## 1 Introduction  
Solar-electrically powered fixed-wing unmanned aerial vehicles (UAVs), regarded as a substitute for satellites, have wide applications in communication, disaster relief, supervisory control, and so on. In order to obtain a larger lift-to-drag ratio and solar panel area, this vehicle usually has a high-aspect ratio wing, which requires a large size and very flexible wing-structure. Because of their strict weight limitations, these UAVs often have extreme structural optimization requirements to balance their deformation and material distribution. With the development of solar cell packaging technology, shown in Figure 1, these solar cell modules have become much lighter and more flexible than before, which also means that these structures may fail before the wing achieves maximum tip displacement owing to local buckling on its compressive side. Thus, it is challenging to both minimize the structure's weight and improve the performance of the structure. Laminated composite structures with unidirectional glass or carbon fiber reinforced polymer (GFRP/CFRP) materials are usually used as a substrate and are connected together by a resin with solar cell modules to improve their strength and stiffness characteristics [1; 2; 3]. Generally, this laminated structure with a large thickness may cause the weight to exceed its limits, while a thinner thickness may reduce its stiffness. Consequently, a large number of cutouts exist in this thin-wall structure as a direct method to save the weight. However, the presence of cutouts will destroy the integrity of the structure, reducing buckling resistance and also possibly generating unpredictable flaws. In order to obtain a lighter weight structure, the traditional optimization method is to optimize the laminate stacking sequence. As the development of automated fiber placement techniques makes curved fiber paths affordable, the concept of variable stiffness laminates with curved fiber paths is proposed. With the development of manufacturing technologies, more efficient structural forms have become available. These new structures offer tremendous opportunities for combination with optimization technologies to obtain satisfactory structural weight reduction. As illustrated in Figure 2, grid-stiffened structures or shell structures preserve a high strength because of their exceptional mechanical properties along the fiber direction and are now widely used in both aerospace engineering and ground transportation vehicles. On the basis of the concept of variable stiffness laminates with curved fiber paths proposed by [PERSON] [4], a new idea for arbitrarily distributed curved stiffener layouts has inspired a new wave of research. [PERSON] made full use of these streamlined characteristic to design a fiber path, and these non-uniform curved grid-stiffened composite (NCGC) structures have achieved a remarkable increase in their buckling load [5; 6]. [PERSON] engaged in extensive research on the shape optimization of curved stiffeners for composite structures [7], while [PERSON] proposed a variable stiffness composite cylinder by fiber steering to maximum the buckling load under pure bending [8]. Beyond the shape or size optimization of the stiffeners or varying the path of the stiffeners' distribution, there are several novel features in this paper's surface design. Firstly, this design offers a more flexible surface. Secondly, the grid-like structure is developed, with more grids added in the high stress area to improve safety margins. Thirdly, this design features a non-uniform load distribution. These particularities have yielded a new concept distinct from current optimization methods and also offer an opportunity to determine new structural types with high efficiency. Figure 1: Flexible solar-powered battery plate. Figure 2: Illustration of grid-stiffened composite structures. (**a**) Grid-stiffened panel. (**b**) Grid-stiffened cylinder. (**c**) Curved grid-stiffened panel. The remainder of the paper is organized as follows.