# Technology Identification, Evaluation, Selection, and Optimization of a HALE Solar Aircraft
## 4 General Methodology
### Baseline and Alternative Concepts Identification  
There are various subsystems meeting the design requirements in the aircraft conceptual design stage, and comparison analysis is implemented for a large number of combinations. Since the main design requirement for a solar aircraft is long endurance, it is necessary to calculate the required power for a mission and the available power. The main independent design variables necessary for calculating this required power and available power for a solar aircraft are shown in Table 8. As many independent design variables change flexibly in the conceptual design stage as well, calculation should be repeated many times in a specific range in order to find the optimal design conditions [12]. Table 8 shows the major independent design variables and specific ranges of the minimum and maximum values of each variable.  
Since a solar aircraft obtains energy from a solar cell attached to a wing, wing area and aspect ratio are important design variables, and flight speed is also an important design variable to obtain sufficient lift because it operates at high altitude with low atmospheric density. Solar aircraft uses an  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Solar Aircraft** & **Value (kg)** \\ \hline Solar Riser & 124.7 \\ Solair 1 & 200 \\ Gossamer Penguin & 67.7 \\ Solar Challenger & 153 \\ Pathfinder & 252 \\ Solair 2 & 230 \\ Icare 2 & 360 \\ Solarflugzeug & 280 \\ O sole mio & 220 \\ Pathfinder plus & 315 \\ Average & 220.5 \\ \hline \hline \end{tabular}
\end{table}
Table 7: Maximum take-off weight of solar aricrafts.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Design Variables** & **Min** & **Max** & **Unit** \\ \hline Wing area & 20 & 50 & m\({}^{2}\) \\ Aspect ratio & 10 & 30 & – \\ Flight speed & 18 & 26 & m/s \\ Battery energy & 5 & 10 & kWh \\ Battery specific energy density & 100 & 300 & Wh/kg \\ Battery efficiency & 0.9 & 0.95 & – \\ Solar cell efficiency & 0.15 & 0.24 & – \\ Solar cell specific mass & 0.2 & 0.7 & kg/m\({}^{2}\) \\ Solar cell fill factor & 0.78 & 0.8 & – \\ Airframe weight adjustment & 0.5 & 1.2 & – \\ factor & & & \\ Payload weight & 5 & 10 & kg \\ Payload power & 50 & 100 & W \\ \hline \hline \end{tabular}
\end{table}
Table 8: Define the design space.  
electric propulsion system, which uses electric motors and batteries to generate power required for flight. The ultimate goal of solar aircraft, long endurance, depends on the sustainability of night flights. Since the solar energy obtained during the daylight hours is used for daytime flight and the remaining energy is stored in the battery, the main variables of night flight performance are battery energy, battery specific energy density, battery efficiency. Since the main power source is solar energy, the solar cell efficiency, solar cell specific mass, and solar cell fill factor associated with the solar cell are design variables that greatly affect the solar energy calculation. The airframe weight adjustment factor is a coefficient for forcibly adjusting the weight of the structure, and is one of the variables to be considered because the method for calculating the weight of the structure is different depending on the weight estimation formula. In order to overcome over estimating problem, a prediction equation derived from a human-powered aircraft should be modified. This empirical adjustment factor is multiplied to match the structural weight of the human-powered aircraft to the solar aircraft structural weight. Payload weight includes gimbal system equipment such as avionics and cameras, and payload power is an important design variable because it directly affects the flight demand power. Usually, the payload is a given value from customer requirements. However, in this study, we assumed the payload weight can be changed to minimize the total weight.