## 1 Introduction
The research indicates that the combination of wingtip vortex and detached vortex brings a complex flow field structure and aerodynamic characteristics to the wing. Combined with the deformation and aerodynamic data acquisition methods above, scholars have studied the fluid structure coupling problems of membrane structure in the aviation field. With respect to lift drag characteristics, [PERSON] et al. [12] conducted wind tunnel experiments on the deformed membrane wing. The results indicate that the active deformation makes the aircraft maintain a high lift drag ratio under multiple flight conditions. [PERSON] et al. [19] studied the influence of membrane deformation on the aerodynamic performance of flapping wings by using a three-dimensional numerical simulation method. It was discovered that compared with rigid wings and prestressed membrane wings, highly flexible wings presented the best time-averaged lift-drag ratio. Additionally, some scholars have studied the stall characteristics of membrane wings. [PERSON] et al. [20] studied the membrane airfoil by an experimental method, and quantitatively analyzed the transient behavior of a vortex on the airfoil surface by using an image velocimetry system. The research indicates that the membrane airfoil can automatically adjust the inflow by changing the radian to balance the pressure difference between the upper and lower regions, inhibit the air flow separation on the upper surface, and delay the stall of the airfoil. [PERSON] et al. [21] conducted wind tunnel experiments on a simplified aircraft model equipped with flexible membrane wings. The research indicates that under optimal conditions, the membrane wing achieves a stall delay of 5\({}^{\circ}\) compared with the rigid wing, which is closely related to the flow coupling caused by membrane deformation and vibration. Furthermore, some scholars performed research on the aeroelasticity of membrane wings. [PERSON] [10] established the coupling mathematical model of large deformation membrane structure and three-dimensional flow field, analyzed the coupling aerodynamic characteristics of inflatable airship, and studied the aeroelastic characteristics of stratospheric airship in level flight. [PERSON] et al. [22] performed experimental research on the aerodynamic and static aeroelastic problems of the deformed membrane wing. The research indicates that Young's modulus of the membrane material is the key parameter affecting the aerodynamic characteristics of the membrane wing. [PERSON] et al. [23] studied the effects of membrane materials on the aerodynamic characteristics and deformation process of flexible flapping wings. It was discovered that wings with a higher-elastic modulus membrane could generate more lift but at the cost of more power. Although research has explained the aerodynamic generation mechanism of membrane wings, there are still some deficiencies, mainly including two aspects. Firstly, the existing research on the aerodynamic mechanism of membrane wing is generally performed for small-scale models. For example, the Reynolds number of the research model is generally lower than \(10^{5}\) and the turbulence is less than 0.1%, while the Reynolds number of long-endurance low-speed aircraft in flight is usually \(10^{5}\)-\(10^{6}\), and the real membrane skin, such as flexible photovoltaic skin, is generally rough, which will increase the turbulence. Conversely, the current studies are generally performed for single-membrane wings, which means the thickness of the wing is ignored. However, this kind of wing cannot reflect the real application of long-endurance low-speed UAV. Therefore, this paper performs experimental and numerical research on double-membrane wings with actual size, of which the chord length is 0.91 m. The second chapter introduces the wind tunnel experiment device and highlights the deformation measurement method based on the principle of binocular vision.