# Design and Verification of Large-Scaled Flapping Wings for High Altitude Environment
## 1 Introduction
Based on the observation of flying organisms, high altitudes present various physiological challenges to organisms, including respiration, circulation, temperature regulation, and so on. [PERSON] and [PERSON] et al. [7; 8; 9] pointed out that low-density conditions increase oxygen delivery to flight muscles when organisms fly at high altitudes to compensate for their reduced atmospheric partial pressures. They also stated that the low-density and low-oxygen content of the air will increase metabolic demand as well as create a reduced ability to generate aerodynamic force, limiting the ability to move at high altitudes. During their annual migration, birds select their flight altitudes according to certain criteria, such as minimum energy consumption and favorable wind gusts [10]. Different altitudes will bring about drastic changes in the aerodynamics and biological behavior of birds. The aerodynamic performance of aircraft depends heavily on the surrounding environment, with the lift and drag coefficient directly proportional to air density [11]. Air density decreases with an increase in altitude, thus reducing the air drag that impedes the movement of birds and reduces the lift force required for flight. Therefore, some birds can adjust their biological behavior according to the physical properties of the air, varying with altitude [12; 13]. For example, when birds are in low-density environments at high altitudes, their flapping frequency, amplitude, and speed will increase to provide sufficient lift [14; 15; 16]. The focus of these experiments was to study the physiological adaptation of birds to altitudes while discarding the aerodynamic changes caused by changes in the physical properties of the air. Even today, measuring the aerodynamic coefficients of birds floating in the air is still a challenge, and the evolution of drag and lift is estimated through theoretical mathematical models and flapping-wing kinematics analysis. Numerical simulation and more accurate computational fluid dynamics (CFD) have been proven to be effective methods for studying the aerodynamics of fixed or flapping wings [17; 18]. Most of the CFD studies of flapping wings have focused on the geometrical parameters of the wing [19], others on the influence of the ceiling [20] or the ground [21], but few seem to have addressed the influence of altitude on the aerodynamics of a flapping wing. [PERSON] et al. [22] established the design space of the high-altitude fluid-structure coupling effect of flapping wings, with atmospheric density and the stroke plane angle as design variables. The results show that the lift increases with the increment of the stroke plane angle and air density, while the net thrust remains close to zero. The required averaged power decreases with increasing altitude and eventually becomes negative at 3000 m above sea level. They also investigated the influence of vein structures on the fluid-structure interaction of flexible flapping wings at high altitudes [23]. The paper proposed that the lift and thrust improved with the increment of wing stiffness and stroke plane angle. For a more flexible wing, the force depends on the air density and decreases with an increase in altitude; additionally, the power consumption increases with increasing stiffness. [PERSON] et al. [24] studied the aerodynamic effects of flapping amplitude, frequency, and airspeed on rigid Canada geese wings at different altitudes. The results showed that an increase in speed caused higher energy consumption. The 20% increase in amplitude increased the lift-to-drag ratio by about 22%; a 25% increase in frequency increased the lift-to-drag ratio by 158%. In previous studies, there has been a lack of research on the impact of altitude on flying animals, and the relationship between the flapping-wing design and altitude has not been considered. Wind tunnel experimental techniques, CFD, and flight tests were used to investigate and verify the aerodynamic performance of flapping wings.