## 3 Experiment Results  
### Membrane Deformation in Cruise State  
When the solar UAV displayed in Figure 1 flies at the altitude of 0 m, the membrane deformation monitored by the binocular vision camera is portrayed as in Figure 7. The aerodynamic loads on the upper and the lower surfaces are suction and pressure, respectively. Therefore, the upper membrane is convex, and the lower membrane is concave. However, the deformation nephogram demonstrates some phenomena inconsistent with conventional cognition. First, although the front of the upper surface bears a large suction, the membrane demonstrates a slight depression. Second, the pressure in the front of the lower surface is less than that in the rear, but the extreme value of deformation appears in the forward region. Figure 5: Balance structure and installation method: (**a**) Three-component force-measuring balance; (**b**) Installation of balance and wing. Figure 6: Membrane deformation measurement method: (**a**) Deformation-measurement mechanism; (**b**) Deformation measurement points on the membrane. In fact, the membrane deformation is the result of the joint action of two kinds of loads: prestressing force and aerodynamic force. The deformation behavior is also affected by the curvature of the wing surface, and the deformation properties vary from surface configurations. The deformation properties primarily include: (a) when chordal prestress is applied, the curvature of the surface tends to decrease; (b) for convex surfaces, convexity is more difficult than concavity, and the greater the curvature, the more difficulty of convexity; and (c) the farther the surface is from the border, the less difficult it is to deform. With the above deformation properties, the membrane deformation mechanism can be explained. The basic shape of the upper surface is a convex surface, and the curvature of the front part is larger than that of the rear part. Under the action of chordal prestress, the front convex surface appears to trend concave. Conversely, the larger curvature and closer distance from the frame increase the difficulty of convexity. Therefore, although there is a large suction in the front, it is still insufficient to offset the depression caused by prestress. The situation of the rear region is the opposite. Due to its small curvature, the deformation caused by prestress is not obvious. Additionally, smaller curvature and longer distance from the frame make it easier to bulge under distribution load. Therefore, the convexity deformation caused by aerodynamic force occupies a dominant position. From front to back, the lower surface of the wing is convex surface, approximate plane, and concave surface, respectively. The prestress makes the front and rear surfaces concave and convex, respectively. Under the distributed aerodynamic force, the convex surface in the front is easier than the concave surface in the rear, which further increases the concave amount in the front region. Under the combined action of the above two factors, the maximum deformation area moves forward. As demonstrated in Figure 8, the deformation of membrane skin will significantly change the section airfoil curve. Firstly, because the membranes in the upper and lower surfaces are convex and concave, respectively, the camber of the airfoil increases. Secondly, near 30% chord length of the airfoil, the upper and lower surfaces both deform inward, resulting in the reduction of the maximum relative thickness of the airfoil. Concurrently, in the rear area, the protrusion of the upper surface is greater than the depression of the lower surface, so the thickness at the rear region increases. Third, the trailing edge of the airfoil moves upward, resulting in a slight decrease in the angle of attack. Figure 7: Membrane deformation measured by binocular vision system: (**a**) Upper surface; (**b**) Lower surface. As demonstrated in Figure 9, the upper surface has slight depression in the area of 0-32% chord length, and the deformation is less than 1 mm.