# Design and Verification of Large-Scaled Flapping Wings for High Altitude Environment
## 1 Introduction  
China has a vast and complex territory, where plateau areas account for about 26% of the total territory, mainly covering the frontier areas of key defense, where the environment is harsh and the airflow is unstable. At present, fixed-wing and rotary-wing UAVs have problems such as low energy utilization, poor mobility, and weak stealth capability, while bionic flapping-wing vehicles can actively adapt to and make use of different airflow environments. It has great development potential in terms of flight efficiency, mobility, and bionic concealment [1; 2]. It can be used for border patrol, rescue and disaster relief, biodiversity exploration, and so on. As one of the core components of the plateau bionic flapping-wing vehicle, the performance of flapping wings greatly affects the flight properties and determines the flight performance of the vehicles. For flapping-wing vehicles, different structures and materials of the wings can result in significant variation in the aerodynamic performance of the flapping wings. However, as we are limited by the backwardness of materials, actuators, propulsion systems, etc., directly replicating the kinematics and morphology of biological wings is far from sufficient to manufacture a flapping-wing vehicle with high lift and low power consumption. Therefore, the flapping-wing design, inspired by flying creatures, can only be used as a preliminary conceptual design, and the geometric parameters of the wing need to be further optimized, including wing shape, aspect ratio, root tip ratio, and other parameters. In the past, most of the research on flapping-wing aircraft have focused on the small and micro scale fields. Although such flapping-wing vehicles have high lift efficiency andcan perform missions in confined spaces, they have the limitations of poor load capacity and short flight distance. Therefore, large-scaled flapping-wing vehicles can meet the mission requirements for efficient low-speed cruise flights and far-range long-endurance flights. Among the flapping-wing vehicles with good flight performance that imitate birds of medium and large scale, \"Smartbird\" of the Festo company in Germany [3], \"RoboRaven\" of the University of Maryland [4; 5], and \"HIT-Hawk\" of Harbin Institute of Technology [6] are outstanding. \"SmartBird\" has a wingspan of 2 m and a weight of 450 g; \"RoboRaven\" has a wingspan of 1.168 m and a weight of 290 g; and \"HIT-Hawk\" has a wingspan of 1.1 m and a weight of 455 g. \"RoboRaven\" adopts a single-stage flexible film flapping wing. The wing material is composed of polyester film and carbon fiber rods. The flapping wing with optimal aerodynamic performance is selected by adjusting the planform and iterating wind tunnel experiments. \"HIT-Hawk\" still adopts a single-stage flapping wing. The wing is composed of kite cloth and mast. Similarly, through experimental iteration, the wing film material, layout form, and planform of the wing are optimized. The flapping wing is the main feature that distinguishes flapping-wing vehicles from other types of vehicles. The flapping wing provides all of the thrust and most of the lift of the vehicle. The aerodynamic characteristics of flapping wings determine the performance of the vehicle to a great extent. Although the existing flapping-wing designs and structures vary greatly, the single-stage flexible film flapping wing is adopted by a large number of flapping-wing vehicle prototypes because of its simple structure, light weight, and low requirements for a driving system. Meanwhile, this structural form of flapping wing has been proved to be the most practical configuration through the successful flight of a large number of flapping-wing vehicles. Due to the limitation of the accuracy of theoretical knowledge and the limitations of numerical simulation, the design of flapping-wing vehicles has always been mainly carried out through experimental iteration.