# Design of a Hand-Launched Solar-Powered Unmanned Aerial Vehicle (UAV) System for Plateau
## 2 Conceptual Design Method
### Weight Estimation Model  
The total takeoff weight of an aircraft consists of the weight of components, which directly affects the flight power. In order to calculate the flight power accurately, the weight estimation model should be precise as much as possible. The weight of the main components of the UAV includes weight of  
Figure 2: (**a**) Comparison of predicted and measured values of solar irradiance on 27 May 2017, Xi’an, China (34°16′ N 108°54′ E). (**b**) Measuring instrument of solar irradiance. mission payloads \(m_{pld}\) and avionics \(m_{av}\), structural weight \(m_{af}\), weight of solar cells \(m_{sc}\), weight of MPPT \(m_{mppt}\), weight of batteries \(m_{bat}\), and weight of propulsion system \(m_{prop}\). Payload and avionics are fixed weights, which are related to mission requirements and shown in Formula (12). The weight of other components is related to the parameters of UAV size, flight power, etc. \[m_{fixed}=m_{av}+m_{pld} \tag{12}\]  
The weight of aircraft structures is difficult to model accurately. Most structural weight estimation models are empirical formulas, which are based on a large amount of data from existing aircraft. [PERSON] [11], [PERSON] [12], and [PERSON] [13] proposed several weight estimation models for large wingspan, solar-powered aircraft, which used wing area and wingspan as variables to estimate the structural weight of the aircraft. [PERSON] [7] screened the data by wingload and structure weight to modify the parameters of the weight estimation model, which can apply to the small, light-structure, solar-powered UAV (Formula (13)). \[m_{af}=k_{af}AR^{x_{2}}b^{x_{1}} \tag{13}\]  
The power captured by the solar cells is linear with the laid area. The area of the solar cells determines whether the energy captured from sun is enough to balance the energy consumed in flight. The area and weight of solar cells can be calculated based on the energy-power balance, which is shown in Formula (14). \[\left\{ \begin{array}{l}A_{sc}=\frac{1}{\eta_{sc}l_{dbr}l_{mppt}l_{meqt}l_{ lat}}\Big{(}T_{day}+\frac{T_{night}}{\eta_{long}h_{delay}k_{tmp}}\Big{)}P_{ elec\_tot}\\ m_{sc}=A_{sc}(k_{sc}+k_{enc})\end{array} \right. \tag{14}\]  
where \(k_{sc}\) presents the surface density of solar cells. However, exposed solar cells, which are fragile and oxidizing, need to be encapsulated for practical use. Surface density of packaging \(k_{enc}\) also needs to be considered. MPPT is used to track the maximum power by adjusting the voltage of the solar cells, and its weight is proportional to the maximum power, that is, proportional to the area of the solar cells. The weight of MPPT is shown in Formula (15), where \(P_{solar}\) presents the maximum input power of solar cells, obtained by measurement. \[m_{mppt}=k_{mppt}P_{solar}=k_{mppt}I_{\max}\eta_{sc}\eta_{cbr}\eta_{mppt}A_{sc} \tag{15}\]  
For batteries, their weight is proportional to the energy stored and inversely proportional to energy density \(k_{bat}\), which is an important parameter to evaluate battery performance. However, the energy density of the battery is inversely proportional to the discharge power. For solar-powered aircraft that focus on cruise, the constraint of discharge power can be ignored. Therefore, high-energy density batteries, such as lithium-ion batteries, should be selected. For perpetual flight, the requirement of minimum battery weight is that the energy stored by batteries is sufficient for night flight, which is shown in Formula (16). \[m_{bat}=\frac{T_{night}}{\eta_{dcms}k_{bat}}P_{dec\_tot} \tag{16}\]  
Another factor that cannot be ignored that affects battery performance is ambient temperature. For the lithium-ion batteries concerned in this paper, generally speaking, the most suitable operating temperature is 0-40 \({}^{\circ}\)C. When the temperature is lower than 0 \({}^{\circ}\)C or higher than 40 \({}^{\circ}\)C, the battery discharge efficiency will decrease sharply, which means the reduction of battery capacity or the increase of battery weight required for aircraft parameters.