## 1 Introduction  
In the last few decades, the efficiency of air transportation has become a primary aspect for aeronautical designers, due to the constant growth of the aviation industry throughout the last century and its expected further increase in the near future [1]. In turn, the CO\({}_{2}\) emissions due to the aviation industry are foreseen to grow in the next future--a 75% increment was recorded between 1990 and 2012 and it is projected to grow 300% by 2050 [2]. At this purpose, the international institutions have set quantitative goals for limiting chemical pollution and perceived noise to be fulfilled by the future aviation. Specifically, with respect to the average emissions at 2000, a 75% reduction in CO\({}_{2}\), a 90% reduction in NOx and a 65% one in perceived noise of flying aircraft have been settled for 2050 [3]. As a consequence, two concurring facts are biasing the long-term development of commercial aircraft: the increasing operational constraints to mitigate the environmental impact, and the saturation of the potential development of standard technologies and concepts. These factors impose the identification of breakthrough concepts and highly-innovative technologies to guarantee the air transportation system sustainable development. Most of the solutions currently under analysis present unconventional characteristics in both the aircraft layout and its propulsion system [4; 5; 6]. It is worth noting that, besides environmental issues, financial interests also drive the aviation industry towards unconventional airframe layouts and novel propulsion systems. As a matter of fact, the typical fluctuation of fuel price make the amount of fuel burnt per flight an uncertain parameter, since in the early design phases. Differently from the past where structures and propulsion were seen as two different design threads, the challenge nowadays is given by coupling them into a unique subject. Promising concepts are characterised by hybrid-electric power plant configurations with distributed propulsion and novel aircraft configurations such as the _Blended-Wing-Body_ (BWB). The BWB concept seems to be one of the most promising candidates for the future subsonic jetliners. This layout turns out to have a reduced wetted surface to volume ratio, exploiting a greater useful volume compared to the standard configurations. It is characterised by a large centre body surface which lends itself well to mount distributed electric-driven propellers and _Boundary-Layer-Injection_ (BLI) layouts [4; 7; 8]. Several theoretical studies and experimental campaigns have demonstrated that the BWB aerodynamic efficiency can be considerably improved [9], also due to the reduced interference drag. Accordingly, a remarkable reduction of green-house gases emissions and a substantial abatement of the acoustic impact on the community is foreseen [10]. As a consequence, the BWB layout coupled with the distributed electric-driven propulsion, could lead to strong benefits in terms of fuel efficiency and Direct Operational Costs (DOC). In Reference [11], a general definition of _Distributed-Propulsion_ (DP) can be found, and in particular _Distributed-Electric-Propulsion_ (DEP), which helps the reader to classify these novel architectures. In aviation, DP is used for a propulsion system where the thrust is generated by propeller arrays located on the vehicle. Nevertheless, to be formally classified as DP, the system should have improved efficiency, capabilities, and performance with regards to a classical propulsion system. On the other hand, DEP stands for a particular DP configuration in which multiple devices are powered via an electric transmission system. In some prototypes, the multiple small cores are independently electrically-driven to enhance the vehicle safety (by means of redundant engines) and to allow their use as flight control actuators. Several concepts of DEP exist in the literature [11; 12; 13], but most of them are analysed for their applicability in designing BLI configurations [14; 15].