## 2 Design and Fabrication  
In order to shorten the development process, the design of the ultra-light UAV wing was based on the aerodynamic shape developed for Hirus, an operational UAV from the industrial partner involved in this work. The initial wing structure of Hirus was based on glass fibre fabric embedded into an epoxy resin. In the original design, two composite layers of fibre glass fabric were placed over a layer of aerogel (Rohacell-Evonik Industries AG, Essen, Germany). To compensate for the PV and the addition weight of the wiring, instead of fibre glass fabric, which has a relatively high weight, it was decided that a new composite material based on carbon fibre (Vuhlets(r), Texpack(r) S.r.l. Via Galileo Galilei, 24 25030 Adro (BS) Italy) with a lower weight (40 g/m\({}^{2}\)) and higher resistance would be used. Thus, the initial mass of the wing, which was around 850 g when made of glass fibre, was decreased to 550 g, providing 300 g of additional weight for the photovoltaic cells and wires. Figure 1 presents a schematic transversal section through the UAV's wing, which presents a detailed sandwich structure: carbon fibre fabric, aerogel, carbon fibre fabric. The next step was to establish the best dimensions and technology for photovoltaic cell fabrication. For this step a thorough calculation of efforts and wing bending was conducted. Figure 1: Transversal section through the wing with detailed sandwich structure: carbon fibre fabric, aerogel, carbon fibre fabric. A simplified evaluation of the deformations according to the pressure difference applied on the surface of the photovoltaic element (using average values at ambient temperature [6]) was performed using the calculation theory used at bars on the two main directions of the photovoltaic element (length, width). The following values were used to verify the determinations that were made and to estimate (coarsely) the maximum deformations of the wing on the two directions due to the pressure that was on it during the flight:  
- \(\mathrm{L}=20\) mm, length of the element;  
- \(\mathrm{d}=10\) mm, width of the element;  
- \(\mathrm{g}=0.3\) mm, element thickness;  
- \(\mathrm{E}=160\) GPa, silicon modulus of elasticity (for semiconductors) [7]. The formula used for simulating the wing bending was [1]  
\[y(x)=\frac{w}{24\cdot E\cdot I}\cdot\Big{(}x^{4}-2\cdot D\cdot x^{3}+D^{3}\cdot x \Big{)}.\]  
where:  
- \(y(x)\) represents the deformation of the bar as a function of the position \(x\) along the bar;  
- \(w\) represents the load evenly distributed along the bar;  
- \(E\) is the mode of elasticity;  
- \(I\) is the moment of inertia of the section;  
- \(x\) is the position along the bar;  
- and \(D\) is the length of the bar. Figure 2 presents a graphic representation of the UAV's wing deformation on the two axes. As one can observe, Figure 2 shows the progressive load simulated using the fixed-bar principle to be applied on the wing in order to establish the bending amplitude (on both sides L--length, and d--wide). It can be observed that the maximum deformation of the wing on the length (L) at a pressure of 1 atm lies below 0.6 mm. For a bar supported at the ends and that is uniformly loaded, the maximum deformation (maximum arrow) occurs at half the distance between the ends. (For a wing that is up to 1.5 m wide and with UAV  
Figure 2: UAV’s wing deformation on the two axes. speeds of up to 250 km/h, it was determined that the dimensions of the photovoltaic cells should be L = 20 mm, d = 10 mm.) In order to verify the maximum allowable deformation of the photovoltaic cells, destructive tests were also performed using the MECMESIN MultiTest 2.5 Software-controlled force test system (Newton House, Spring Copse Business Park, Slinfold, West Sus Sussex, United Kingdom), the results of which will be presented further on in Section 3. Following the practical tests, a maximum deformation arrow of 0.8 mm was found to be acceptable in the \"L\" direction, which is part of the calculation of the maximum deformation of the wing.