Radical innovations for all aircraft systems and subsystems are needed for realizing future carbon-neutral aircraft, with hybrid-electric aircraft due to be delivered after 2035, initially in the regional aircraft segment of the industry. Electrical energy storage is one key element here, demanding safe, energy-dense, lightweight technologies. Combining load-bearing with energy storage capabilities to create multifunctional structural batteries is a promising way to minimize the detrimental impact of battery weight on the aircraft. However, despite the various concepts developed in recent years, their viability has been demonstrated mostly at the material or coupon level, leaving many open questions concerning their applicability to structural elements of a relevant size for implementation into the airframe. This review aims at providing an overview of recent approaches for structural batteries, assessing their multifunctional performance, and identifying gaps in technology development toward their introduction for commercial aeronautic applications. The main areas where substantial progress needs to be achieved are materials, for better energy storage capabilities; structural integration and aircraft design, for optimizing the mechanical-electrical performance and lifetime; aeronautically compatible manufacturing techniques; and the testing and monitoring of multifunctional structures. Finally, structural batteries will introduce novel aspects to the certification framework.

With the drastically increasing traffic in the last decades, crucial environmental problems have been caused, such as greenhouse gas emission and traffic noise pollution. These problems have adversely affected our life quality and health conditions. In this paper, modelling of traffic noise employing deep learning is investigated. The goal is to identify the best machine-learning model for predicting traffic noise from real-life traffic data with multivariate traffic features as input. An extensive study on recurrent neural network (RNN) is performed in this work for modelling time series traffic data, which was collected through an experimental campaign at an inner city roundabout, including both video traffic data and audio data. The preprocessing of the data, namely how to generate the appropriate input and output for deep learning model, is detailed in this paper. A selection of different architectures of RNN, such as many-to-one, many-to-many, encoder–decoder architectures, was investigated. Moreover, gated recurrent unit (GRU) and long short-term memory (LSTM) were further discussed. The results revealed that a multivariate bi-directional GRU model with many-to-many architecture achieved the best performance with both high accuracy and computation efficiency. The trained model could be promising for a future smart city concept; with the proposed model, real-time traffic noise predictions can be potentially feasible using only traffic data collected by different sensors in the city, thanks to the generated big data by smart cities. The forecast of excessive noise exposure can help the regulation and policy makers to make early decisions, in order to mitigate the noise level.

Tonal airborne noise of aerofoils appears in a limited range of moderate Reynolds numbers and angles of attack. In these specific conditions, the aerofoil is characterised by a large region of laminar flow over the aerodynamic surface, typically resulting in two-dimensional laminar instabilities in the boundary layer, generating one or more acoustic tones. The numerical simulation of such phenomenon requires, beside an accurate prediction of the unsteady flow field, a proper modelling of the laminar to turbulent transition of the boundary layer, which generally imposes the use of highly CPU demanding approaches such as large eddy simulation (LES) or direct numerical simulation (DNS). This paper aims at presenting the results of numerical experiments for evaluating the capability of capturing the tonal airborne noise by using an advanced, yet low computationally demanding, unsteady Reynolds-averaged Navier-Stokes (URANS) turbulence model augmented with a transitional model to account for the laminar to turbulent transition. This approach, coupled with the Ffowcs Williams and Hawkings (FW-H) acoustic analogy, is adopted for predicting the far-field acoustic sound pressure of a NACA 0012 aerofoil with Reynolds number ranging from 0.39 · 10⁶ to 1.09 · 10⁶. The results show a main tone located approximately at 1.6-1.8 kHz for a Reynolds number equal to 0.62 · 10⁶, increasing to 2.4 kHz at Reynolds number equal to 0.85 · 10⁶ and 3.4 kHz at 1.09 · 10⁶, while no main tones are observed at 0.39 · 10⁶. The computed spectra confirm that the acoustic emission of the aerofoil is dominated by tonal structures and that the frequency of the main tone depends on the Reynolds number consistently with the ladder-like tonal structure suggested by Paterson et al. Moreover, in specific conditions, the acoustic spectra exhibit a multi-tonal structure visible in narrowband spectra, in line with the findings of Arbey and Bataille. The presented results demonstrate the capability of the numerical model of predicting the physics of the tonal airborne noise generation.

Hybrid electric propulsion is likely to play a more prominent role for regional aircraft with 40+ passengers in the future air transport system with reduced climatic impact. In IMOTHEP, two hybrid-electric regional (HER) aircraft concepts, a conservative and a radical, are developed. Energy dense battery technologies are needed to enable hybrid-electric propulsion of regional aircraft. Furthermore, these batteries need to become commercially available within the planned development time of the new aircraft, i.e. until 2030. This paper will discuss general requirements for the HER battery and aims at providing an overview on the most promising industrial approaches for energy dense battery cell technologies – from advanced Li-ion to all-solid-state – with view of their application in air transport and forecasted availability on the market. Furthermore, results will be presented from the study performed in IMOTHEP on hybrid polymer-ceramic all-solid-state battery electrochemistry that combines lab trials with electrochemical numerical simulation.

In this work, a battery layup consisting of a poorly flammable ionic liquid electrolyte and a poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) thermoplastic has been developed along with composite anode and cathode electrodes. The developed gel electrolyte exhibits feasible ionic conductivity of about 1 mS/cm at 30 degrees C. State-of-the-art active electrode materials, i.e., LiNi0.8Mn0.1Co0.1O2 (NMC811) and graphite, have been employed. Full cells were tested in coin and pouch cell format, obtaining capacities of about 120 and 100 mA h/gNMC811, respectively, at a C-rate of C/10. Thereby, it was observed that good contact between the individual cell layers is crucial. Recently, it was shown that the mechanical properties of structural batteries, realized by integrating battery cells into carbon fiber-reinforced polymer (CFRP) laminates, depend significantly on the mechanical properties of the cell itself. Hence, to promote the realization of such a structural battery concept, tensile tests were carried out to investigate the mechanical properties of cells as well as the individual components developed in this work. The full cell showed values of 10 GPa and 49 MPa for the Young's modulus and tensile strength, respectively. Thus, feasible multifunctionality could be verified on the cell level. However, regarding the contributions of the different components, it could be shown that mainly the current collector foils contribute to the mechanical properties, in contrast to the electrode loadings and the gel electrolyte. Additionally, the thermal and chemical stability of the developed system was evaluated, highlighting the importance of these secondary properties for the fabrication of structural batteries, i.e., the integration of cells into load-bearing CFRP laminates. Specifically, it was observed that the developed system is thermally stable up to 150 degrees C and no HF release was detected upon exposure to ambient conditions.