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The aviation sector has been forced to deal with a strong transformative phase driven by the urgent and non-negotiable goal of reducing its impact on the climate [...]

This article discusses the mission performance of regional aircraft with hybrid-electric propulsion. The performance analyses are provided by mission simulations tools specifically developed for hybrid-electric aircraft flight dynamics. The hybrid-electric aircraft mission performance is assessed for the design point, identified by top level requirements, and for off-design missions, within the whole operating envelope. This work highlights that the operating features of hybrid-electric aircraft differ from those of aircraft of the same category with conventional thermal propulsion. This assessment is processed by properly analysing the aircraft payload–range diagram, which is a very effective tool to assess the operating performance. The payload–range diagram shape of hybrid-electric aircraft can vary as multiple combinations of the masses of batteries, fuel and payload to be transported on board are possible. The trade-off in the power supply strategies of the two power sources to reduce fuel consumption or to extend the maximum flight distance is described in detail. The results show that the hybrid-electric propulsion integrated on regional aircraft can lead to benefits in terms of environmental performance, through savings in direct fuel consumption, or alternatively in operating terms, through a significant extension of the operating envelope.

The adoption of hybrid–electric propulsion, allowing us to partially replace fuel with batteries and to reduce aircraft in-flight emissions, represents one of the main investigated solutions to mitigate the aviation climate impact. Despite its environmental potential being appealing for a practical application, two main drawbacks limit the actual implementation of this technology: first, the low gravimetric energy density of the batteries restricts hybrid–electric aircraft payload and range capabilities; second, the production of electricity is currently not entirely based on renewable energy sources, hence a non-direct emissions budget may limit the benefit in terms of overall decarbonization. When designing hybrid–electric aircraft, even projecting its actual entry into service in the next decades, it is necessary to take these limitations into account depending on both the more reliable technological forecasts on the development of electric components and on the estimates of electricity production. A proper analysis of the figure of merits related to the operation of such an aircraft, therefore, becomes crucial in assessing the impact related to its introduction into service. In this context, trade-offs between different performance metrics may be needed to efficiently exploit the environmental benefits of such an advanced concept, while limiting the possible drawbacks coming from its utilisation. This paper provides a performance analysis of hybrid–electric aircraft through an assessment of the relevant figures of merit characterizing its operations. In particular, direct and non-direct emissions, climate impact, ground pollution, operating costs, fuel consumption, weight, and a combination of these figures of merit allow us to define a proper development perimeter in which a possible (future) hybrid–electric aircraft can express its maximum potential towards all the aspects of its utilisation. The trade-off analyses provided in this paper allow us to identify more effective paths for the actual development of hybrid–electric aircraft, highlighting the impact of the selected design variables on the performance metrics, and bringing to light also the possible related limitations.

Recent progress of electric systems has raised attention towards hybrid-electric and full-electric aircraft. Nevertheless, the current low battery energy density limits the application of these propulsive architectures to large transport aircraft. In the context of the general aviation category, full-electric aircraft for the so-called Urban Air Mobility scenario are gaining increasing interest. These air taxis, also called e-VTOL, are conceived to exploit vertical take-off and landing capabilities, to carry people from one point to another, typically within the same city. In this paper, a new conceptual design methodology for urban air vehicles is presented and applied to an innovative convertiplane, called TiltOne, based on a box-wing architecture coupled with tilt-wing mechanisms. Several TiltOne configurations have been designed according to the current regulations imposed by European Union Aviation Safety Agency, and sensitivity analyses have been carried out on the varying main design parameters, such as wing loading and propellers’ disk loading, as well as main top-level aircraft requirements. The results provide an overview for today’s operational capabilities of such aircraft and, in addition, depict possible scenarios for a near-future horizon, based on the assumption of increased performance levels for the electric powertrain components. In such scenario, two different concepts of operations are analysed and discussed: the first is based on a given design range, long enough to cover the urban distances; the second is conceived to exploit the capability of flying multiple shorter missions with a single battery charge. The designed TiltOne configurations derived from these different approaches are presented, highlighting their potential capabilities and possible drawbacks.

This paper presents an overall performance assessment of hybrid-electric medium-range transport aircraft, with the aim to evaluate the potential of such a propulsion technology towards the reduction in the environmental impact of aviation transport, in terms of both local air quality degradation in airport areas and climate change. The proposed approach presents distinct analyses of the environmental impact of transport aircraft, distinguishing climate-changing effects from local pollution effects so that the integration of hybrid-electric propulsion is carried out to face the two issues specifically. The proposed analysis, although of conceptual nature, presents a clear scenario in which, given the technological limitations of batteries, the use of hybrid-electric propulsion on medium-haul aircraft can only be useful to reduce local pollution. In contrast, other solutions are needed to mitigate the climate-changing impact.

This paper proposes a performance analysis of a medium-range airliner powered by liquid hydrogen (LH2) propulsion. The focus is on operating performance in terms of achievable payload and range. A non-conventional box-wing architecture was selected to maximize operating performance. An optimization-based multidisciplinary design framework was developed to retrofit a baseline medium-range box-wing aircraft by designing and integrating the fuel tanks needed to store the LH2; several solutions were investigated for tank arrangement and layout by means of sensitivity analyses. As a main outcome, a performance analysis of the proposed LH2-powered box-wing aircraft is provided, highlighting the impact of the introduction of this energy carrier (and the integration of the related tank systems) on aircraft operating performance; a comparative study with respect to a competitor LH2-retrofitted tube-and-wing aircraft is also provided, to highlight the main possible operating differences between the two architectures. The findings reveal that the retrofitted box-wing can achieve long-range flights at the cost of a substantially reduced payload, mainly due to the volume limitations imposed by the installation of LH2 tanks, or it can preserve payload capacity at the expense of a significant reduction in range, as the trade-off implies a reduction in on-board LH2 mass. Specifically, the studied box-wing configuration can achieve a range of 7100 km transporting 150 passengers, or shorter ranges of 2300 km transporting 230 passengers. The competitor LH2-retrofitted tube-and-wing aircraft, operating in the same category and compatible with the same airport apron constraints, could achieve a distance of 1500 km transporting 110 passengers.

This paper proposes a conceptual analysis of the limitations related to the development (and integration) of hybrid–electric propulsion on regional transport aircraft, with the aim to identify a feasibility space for this innovative aircraft concept. Hybrid–electric aircraft have attracted the interest of aeronautical research as these have the potential to reduce fuel consumption and, thus, the related greenhouse gas emissions. Nevertheless, considering the development of such an aircraft configuration while keeping the constraints deriving from technological and/or operating aspects loose could lead to the analysis of concepts that are unlikely to be realised. In this paper, specifically to outline the boundaries constraining the actual development of such aircraft, the influence on overall aircraft design and performance of the main technological, operating, and design factors characterising the development of such a configuration is analysed and discussed at a conceptual level. Specifically, the current achievable gravimetric battery energy density (BED) is identified as the main limiting factor for the development of regional hybrid–electric aircraft, and a sensitivity analysis shows the correlation of this important technological parameter with aircraft performance in terms of both fuel consumption and energy efficiency. In this context, minimum technological development thresholds are therefore identified to enable the effective development of this type of aircraft; namely, a minimum of BED = 500 Wh/kg at battery pack level is identified as necessary to provide tangible benefits. From an operating point of view, flight distance is the most limiting design requirement, and a proper assessment of the design range is necessary if a hybrid–electric aircraft is to be designed to achieve lower emissions than the state of the art; flight ranges equal to or lower than 600 nm are to be considered for this type of aircraft. As a bridging of both of the previous constraints, a change in the design paradigm with respect to established practices for state-of-the-art aircraft is necessary. More specifically, penalisations in maximum take-off weight and overall aircraft energy efficiency may be necessary if the aim is to reduce direct in-flight consumption by means of integration of hybrid–electric powertrains.

A way to face the challenge of moving towards a new greener aviation is to exploit disruptive aircraft architectures; one of the most promising concept is the PrandtlPlane, a box-wing aircraft based on the Prandtl's studies on multiplane lifting systems. A box-wing designed accordingly the Prandtl “best wing system” minimizes the induced drag for given lift and span, and thus it has the potential to reduce fuel consumption and noxious emissions. For disruptive aerodynamic concepts, physic-based aerodynamic design is needed from the very early stages of the design process, because of the lack of available statistical data; this paper describes two different in-house developed aerodynamic design tools for the PrandtlPlane conceptual aerodynamic design: AEROSTATE, for the design of the box-wing lifting system in cruise condition, and THeLMA, aiming to define the layout of control surfaces and high lift devices. These two tools have been extensively used to explore the feasible space for the aerodynamic design of the box-wing architecture, aiming to define preliminary correlations between performance and design variables, and guidelines to properly initialize the design process. As a result, relevant correlations have been identified between the rear-front wing loading ratio and the performance in cruise condition, and for the rear-front flap deflections and the aeromechanic characteristics in low speed condition.

The combination of new airframes with electric and hybrid-electric propulsion is a potential solution to decarbonize aviation. In this context, recent studies have proven that the box-wing airframe, if integrated on a hybrid-electric aircraft belonging to the regional category, can provide significant reductions in fuel consumption. In light of these promising results, this paper aims to present a broader comparison between the box-wing aircraft and the conventional tube-and-wing aircraft, in the context of regional hybrid-electric air transport. An economic analysis is assessed, and the effects deriving from the box-wing introduction, in terms of direct operating costs, are quantitatively evaluated by applying cost models that consider the integration of hybrid-electric propulsion. In parallel, a comparative analysis of greenhouse emissions is proposed, considering both flight- and production-related emissions. The environmental, economic, and operating improvements that the introduction of the box-wing configuration may provide in the context of future regional hybrid-electric aviation are critically detailed. In this regard, the proposed results show that a box-wing hybrid-electric aircraft can reduce cost and emission without affecting compliance with current airport aprons. Finally, a general summary is presented, providing a solution that represents a practical, incremental, and technological step in the path of commercial aviation decarbonization.

This paper presents an effective simplified model to optimize the mission power management supply for hybrid-electric aircraft in the conceptual design phase. The main aim is to show that, by using simplified representations of the aircraft dynamics, it is possible to achieve reliable results and identify trends useful for early-stage design, avoiding the use of more expensive and advanced methods. This model has been integrated into a multidisciplinary design framework, where the mission analysis, based on a simplified point mass dynamic model, focuses on splitting the power supply between electric and thermal power throughout the flight. An optimization algorithm identifies the time profiles of the supplied power, thermal and electric, to minimize fuel consumption. The power supplied by the thermal engine, modeled as a time piecewise function, is a design variable; a parametric study on the number of intervals composing this function is performed. The framework is used to propose a generalized approach for hybrid-electric power management optimization during the conceptual design iterations. This study showed that, for regional hybrid-electric aircraft, dividing the airborne mission into climb, cruise and descent is sufficient to define the optimum power split supply profiles. This allows for the avoiding of finer mission discretization, or the adoption of more complex simulative models, providing a very efficient model.