# Analysis of Motion Characteristics of Bionic Morphing Wing Based on Sarrus Linkages
## 1 Introduction  
The bionic morphing wing studied in this paper was suitable for an underwater-aerial transmedia aircraft [1]. The underwater-aerial transmedia aircraft was designed in imitation of a kingfisher's posture, and its corresponding relationship of aerodynamic layout is shown in Figure 1. Figure 1a-d show four typical postures of kingfisher when entering and leaving water. According to the special requirements of large lift force in the air, small resistance in entering water, lowered posture in water, and lift force in coming out of water, our research group designed the stances of aircraft corresponding to kingfisher's flight posture, as shown in Figure 1e-h. The body shape of the aircraft is the streamlined shape of the smooth kingfisher body. The body length was enlarged in proportion to the kingfisher body (the ratio is 1:3.45), and the wingspan was appropriately increased relative to the kingfisher wings to meet the requirements of lift. The morphing wing is the main form required to realize the variant of the aircraft. A reasonable morphing wing can make the aircraft have better aerodynamic and hydrodynamic performance, improve the adaptability of the aircraft, and increase the endurance of the aircraft [2]. After years of rapid development, there are many types of morphing wings. According to the deformation principle and characteristics, they can be divided into the following categories: telescopic wing, swept-back wing, folding wing, and flexible intelligent variant wing [3, 4]. Different morphing wings have their own advantages, but also have their own difficulties and disadvantages. In the extended state, the retractable wing can play the role of endurance and drag reduction and has good aerodynamic performance. In the retracted state, although the aerodynamic performance is poor, it has better maneuverability. However, the expansion mode will change the position of the aircraft's center of gravity and will increase the size of the aircraft, which is not conducive to underwater navigation [5; 6]. The swept wing is a morphing wing that can change its sweep angle. When flying in the air, it can use a larger sweep angle to enhance aerodynamic performance, and when entering water, it can use a smaller sweep angle to make the wingtip close to the fuselage to obtain smaller wingspan and contact area, which can effectively reduce the impact of entering water and the resistance of diving in water. The structure is simple and easy to control. However, larger driving torque is needed at the wing root hinge to achieve the variant, and the wing as a whole is not easy to integrate into the fuselage, so there will still be greater resistance [7; 8; 9]. The folding mechanisms used in folding-wing vehicles are mainly: linkage mechanism, gear mechanism, and torsion spring-pulley mechanism. In order to improve the flight performance of collapsible aircraft, [PERSON], [PERSON] et al. [10] designed a novel mechanism of bionic foldable wings similar to a beetle's based on the four-plate mechanism theory. Inspired by the beetles, [PERSON], [PERSON] et al. [11] designed a new type of bionic folding wing for the flapping-wing micro air vehicle (MAV). [PERSON], [PERSON] et al. [12] designed two wing mechanisms based on a four-bar linkage structure: one is only for flapping motion (FM), and the other is for simultaneous flapping and folding motion (FFM) during a wing stroke. [PERSON] et al. [13] develop an artificial foldable wing that mimics the hind wing of a beetle. This paper proposed a three-stage lateral-folding morphing wing based on Sarrus linkages. The morphing wing is unfolded when flying in the air, with good aerodynamic performance. The wing is folded when entering water and diving, which can effectively reduce the impact force and resistance of water flow. However, large driving torque is needed to drive the folding mechanism, and the impact force sustained at the moment of entering water may directly damage the hinge, which is the most vulnerable part of the whole mechanism [14; 15; 16].