## 1 Introduction
### Context of the Work and Literature Results  
The results recently discussed in [29] are the initial pivot for the development of the research work illustrated in this paper. In [29], a multidisciplinary design and optimization methodology for regional hybrid-electric aircraft with BW and TW configurations is first described, the results obtained from the application of this methodology are proposed and commented on, and the performance comparison between the two different lifting architectures is critically discussed. The main result that emerges is that the BW architecture introduces structural weight reductions compared to the corresponding TW with the same maximum take-off weight, together with an increase in aerodynamic performance in terms of classical lift-to-drag ratio. The main results in this regard, referring to a population of about \(10^{3}\) configurations of hybrid-electric TW and BW sizes, varying the maximum takeoff weight (MTOW) and the on-board power utilization, are reported in Figure 1. In particular, Figure 1 represents the statistical analysis of the numerical results obtained from the sensitivity study involving the design of hybrid-electric TW and BW. This representation allows the designer to extract general trends, hence, to identify the main differences between the two possible architectures. The left part of Figure 1 shows the average lifting system structural weight \(m_{w}\) reductions achievable by properly integrating the BW architecture, whereas the right part of Figure 1 reports the differences in terms of lift-to-drag ratio \(L/D\) at the design point. For the accurate and detailed explanation of these results, the reader can refer to [29]. In fact, reductions in \(m_{w}\) introduce the most significant performance gains for BW architecture. In particular, these reductions are achieved as the BW lifting system allows the redistribution of the lifting load on two different wings, which also have reduced wingspan than the corresponding TW and are over-constrained to the fuselage structure. These features provide lower bending stress for the wings, enabling the design of lighter wing box. Furthermore, BW does not need an additional lifting surface (i.e., the horizontal stabilizer) for longitudinal stability and control functions, as these are intrinsically provided by a proper design and arrangement of the two horizontal wings. The lower structural mass allows the BW to load a larger amount of battery onboard, if the same MTOW of the corresponding TW configurations is considered. This aspect enables a larger utilization of the electric power during the aircraft mission, providing both a reduction in thermal power request and an increase in the overall propulsive efficiency [29]. Such that feature, together with the average higher cruise lift-to-drag ratio, makes the BW configuration more energy-efficient than the corresponding TW. Overall, these benefits translate into substantial reductions in block fuel consumption \(m_{fb}\) (i.e., the fuel needed for the standard mission, without considering the amount for diversion and reserve) for the different hybrid-electric configurations presented in [29], as illustrated in Figure 2.  
Fuel consumption gains are for sure one of the key aspects in aircraft optimization (i.e., the set of mathematical methodologies that aims at optimizing aircraft configuration in terms of aerodynamics, structure, and performance) when introducing new, even disruptive, technological elements such as hybrid-electric propulsion and unconventional lifting configurations. The performance analysis should not be limited to the evaluation of only this key figure of merit (FoM) but should expand into a more general assessment of the overall impact that the introduction of innovative aircraft can have, even at a conceptual level. Specifically, how do the technical/performance advantages reflect in terms of environmental impact, emissions, and climate-changing effects? Are there trade-offs to be accepted between environmental and economic metrics?