Solar-Powered UAV Iterative Design Process: A Low-Altitude Demonstrator Toward HAPS

Solar energy provides the only viable path to multi-day endurance for lightweight unmanned aerial vehicles (UAVs). At the system level, the challenge is to close the diurnal energy cycle: solar input during the day must sustain flight and recharge batteries sufficiently to endure the night. Achieving this balance requires a design process in which mass, wing geometry, propulsion, battery capacity, and solar array area are tightly coupled. This paper presents a structured methodology for the conceptual design of solar-powered UAVs. The process begins with a preliminary mass assumption and early geometry definition, linking aspect ratio and chord directly to solar cell tiling. Loiter power is then derived from aerodynamic first principles, which feeds into battery sizing from night-time energy requirements. Daily solar input is evaluated using a simplified irradiance model based on maximum solar elevation and cosine law, serving as a verification check against daytime loads and night recharge. The loop is iterated until mass, geometry, and energy balance converge. The methodology is demonstrated on the Aurora low-altitude demonstrator, designed for continuous flight with 0.5 kg payload, during the 8.5-hour summer nights. The process converges on a seven-kilogram aircraft with a 5.3 m wingspan, showing an 19% daily energy margin. These results confirm feasibility and provide a foundation for future stratospheric high-altitude pseudo-satellite development.