Theoretical study on energy performance of a stratospheric solar aircraft with optimum Λ-shaped rotatable wing

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The sun-tracking rotatable wing can be used to improve energy performance of solar aircraft at low sun elevation angles. This work mainly focuses on studying the energy performance of a symmetric Λ-shaped rotatable wing solar aircraft through developing net energy optimization model, which considers the coupling of additional solar energy conversion and extra energy consumption. The derived energy conversion models reveal that, only when the morphing angle is larger than the sun elevation angle, the Λ-shaped solar aircraft is capable of achieving more energy conversion than that of planar wing. The optimum flight attitude control is adjusting its yawing angle to make wingspan axis coincide with the horizontal projection of sunlight. The numerical results of energy performance demonstrate that, with solar cell efficiency of 0.3, the optimum Λ-shaped solar aircraft can achieve 27 kWh more net energy nearby 55°N in winter, corresponding to more than 8 hours increase of flight endurance. Meanwhile, the yearly maximum perpetual flight date at 65°N increases by 34 days. In addition, the Λ-shaped solar aircraft is more effective to improve energy performance with higher conversion efficiency of solar cell. For the solar cell efficiency of 0.45, the net energy is more than 45 kWh and the increment of maximum flight latitude is 10.7° on winter solstice. The results indicate that the optimum Λ-shaped rotatable wing is an effective method to improve energy performance of solar aircrafts.