Elsevier

Applied Acoustics

Volume 182, November 2021, 108263
Applied Acoustics

Feasibility of measuring noise sources on a detailed nose landing gear at low cost development

https://doi.org/10.1016/j.apacoust.2021.108263Get rights and content

Abstract

This work aimed to develop an aeroacoustics analysis of a nose landing gear through rational and low-cost wind tunnel tests to develop potential noise mitigation technologies of aircraft components. This approach led to using a nose landing gear model of the Boeing 777 aircraft, whose more complex parts were 3D-printed. The aeroacoustics and aerodynamics tests were performed in a hard-wall wind tunnel with the capability to perform aeroacoustics tests after corner-vanes and fan section treatment to reduce background noise. Improved beamforming techniques were applied for noise source localization through microphone array measurements. Also, near field noise analysis was performed by microphones installed inside model cavities as part of the correlation procedures for noise investigation and creation of reference data for further CAA simulations. Additionally, using wake-flow mapping and lateral aeroacoustics measurements allowed the characterization of the flow field and noise source positions at low subsonic speeds in the approach condition. The flexibility of the model construction allowed a systematic assembly of the main parts and structures, and therefore, a noise build-up analysis showed the contribution of each component to the overall noise of the nose landing gear. Near-field measurements provided better insights into the noise generation mechanisms of each component. Despite some limitations identified by following this approach, the results were consistent and revealed that the noise sources are concentrated in the bay cavity, at the upper structure, and between the wheels. The feasibility of measuring noise sources on a detailed nose landing gear has been achieved with relatively low-cost development, providing useful data for further CAA computations as well as noise mitigation development.

Introduction

Despite all the technological advances found in modern and large transport aircraft, the external noise still concerns authorities and civil institutions. Indeed, aviation authorities establish and apply noise restrictions to aircraft certification and operations [1]. The aeronautic industry has been, for decades, developing means to reduce aircraft noise to comply with these increasingly stringent regulations. First, the considerable noise reduction achieved for the turbofan engines reached, nowadays, the same noise level as the airframe. Therefore, regarding noise, engine optimization is no longer the exclusive preoccupation of the aeronautic industry by now. Airframe noise emanating mainly from high-lift systems and landing gears are responsible for the total aircraft noise for approach-to-land operation.

Moreover, landing gear noise is dominant for wide-body airplanes, followed by aerodynamic noise originating from deployed high-lift devices, according to [2]. Somehow, landing gear design is primarily driven by the structural and mechanical needs of the parts. Aerodynamics is mostly ignored due to the short interval of exposure to the flow during takeoff, and at landing, the drag is a positive side effect. However, from the aerodynamic standpoint, the flow developing over this complex geometry allows for highly unstable pressure loading and velocity fluctuations. The intrinsic presence of small details such as screws, nuts, pins, hose, and connections results in an even more complex flow pattern [3]. Therefore, the difficulty of predicting the sound sources on a landing gear is related to the fact that the geometric details are extremely relevant to the entire problem. Predicting or measuring the landing gear's noise without considering all the components can lead to large errors in developing technologies for noise mitigation.

Therefore, one of the challenges is related to the accuracy of noise measurements of such landing gear devices and how to use these data for engineering development. Another critical aspect is associated with the amount of investment or cost associated with performing such measurements. It is tough to replicate in a controlled environment (laboratory) the exact flight conditions for an aircraft, primarily related to external noise measurements of airframe components such as landing gears and high-lift systems, mainly due to Reynolds number and installation effects. The search for accuracy leads to high costs, and without a doubt, flying the actual configuration is the best choice in terms of analysis since you are dealing with the full-scale and not with a model. However, this is inaccessible when treating various design configurations, especially considering that in some cases, an hour of flight can reach exorbitant tens of thousands of dollars (roughly estimated). Simplification of the problem is then necessary by removing the aircraft, avoiding flying, geometrically simplifying the model, scaling down, and testing in a wind tunnel with open-jet or hard-wall closed test-section. This approach has brought many challenges to engineering and according to [4], noise prediction models based solely on wind-tunnel tests featuring scale models are prone to underestimate airframe noise levels because simplified wind tunnels do not capture all the geometric details.

Moreover, the acquired data is also difficult to extrapolate to flight, and aircraft noise levels cannot be recalculated. Not surprisingly, the prediction of landing gear noise, by itself, represents a significant challenge because of the geometry features, complicated flow field, and potentially considerable installation effects. This required complexity also poses a series of restrictions to the application of Computational Aeroacoustics (CAA) simulations to the study of noise from landing gears. Attempts are found in several works such as [5], [6], [7], among others. Otherwise, early studies [8], [9], have focused on acquiring noise data for large models of main and nose landing gears.

Nevertheless, such test’s lack of details made the results not directly comparable with data from real geometries. Albeit brought the understanding of the primary noise sources present in the problem. Thus, recent campaigns are demonstrating different approaches: a) simplified landing gear models are being used for CFD/CAA validations such as the LAGOON project [10] or generic nose landing gear [11], [12]; or basic technologies testing as seen in [13], [14], [15]; b) complex geometries are applied for element noise estimations and development of new noise reduction technologies such as ALLEGRA [16] or by [9] which showed results for two full-scale A320 landing gear geometries (2 and 4 wheels design), with data being extrapolated for in-flight conditions used for real EPNdB predictions. Furthermore, [17] presented detailed investigations that included flight tests conducted for the QTD (Quiet Technology Demonstrator) program to study the effect of fairing the landing gear main axle. The SILENCER project [2] also included tests with different landing gear concepts designed for low noise. Other two important and thorough airframe noise investigations, with a detailed nose, and main landing gear aerodynamics and aeroacoustics, were the high fidelity 1:4-scale model of Gulfstream G550 aircraft nose landing gear Gulfstream G550 [17] and the Boeing 777 campaigns [19]. The tests were conducted in several facilities, with complex fully instrumented models. These studies were carried out to understand the contributions of the landing gear to the overall airframe noise and create aeroacoustic datasets for a nose gear [18]. Given the importance of the details for the overall noise, wind tunnel models have become more complex to encompass the fundamentals of landing gear noise without overloading the study with small details and multiple noise sources. For the model to become a benchmark airframe configuration, its geometry needed to be realistic enough, but still with some simplicity to reach the aeroacoustic research community as widely as possible [20]. However, there is increasing use of more sophisticated wind tunnels with full-scale models of the nose and main landing gear by including part of fuselage and bay cavities as well, leading back to high economic cost and limiting the number of configurations available for study [16], [21]. In these works, complete full-scale landing gear was tested with fixtures, small details, and associated structures such as the wheel bay, bay doors, and hydraulic dressings, with the main objective of noise characterization of each component. Other works by following this approach are, as presented by [22], considered the noise characterization of a full-scale nose landing gear.

Additionally, the study presented by [23] evaluated the pressure field in the wheel-bay of a full-scale nose landing gear through numerical analysis, identifying the frequencies at which the Helmholtz resonance and the 3-D standing wave duct modes were excited by instabilities in the bay opening shear layer. Numerical data was obtained and compared to those gathered from wind tunnel experiments of a highly detailed full-scale model. Following further developments, [24] presented noise emissions comparisons among this full-scale model with results obtained from computational simulations and data registered for three regional aircraft types. These costly approaches with sophisticated wind tunnels and full-scale (or near) models are essential for obtaining more accurate data on external landing gear noise. However, cost-benefit tests can be achieved to minimize development costs while obtaining reasonably accurate data, which can be useful for engineering.

Thus, the idea of this work is to make it possible to obtain relatively reliable data, with low cost and fast execution time to support preliminary development campaigns for low noise aircraft components such as nose landing gear (NLG). The feasibility of measuring noise sources on a detailed nose landing gear with a relatively low cost was tackled by performing hard-wall closed test-section wind tunnel measurements. A detailed model, corresponding to a wide-body aircraft's nose landing gear, was scaled-down to 1:6 with the exclusion of parts whose dimension is smaller than 2 mm. The model comprising the bay-cavity, doors, the main structure with all relevant components, axes, and wheels were evaluated through aerodynamic and aeroacoustics tests. The aerodynamic measurements, such as steady pressure distribution on the wheels and wake flow-field mapping, were performed and could be used to adjust comparisons against numerical simulations. Because the components can be easily assembled or disassembled, a noise build-up process could be pursued by evaluating each NLG component's contribution to the overall model noise. These results were complemented with the source localization maps obtained from an array of 61 microphones through a beamforming technique applied for sideline measurements. The whole test campaign was carried out under a low budget and the data was consistent and potentially useful for further engineering developments for noise mitigation of nose landing gear.

The work is structured as follows: Section 2 explains the experimental setups for the wind-tunnel, and NLG geometries. The aerodynamic and aeroacoustic measurement techniques are also described in Section 2. The results and discussion presented in Sec. 3 are separated by aerodynamics and aeroacoustics subsections, illustrating pressure distribution, wake mapping, wind tunnel background noise, dressed NLG noise, beamforming maps and NLG noise build-up components. Sec. 4 gathers the most important conclusions.

Section snippets

Experimental setup

The experiments were performed in the Laboratory of Aerodynamics (LAE) in the Aeronautical Engineering Department of the School of Engineering at University of São Paulo (USP), Brazil. One of the wind tunnels (WT), the LAE-1 test facility has a test section of 1.30 m high, 1.70 m wide and 3.0 m long. The eight-blade fan with five straighteners is driven by an 82 kW electrical motor designed to achieve flow speed at the test-section of 50 m/s, as described by [25] The turbulence level confirmed

Results

This section presents the results and discussion applied to quantify both aerodynamics and aeroacoustics of a scaled Boeing 777 nose landing gear. Aerodynamics measurements are presented by considering the steady pressure distribution on the wheels and wake velocity profiles and flow mapping. Aeroacoustics measurements include the wind tunnel background noise data, nearfield measurements at bay-cavity and wheel-cavities, followed by complete nose landing gear noise spectra, beamforming maps and

Conclusions

A high-detailed 1:6 scaled model, corresponding to the nose landing gear of a Boeing 777 aircraft, was evaluated through aerodynamic and aeroacoustics tests. The scale model wind tunnel methodology was aimed to gather reliable data while keeping the development cost low enough with fast turnaround. Thus, the tests were performed in Brazil with a modified WT with hard-wall test-section, taking advantage of traditional aerodynamics techniques and improved beamforming apparatus.

The association of

Funding

This work was supported by Boeing Research and Technology – Brazil (BR&TB) through the project: Advanced Experimental Aeroacoustics of Nose Landing Gear and Wind Tunnel Upgrade, during the years of 2018 – 2020 carried out in the Aeronautical Engineering Department – São Carlos School of Engineering (EESC-USP), Brazil.

CRediT authorship contribution statement

Lourenço Tércio Lima Pereira: Investigation, Data curation, Writing - original draft, Software. Fernando Martini Catalano: Conceptualization, Methodology, Resources, Visualization, Supervision. Odenir de Almeida: Investigation, Data curation, Writing - original draft, Visualization, Writing - review & editing. Paul Bent: Conceptualization, Methodology, Resources, Supervision.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors would like to acknowledge such cooperation among EESC-USP, CPAERO-UFU and UNICAMP through fruitful discussions during the research. They would like also to thank Boeing BR&TB for funding the project and granting the scholarship for concluding Master’s and post-doc researches into this field.

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