# A Conceptual Design Methodology for e-VTOL Aircraft for Urban Air Mobility
## 2 Design Methodology  
The workflow developed for the conceptual design of e-VTOL aircraft is depicted in Figure 2. It takes all the main aspects of the aircraft design (e.g., aerodynamics, mission analysis, structural design, propulsion sizing) into account; in particular, the propulsion sizing block has been developed specifically to deal with electric propulsion. Accordingly, five main blocks can be identified:  
1. _TLARs and design parameters_: it defines Top-Level Aircraft Requirements (TLARs) and main design variables;
2. _Aerodynamics_: it estimates aircraft drag polar by using a Vortex Lattice Method (AVL [57]) and component build-up method [49,58];
3. _Propulsion system_: it calculates the power supplied by the propulsion system by means of the matching chart [58,59];
4. _Mission performance: it evaluates aircraft performance by using flight mechanics equations_;
5. _Weight estimation_: it estimates mass breakdown of the configuration by using NASA weight estimation for the general aviation category [60] and literature models for electrical components (e.g., battery mass). The proposed workflow can be applied to VTOL aircraft of different typologies: tilting, tilttor, \"lift + cruise\" and multirotors. Moreover, it allows to design aircraft with different architectures, such as the conventional wing-tail or the innovative box-wing one. This latter has two staggered main horizontal wings connected by lateral wing tips and can be provided with VTOL capabilities by implementing a tilt-wing mechanism. Therefore, in far-to-ground operations, the aircraft moves horizontally in \"fixed-wing\" mode (Figure 3b), whereas in close-to-ground operations it tilts the wings to hover and move vertically, sustained by the thrust generated by the propellers (\"multi-rotor mode\", Figure 3a). Hereafter, this e-VTOL box-wing aircraft is simply referred as _TiltOne_. Figure 2: Conceptual design workflow for e-VTOL aircraft. It is worth noting that the _TiltOne_, in fixed-wing mode, can exploit the aerodynamic behaviour of the box-wing architecture, artistically represented in Figure 3, also called \"PrandtlPlane\". The box-wing architecture, for the first time applied in 1907 by the pioneer [PERSON] on his prototype \"14 bis\", inspired the studies carried out in the 1920s by [PERSON] [61], who first indicated it as the \"best wing system\", i.e., a lifting system capable to minimize the induced drag for given lift and wingspan. Following studies have provided an exact mathematical solution to describe the optimal lift distribution [62] (Figure 4a), hence providing a theoretical support to the aircraft design activities. An example of PrandtlPlane is depicted in Figure 4b, which refers to the case of the two-seater amphibian from the Italian project \"IDINTOS\". As shown, the PrandtlPlane has a tailless box-wing architecture with a significant negative stagger (or forward, although less common) [63], which allows the use of the rearmost wing both as lifting surface and horizontal stabilizer. This change allows the architecture to reduce the induced drag but introduces the need for accurate studies on static and dynamic flight stability, as well as new approaches for the structural design, the box-wing being a hyperstatic system. As detailed in the following, the approach adopted for an initial investigation on the design of a tilt-wing aircraft is based on two main design parameters: the wing loading _W/S_ and the propellers' disk-loading _T/A_, defined as in Equations (1) and (2), respectively. \[\frac{W}{S}=W_{\text{aircraft}}/S_{ref} \tag{1}\]  
\[\frac{T}{A}=T_{propeller}/A_{propeller} \tag{2}\]  
Figure 4: Example of box-wing architecture: (**a**) Lift distribution of the “best wing system” [62]; (**b**) Views of the 2-seater amphibious PrandtlPlane “IDINTOS” [64,65]. Figure 3: Artistic views of the _TiltOne_: (**a**) hovering and vertical flight mode (or “multi-rotor” mode); (**b**) horizontal flight mode (or “fixed-wing” mode).