Making use of solar energy to fly is an up-and-coming technology in the human aviation field since solar energy is renewable and inexhaustible, and more and more attention and efforts have been directed to the development of high-altitude solar aircraft (HSA). Due to the technical constraints of the rechargeable battery, the HSA must carry sufficient batteries to meet the flight power consumption at night, which seriously limits the flight endurance of HSA. To solve this contradiction, the paper has proposed a new energy management strategy (EMS) of multiple flight phases for HSA based on the gravitational energy storage and mission altitude, which aims to achieve the goal of long-endurance flight for HSA. The integrated model of this new EMS includes the aerodynamic model, the kinematic model, the solar irradiation model, the battery model, and the energy management model. Compared with the current EMS of level flight, the flight path of HSA in the new EMS has been divided into five phases: the lower altitude level flight at night, the maximum power ascending for mission altitude, the level flight at mission altitude, the maximum power ascending for higher altitude, and the longest gliding endurance. At last, the calculation of the new EMS for Zephyr 7 is studied by MATLAB/Simulink, and the calculation results indicate that about 22.9% of energy surplus can be stored in battery with the new EMS for Zephyr 7 compared with the current EMS, which is equal to reducing the rechargeable battery weight from 16.0 kg to 12.3 kg. Besides, the results of simulation in the four seasons also show that the new EMS is a very promising way to achieve the long-endurance goal for high-altitude HSA when the flight conditions satisfy some constraints like the deficiency of solar flux and the limit of battery mass.

High-altitude long-duration (HALE) flight capability is one of the ultimate goals pursued by human aviation technology, and the high-altitude solar-powered aircraft (SPA) is the most promising technical approach to achieve this target as well as wide application prospects. Due to the particularity of the energy system, the flight strategy optimization through the storage of gravity potential energy and other methods is a significant way to enhance the flight and application abilities for the SPA. In this study, a flight strategy optimization model has been proposed for the aim of HALE flight capability, which is based on the gravity energy reserving and mission altitude in practical engineering applications. This integrated model contains the five flight path phase model, the three-dimensional kinematic model, aerodynamic model, solar irradiation model and energy store and loss model. To solve the optimization problem of three-dimensional flight strategy, the Gauss pseudo-spectral Method (GPM) was employed to establish and calculate the optimal target as its advantages in treating process constraints and terminal constraints for the multiphase optimization problem. At last, the flight trajectory optimization with minimal battery mass for Zephyr 7 was studied by the GPOPS with some function files in MATLAB. The results indicate that the Zephyr 7 can reduce the battery mass from 16 kg to 12.61 kg for the day and night cycle flight and missions, which equals to increasing the battery specific energy by 23.1%. Meanwhile, the optimization results also show that the attitude angel may contribute to increasing the energy gained by photovoltaic cells. In addition, this optimized flight strategy brings the possibility of monthly or annual continuous flight for SPA as the simulation date is set to the autumnal day.

Solar-powered aircraft can perform long-term flights with clean solar energy. However, the energy derived from solar irradiation is influenced by the time of year and latitude, which limits the energy acquisition ability of solar aircraft with a straight-wing configuration. Hence, unconventional configurations based on increasing wing dihedral to track the sun are proposed to improve energy acquisition at high-latitude regions in winter, which may involve power loss caused by mismatch in the photovoltaic system. However, mismatch loss is seldom considered and may cause energy to be overestimated. In this paper, the energy acquisition characteristics of a joint-wing configuration are presented based on the simulation of an energy system to investigate the mismatch power loss. The results indicate a 4~15% deviation from the frequently used estimation method and show that the mismatch loss is influenced by the curved upper surface, the severity of shading and the circuit configuration. Then, the configuration energy acquisition factor is proposed to represent the energy acquisition ability of the joint-wing configuration. Finally, the matching between the aircraft configuration and flight trajectory is analyzed, demonstrating that the solar-powered aircraft with an unconventional wing configuration is more sensitive to the coupling between configuration and trajectory.