# A Conceptual Design Methodology for e-VTOL Aircraft for Urban Air Mobility
###### Abstract
Many companies (e.g., AIRBUS, Lilium, Joby Aviation) are developing several configurations to satisfy the above requirements [21; 22]; among them, two main categories can be recognized: rotary wing and fixed wing aircraft. Multi-rotors (Figure 1d) or helicopters belong to the first category; tilt-wing (Figure 1a), tilt-rotor (Figure 1c) and \"lift + cruise\" (Figure 1b) to the second one [23; 24]. Figure 1: Different e-VTOL configurations developed from 2016: (**a**) Tilt-Wing (T-W) [25]; (**b**) Lift+Cruise (L+C) [26]; (**c**) Tilt-Rotor (T-R) [27]; (**d**) Multi-Rotor (M-R) [28]. Multi-rotors (M-R) are equipped with three or more propellers, which generate thrust and sustain the vehicle during hovering and forward flight. Tilt-rotor (T-R) and tilt-wing (T-W), also called convertiplanes, mix the capability of rotary-wing and fixed-wing aircraft: in forward flight they are similar to a propeller airplane (achieving higher range and speed with respect to M-R), whereas to take-off/land they are similar to rotary-wing aircraft. Tilt-rotor aircraft perform a rotation of the propulsors to switch from fixed-wing to the rotary-wing layout (and vice versa); in tilt-wing, instead, lifting surfaces and propulsors are integral components, hence they rotate together during the transition. In such case, the movable lifting surface can be only a portion of an entire wing and it can be connected to one or more propulsors. A \"lift + cruise\" (L + C) e-VTOL is an aircraft which exploits some propellers for take-off and landing and other ones to generate thrust during cruise. The main advantages and drawbacks of each configuration are detailed in Table 1, in which the \"lift + cruise\" case has been used as baseline, since it combines some features of the multi-rotor with other of the convertiplanes. If compared to convertiplanes, the \"lift + cruise\" solution allows to reduce the complexity [24], but shows some penalties in terms of cruise efficiency, because of the additional drag introduced by the rotors used in the hovering phase [29]. In terms of internal comfort, the inclination required by multi-rotors to accelerate and decelerate during the transition phases can be a disadvantage, whereas tilt-wing/rotor can have a better response since the fuselage can be kept in a horizontal position during the transitions. For more details, the reader can refer to [24]. To enable UAM, several challenges concerning infrastructure requirements, regulation, air traffic management and public acceptance must be addressed. Specific ground infrastructures, designed for VTOL operations and sometimes called \"vertiports\", are necessary to operate the air taxis [24]. As described in [18; 31], even replacing the existing helicports with vertiports, the ground infrastructures would not be sufficient to meet the envisaged market demand [18], so new vertiports should find their place in the urban context. Currently, the location of new vertiports is an open problem, which needs to be further investigated [31]. Moreover, the readaptation of existing helicports must guarantee safety during take-off and landing phase if multiple departure/arrivals are expected [18]. Starting from helicopter regulations, aviation safety authorities, such as EASA in Europe and FAA in the United States, have started drafting the definition of e-VTOL air taxis certification rules. EASA has produced a first draft in 2019 [32], defining some general requirements such as maximum take-off weight of 3175 kg and maximum number of passengers equal to nine. Recently, a means of compliance has been published where more regulatory aspects are detailed [33]. FAA, together with NASA and stakeholders, has released a first draft defining the main indications for air traffic management [34]. Several manufacturers, such as Airbus, Boeing-Aurora and eHANG, have the long term goal of introducing air taxis capable to fly autonomously (i.e., without the presence of a pilot onboard), but it is more likely to assume that in an initial phase they will have a pilot on-board.