# Effect of Wing Membrane Material on the Aerodynamic Performance of Flexible Flapping Wing
## 1 Introduction  
Flying insects have drawn the attention of both biologists and engineers for their unsurpassed aerodynamic performance and maneuverability. Unsteady aerodynamic mechanisms of insect wings have been widely and deeply investigated [1, 2], and are responsible for the high lift generation. Recently, with the application of high-speed photography technology, the flapping process of the insect wings has been clearly observed during free-flying movement [3]. The wing deformation due to flexibility, such as bending and twisting, was found to be an obvious phenomenon. Furthermore, wing deformation is prevalent among flying insects like hawkmoths, dragonflies, damselflies, etc. Recently, research on the influence of wing flexibility has shown that the flexible wing presents advantages in energy saving and lift generation [4, 5]. In most insects, the wings' deformations are usually composed of bending, twisting, and camber changes [3, 6]. The distribution of the wing veins makes the flexural stiffness in spanwise and chordwise directions very different [7]. The leading edge with thicker vein and membrane forms higher flexural stiffness along the spanwise direction, indicating that twisting deformation and camber change will be more obvious. According to [PERSON] et al. [8], who analyzed the flow around the deformed dragonfly wing, the camber change reduced flow separation on the outboard wing, thus improving the aerodynamic efficiency. [PERSON] et al. [9] experimentally compared the rigid and flexible wing, and the result shows the flexible wing could increase the lift generated at the end of the stroke by releasingits stored elastic energy in the form of twisting and camber. Numerous studies have been performed on the investigation of the flexible flapping wing and the results have proved that flexibility has advantages in force generation and aerodynamic efficiency [10; 11]. Nevertheless, [PERSON] and [PERSON] [12] suggested that there exists an optimum flapping frequency beyond which the efficiency starts to decrease. In the past few years, the Flapping wing Micro Aerial Vehicles (FWMAVs), which mimic birds and insects, have attracted a lot of attention for their outstanding maneuverability and high efficiency. To design the FWMAVs with higher flight ability and aerodynamic performance, similar to natural flyers, is a persistent pursuit for the researcher. Therefore, numerous efforts have been conducted to design and optimize the flapping wing with higher aerodynamic characteristics, which play a critical role in the vehicles' performance [13; 14; 15]. [PERSON] et al. [16] reported the development and characteristics of artificial wings that can reproduce camber and twisting deformations. [PERSON] et al. [17] performed experimental optimization on the shape of flexible flapping wing with the test parameters including camber angle, aspect ratio, etc. [PERSON] et al. [18] developed several deformable wings with different vein structures, and the results presented potential applications on the FWMAVs. [PERSON] et al. [19] and [PERSON] et al. [20] innovatively proposed the design of a wing with distributed corrugations along the wingspan, while this arrangement would slightly augment the lift performance. Recently, [PERSON] et al. [21] presented an experimental design optimization of a flapping wing via a surrogate-based global optimization method. Significant improvement in the lift efficiency confirmed the successful optimization process. Although there has been considerable progress in the investigation of flexible flapping wing for the FWMAVs, there is still a lack of research on the influence of some flexible parameters, such as the influence of wing membrane materials. In this study, an experiment was conducted to investigate the aerodynamic characteristics of the deformable flapping wing with different flexibility. The flexibility of the flapping wing was adjusted by replacing the wing membrane with three different elastic moduli. A flapping mechanism was developed to generate flapping motion.