Solar-Regenerative High-Altitude Long-Endurance (SR-HALE) aircraft are designed to sustain year-round flight at high altitudes indefinitely. No SR-HALE aircraft has yet accomplished this task due to the complex network of environmental, solar, structural, and aerodynamic trade-offs, among which aircraft flexibility plays a key role. A comprehensive SR-HALE aircraft multidisciplinary design optimization framework is developed in which the flexible aircraft analysis tool ASWING is incorporated in order to constrain nonlinear aeroelasticity. Energy, battery, ply thickness, material failure, local buckling, aerodynamic stall, longitudinal stability, and general stability (including nutter) constraints are applied in order to reasonably constrain the optimized SR-HALE aircraft design. An SR-HALE aircraft design with a span length of 60.15 m and a total aircraft weight of 432.2 kg is found which fulfills all SR-HALE mission requirements and minimizes aircraft mass. A further 21% reduction in total aircraft mass is found through the use of high modulus carbon fiber reinforced polymer. Significant decreases in aircraft mass, down to a total aircraft mass of 250.6 kg, are found to be possible if altitude requirements for SR-HALE aircraft are lowered from 18,288 m to 16,764 m. A feasible SR-HALE aircraft with a mass of 357.9 kg was also found to be possible if battery specific energies of 360 Whkg~(-1) are developed.
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