important noise levels. As the nose gear configuration was not treated in each project, but also because the main landing gear is the dominant noise source, the following comments will address the main landing gear noise, only. The potential noise reduction for each of the projects are respectively 4.5dB(A), 1.8 EPNdB, 8dB(A) and associated 7 EPNdB for RAIN, SILENCE(R) and TIMPAN projects, on 4-wheel configurations with combined use of lownoise treatments, including bogie angle modification. The fairings are mainly used on 2-wheel landing gear configurations. The torque link mesh fairing is identified several times as very efficient leading to a potential decrease of more than 2dB(A) (RAIN, OPENAIR). Similar noise reductions are achieved when covering the axle brakes and hub caps area. The other technologies proposed and investigated as low-noise solutions do not show similar level of reproducibility in terms of configuration or noise results. 3. Potential improvements with bogie angle modification From the European project results described above, it appears that the bogie angle is a simple geometrical parameter influencing the noise of a 4 and 6-wheel landing gear. The different observations are summarized in Figure 2, respectively for 4 and 6-wheel configurations. Also, additional data are considered for 6-wheel configurations thanks to the experimental data available for the bogie angle parameter. Indeed, following TIMPAN Smith et al14, installed a 6-wheel landing gear in wind tunnel facilities in order to investigate the interactions between the different components of landing gears. It was found that a toe up angle of about 15° leads to the lowest noise signature, on a 6-wheel configuration with two side-stays. Finally, in the framework of Quiet Demonstrator Technology II, a significant noise reduction of 1 dB to 4 dB according to the frequency range16 was observed on a 26%-scale and high-fidelity landing gear. This confirmed the noise decrease of about 2.5dB when the bogie is aligned with the flow, on the entire frequency range observed on a similar reduced 6-wheel configuration15. However the benefit of aligning the bogie with the flow was not evident during flight tests17.

Figure 2. Summary of the impact of the bogie angle on the noise emissions for 4 and 6-wheel landing gear configurations. On the 4-wheel configurations, it appears from Figure 2 that a negative inclination (toe down) has a positive impact on the noise emission, even if there is not clear explanation of the reasons leading to this. SILENCE(R) tests however show that an important toe down angle, above -15º leads to an increase of the noise. On the 6-wheel configurations, the gathered results suggest that a slight toe up angle must be privileged. 4 American Institute of Aeronautics and Astronautics

Computational Fluid Dynamics (CFD) simulations have been performed on detailed 6-wheel configuration geometry in order to increase the knowledge and the understanding of underlying mechanisms of the noise produced by the bogie at inclination, and will be considered in section IV.


IV. Numerical studies on detailed landing gear geometry Besides the findings taken out of the collaborative projects that mainly – but not only – focus on experimental aspect, MBD is currently carrying out a large-scale numerical work. The latter aimed to understand the capabilities of Computational Fluid Dynamic (CFD) / Computational Aero Acoustic (CAA) and post-processing tools, acquire knowledge on landing gear noise mechanisms and numerically assess and optimize low-noise solutions18. Numerical work benefits from the experimental results mainly provided by the collaborative projects. Most of the findings are numerically assessed. In addition to that, optimization of the low-noise solutions is achieved thanks to the numerical framework. The selected numerical solution is the PowerFLOW software developed and provided by EXA and based on lattice Boltzmann method combined with Very Large Eddy Simulation (LBM-VLES) strategy for turbulence modelling19. The propagation into the far-field is performed with a solid or permeable Ffowcs-Williams-Hawkins approach20. Cartesian mesh approach allows benefiting from a direct bridge from the Computer-Aided Design (CAD) models to the detailed geometry used for aeroacoustics studies. The details of the flow features associated to all the components of the landing gear constitute an essential insight towards the low noise design. The numerical approach allows supplementing the experimental observations but also detailed comparisons with the aims of beamforming of numerical results for example. Two, four and six-wheel landing gears configurations are intensively analysed. Several post-processing techniques allow deep investigations and understanding of the noise sources mechanisms. The numerical results have been analysed and presented so that the comparison with the experimental findings is highlighted. 1. Sources ranking and impact of landing gear size The FW-H integration of distinct components of the landing gear provides the contribution of each one in all directions and on the whole frequency range. Based on separated surface integration, it is powerful to assess, in the far-field, the noise from different sources. But this approach also fails in capturing the interaction effects between the components of the individual surfaces. For this reason, only four assemblies of components, presented in Figure 3 are selected in order to mitigate the loss of mutual interaction phenomena that are not captured.

(a) Four assemblies of components selected for separated FW-H propagation.

(b) Acoustic contribution of the lower part of the gear in OASPL is the largest in the three directions.

Figure 3. Numerical approach for noise ranking of landing gear areas on a 2-wheel configuration. In each of the three directions, more than half of the acoustic power comes from the lower part of the landing gear, including the wheels and the axle. Afterwards, both the main strut and the arm show a contribution of about 20%. The acoustic signature of the main strut is however less significant at 90˚ under the landing gear. The door is the quietest component in this analysis. The noise created by the door is expected to emit at side-line positions rather than in the directions under the landing gear. Similar trend is observed on a 4-wheel configuration where half of the acoustic power is associated to the lower part of the gear. The part of the door is slightly increased with respect to the 2-wheel case, as presented in Figure 4.


(a) Four assemblies of components selected for separated FW-H propagation.

(b) Acoustic contribution of the lower part of the gear in OASPL is the largest (around 60° angle)


Figure 4. Numerical approach for noise ranking of landing gear areas on a 4-wheel configuration. The comparison of noise levels associated to 2, 4 and 6-wheel configurations can be performed as in RAIN project in order to assess the impact of the size of the landing gear. Although the direct link between drag and noise cannot be determined, the drag and noise comparison has been assessed in Figure 5. The noise is determined in EPNL while the drag is un-dimensional. Several modified configurations have been simulated and the figure shows a clear trend. Two wheel landing gears are associated to the lowest noise signatures. Larger landing gears, like 4 and 6-wheel configurations show higher drag and higher noise levels.

Figure 5. Noise and drag comparison for different LG configurations (linear scale for drag). The trend can be associated to the size rather than to the drag itself. Nevertheless, such an assessment is consistent with the observations addressed in RAIN. The sensitivity to the wheel number is highlighted between 2 wheels and more, but between 4 and 6, this is less obvious. In particular the 4-wheel configuration that shows the higher drag has a noise signature that is above the 6-wheels configurations. Hence, in order to decrease the noise signature of the gear, the drag of the landing gear can be modified with a potential noise decrease. However, any relationship between noise and drag must be considered with caution. Indeed, the use of add-on fairings is one of the main solutions to decrease the noise while it is habitually associated to a drag increase. For this reason, the impact of the drag to the noise levels has to be considered on an overall landing gear geometry and on a size point of view. And for this, the compactness of the gear appears to be positive for the noise. 2. Potential improvements with fairings The use of add-on fairings is mainly proposed as low-noise solutions for 2-wheel landing gears corresponding to medium range aircraft. This section presents the results of numerical simulations carried out on a real two-wheel main landing gear in order to reduce its acoustic signature. Three areas of the lower part of the gear were covered by optimized add-on fairings in order to reach a low noise configuration. The outer rims of the wheels are covered in order to eliminate the interactions between the shear layer created on the front of the tire and the cavity. A second effect associated to the use of the fairings is to narrow the wake behind this part of the gear as shown on the instant velocity plots in Figure 6. The decrease of the pressure fluctuations leads to a decrease of the acoustic emissions. A shielding of the brakes is achieved thanks to the use of fairings. The flow is deflected and no impact between the piston housing and the flow occurs.



Figure 6. Instantaneous velocity section maps for configuration without (left) and with (right) fairings. The combined use of rim and brake fairings decreases the size of the wake, avoids the interaction between the rim cavity and the shear layer of the tire and the impact of the flow on the brake pistons. The third area covered by fairings is the torque link. The mechanism responsible of noise in that part of the gear is associated to the flow separation induced by the torque link which impacts the main strut. The purpose of the fairing is to decrease the incoming flow velocity. The flow separation is then reduced and the strut is impacted by the flow at lower speed. A first design of the fairing was composed by a solid surface. However, the flow deflection and acceleration of the fluid due to the presence of the fairing induces strong negative interactions with several parts of the main strut. Figure 7 shows the differences in the flow field just behind the torque link fairing. Behind the solid surface, the instantaneous velocity is lower between the wheels. Conversely, the use of porous fairing increases the velocity of the flow behind the fairing but makes it more homogeneous. In Figure 8, it is notable that the porous surface makes the wake more homogenous. The shear layers on the top and bottom of the fairing surface are smoother and smaller high-velocity areas appear around the shock absorber.

Figure 7. Instantaneous velocity maps. Horizontal view of the space between the wheels. Solid fairing surface (left) and porous surface (right) are presented.

Figure 8. Instantaneous velocity maps. Vertical cut between the wheels. Solid fairing surface (left) and porous surface (right) are presented. In addition to being lighter, the porous fairing improves the acoustic emission of this part of the gear. In order to allow a comparison with the noise reductions of fairings that were observed during the experimental campaigns, the results are presented in Overall Sound Pressure Level (OASPL) according to three directions in Figure 9(a). The use of brake and hub cab fairings (low noise configuration 1) reduces the noise in all directions about more than 2dB, especially just below the landing gear (90°). This corroborates the experimental findings. In this direction, no additional noise acoustic benefit comes when adding the torque link porous fairing (low noise configuration 2). However at forward (45°) and backward angles (135°), a noise reduction is observed with the perforated fairing covering the torque link. It is also interesting to observe here, that the noise signature of the main landing gear is dominant on the forward angles, as detected during the experimental projects. 7 American Institute of Aeronautics and Astronautics


(a) Overall Sound Pressure Levels of the baseline and low noise configurations according to the direction. 45° is forward angle and 135° is backward angle.

(b) Sound pressure levels of the three configurations at 45° (forward angle).

Figure 9. Sound Pressure levels of the three configurations at 45°, in the forward angle. Finally, it is to be mentioned that the most relevant angles for the computation of EPNL levels are precisely the forward angles on which significant noise levels are reached. The noise reductions obtained with the low-noise configurations is thus positive for these values. This is illustrated in Figure 9(b) presenting the spectrum at a forward angle (45°). Except under 100 Hz, the noise level of the low noise 2 configuration –with covered torque link– is reduced with respect to low noise 1. The decrease at frequencies above 1000 Hz is notable. This is vital as it contributes strongly to the perceived noise level. Hence, while the noise reduction induced by the torque link fairing is not pronounced in OAPSL, in EPNL value, it brings a noise decrease of almost 1EPNdB. The decrease of the lowfrequency noise levels with covered wheel caps is similar to RAIN results. The numerical study and design optimization carried out on a two-wheel main landing gear configuration appears to be consistent with experimental conclusions in terms of directivity, noise source locations and potential noise decrease. The first phase of the noise decrease targeted the lower part of the landing gear identified as being the largest contributor to the acoustic signature of the gear. This was done with the use of three optimized fairings, on the wheels, the brakes and the torque link. Further noise decrease can be achieved by targeting other parts of the gear as the ones modified during the experimental studies. On the other hand, MBD is working to allow the integration of low noise solutions on existing landing gears but also in a design process including multiple objectives. This remains a major challenge that will be possible through a complementary and fully integrated design approach. Among the integrated low noise solutions, the modification of the bogie angle appears to be an efficient way to decrease the noise signature with reduced impact on the landing gear structure. This solution is investigated on 4-wheel and 6-wheel configuration (see Section III). 3. Potential improvements with bogie angle modification Several experimental results but also numerical simulations focused on the bogie inclination as a key parameter influencing the noise of a landing gear. Previous investigation based on experiments showed that the bogie angle has a major effect on the landing gear noise. However, no clear trend in the low noise bogie angle orientation could be given, especially for a 6-wheel configuration. Nevertheless, in SILENCE(R), the alignment of the bogie with the flow was identified as a way to reduce the noise emissions, by allowing a flow deflection that shields the 6-wheel bogie beam components according to the interpretation4. It appears that the simulation results of a 6-wheel configuration show that this effect is real and impacts the noise signature. The main flow mechanisms and noise sources are represented in Figure 10, based on total pressure field and beam forming maps (Figure 11).



Figure 10. Simplified view of the flow filed main mechanisms for the two 6 wheels configurations. In the aligned gear configuration only, a large separation ( in Figure 10, left) area appears underneath the bogie, between the first and the second wheel. Moreover, the separation of the flow appearing for the two configurations just on the front top of the bogie is of similar size ( and ), meaning that the toe up angle does not induce large flow separation over the bogie because of its orientation. In other words, the underneath separation of the aligned configuration is not substituted by a larger separation above the bogie for the toe up configuration. The noise of the upper leg components is thus expected to be similar for both configurations.

(a) Total pressure maps

(b) Beamforming for frequencies between 2800 and 3550 Hz.

Figure 11. Flow and noise sources visualization between aligned and toe up configurations. In Figure 12, it can be seen that the flow velocity that impacts the rear axle is significant. On this rear wheel, the flow particles are deflected over the axle, ensuring a re-attachment of the flow on the axle. For this reason, the rear axle/brakes area is probably noisier than the middle axle/brakes area on the toe up configuration. In addition to that, significant noise ensues from the interaction between the attached flow field and the brake rods, on the toe up configuration. Indeed, the brake rods are located close to the boundary layer formed by the flow field around the bogie beam. This is observable on the bottom of Figure 12 (b), on the vorticity magnitude, near the joints between the brake rods and the bogie beam. Beamforming also illustrates such an effect. Figure 11(b) presents the noise levels on a 2D plane for both configuration between 2800 Hz and 3550 Hz. Important noise source appears near the front and rear wheel on the angled configuration. The same effect appears from approximately 2000 Hz. Numerically, the impact of the bogie angle modification is highlighted through the noise source locations and mechanism. This thorough investigation of the stream flow constitutes a way forward in the understanding of the noise creation process, and thus to the optimization of this parameter. According to the configuration, the optimal bogie angle can be identified and integrated into the design process of the landing gear.


(a) Y-planes of velocity.

(b) Y-planes of vorticity


Figure 12. The flow separation underneath the bogie for the aligned configuration prevents the flow to a straight impact on the brakes and brake rods.

V. Conclusion In order to anticipate the potential future requirements targeting airframe noise, MBD has been investigating low noise solutions to improve the landing gear acoustic signature. Internal initiatives together with collaborative projects are opportunities for MBD for increasing the knowledge on the complex fluid dynamics and noise mechanisms of the landing gear. RAIN, SILENCE(R), TIMPAN or other European projects allowed MBD to acquire a significant landing gear noise result database and knowledge. A complementary approach, numerical simulations of detailed landing gears were carried out on a several landing gear configurations leading to consistent results with the experimental findings. Another aspect arises in understanding the environment which imposes the main constraints and opportunities towards quieter landing gears. Namely the aircraft overall noise sources, the aircraft noise and flow interactions, the future noise requirements and local restrictions, the design constraints associated to weight, maintainability, operational aspects. In this framework, it appears that the progress to quieter landing gears has to be encompassed into a global approach. Either for operational design and for acoustic improvement evaluation.

Acknowledgment This work has been performed on the basis of many simulations provided by EXA. The authors would like to thank EXA for continued support and engineering knowledge provided for several years.

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