# Technology Identification, Evaluation, Selection, and Optimization of a HALE Solar Aircraft
## 4 General Methodology
### Problem Definition  
The first stage of TIES is to define the problem in accordance with the design requirements of the HALE solar aircraft. Since the degree of initial design requirement in the conceptual design phase is found based on the analysis of operation and mission requirements, most of this part is determined by considering the mission analysis. Since the main design requirement for a HALE solar aircraft is long endurance, design parameters such as max take-off weight and wing loading are the primary components. Such design parameters are not fixed in the conceptual design stage but changeable within a specific range until the design requirements are determined [25]. The parameters meeting the requirements are called the design objective parameters. In this study, design objective parameters and specific ranges are set as shown in Table 5.  
Wing loading is the most important design objective parameter, and the flight speed should be able to exceed the wind speed at the cruising altitude. In consideration of wind speed, the minimum wing loading at about 20 km altitude is approximately 15 N/m\({}^{2}\). The range of wing loading for solar aircraft mission implementation at high altitudes is 15\(\sim\)30 N/m\({}^{2}\)[26].  
The ratio of power to weight is one of the most important factors determining the aircraft performance, together with the wing loading. This design variable has an important impact, particularly on engine and power selection. In this study, as shown in Table 6, the ratio of power to weight ratio for cruise of another solar aircraft was employed. The average values of developed HALE solar aircraft were set as the target values [27]. In this paper, the required power for flight was calculated using the SI unit system, Watt. However, data of the power to weight in other papers were hp/kg so that we converted the unit using the following equation.  
\[1\;\frac{\text{W}}{\text{kg}}\;=\;\frac{1}{745.7}\;\frac{\text{hp}}{\text{kg}} \tag{25}\]  
The maximum take-off weight is one of the factors influencing not only the aircraft performance but also the cost analysis performed using weight data [12]. In this study, 200 kg, the average maximum take-off weight of conventional solar aircraft as shown in Table 7, was used as an initial assumption value.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Solar Aircraft** & **Value** & **Unit** \\ \hline Pathfinder & 0.034 & \\ Pathfinder plus & 0.042 & hp/kg \\ Centrion & 0.036 & \\ Zephyr & 0.045 & \\ \hline \hline \end{tabular}
\end{table}
Table 6: Power to weight ratios for certain solar aircrafts.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Design Objective Parameter** & **Target Value** & **Unit** \\ \hline Wing loading & \(\leq\)30 & N/m\({}^{2}\) \\ Power to weight & \(\leq\)0.04 & hp/kg \\ Maximum take-off weight & \(\leq\)200 & kg \\ Lift to drag ratio & \(\geq\)35 & – \\ \hline \hline \end{tabular}
\end{table}
Table 5: Design objective parameters and target values.  
Lift-to-drag ratio is a significant indicator representing the performance of a solar aircraft performing a mission at high altitude. The appearance of a HALE aircraft is like a glider or flying wing; this shape is chosen to secure sufficient wing area to generate lift. In the case of a glider form aircraft, the higher the lift-to-drag ratio, the smaller the glide angle and the farther the distance to reach the same altitude. General aviation (GA) aircraft have lift-to-drag ratios of 10-20 in their cruise state. The long distance air vehicle of Virgin Atlantic, the Global Flyer, has a lift-to-drag ratio of 37. Therefore, in this study, the target lift-to-drag ratio was set at 35.