# Modeling and Application of Dynamic Soaring by Unmanned Aerial Vehicle
## 1 Introduction  
The albatross has a remarkable ability to migrate without flapping its wings. The albatross can regain its energy through a unique glide technique by gliding in the horizontal wind gradient. In 1883, [PERSON] first named this flight mode as dynamic soaring [1]. According to research, the albatross is around 10 kg [2, 3]. From the NATO Unmanned Aerial Vehicles (UAVs) classification, the UAVs around this mass are classified as mini UAVs (Class I), which attentions on fight distance and endurance. From the bionics point of view, if UAVs can apply dynamic soaring, it will promote UAVs' flight performance by enhancing the fight distance and endurance. For the application of dynamic soaring, the basic mechanisms of dynamic soaring need to be explored first. The energy harvesting mechanism has been researched since 1925. [PERSON] used a dynamic model to deduce the two-dimension windward climb model and concluded that wind \"allowed to be of some assistance\" [4]. Since then, many researchers followed to deduce the energy harvesting mechanism model with a similar method [5, 6, 7, 8, 9]:  
1. Constructing force equations, \(F=ma\);
2. Constructing and derivate the mechanical energy equation, \(\dot{E}=mV\frac{dV}{dt}+mg\frac{dt}{dt}\);
3. Combine the two equations to obtain the function of energy change rate and force, \(\dot{E}=FV+mg\frac{dt}{dt}\). From the above method, the energy harvesting equation was deduced. This equation explained how energy changed during the dynamic soaring and became the core of the energy harvesting mechanism model. However, in the aspect of the dynamic soaring application, there are still two critical problems: (a) longtime dynamic soaring trajectory optimization and (b) hard to execute dynamic soaring flight tests to verify the feasibility. Pointing at the above problems, researchers did a lot of work on the trajectory optimization and UAVs flight test. The dynamic soaring trajectory optimization is based on optimal control. In the beginning, the optimal time was hundreds of seconds [10]. With the development of the algorithm, the time decreased. In 2014, [PERSON] used GPOPS (a solver based on the Gauss-Pseudo) to optimize dynamic soaring trajectory in 15 s [11]. [PERSON] continues to explore trajectory optimization and narrow the time to seconds [12; 13; 14; 15]. [PERSON] used waypoint navigation to achieve dynamic soaring [16], which gave a new direction to solve the trajectory decision problem. However, the waypoint to establish time was not mentioned in the paper. In the aspect of the dynamic soaring flight test, the most mature flight was in the radio control (RC) glider competition. The RC glider can accelerate to 600 mph by using the wind gradient on the leeward side of the mountain [15; 17]. [PERSON] compared dynamic soaring of human-controlled and auto-controlled methods and concluded that auto-control still needs to be improved to realize the dynamic soaring application [18]. In 2011, [PERSON] gave a dynamic soaring UAV design specification [5]. Similarly, [PERSON] raised a dynamic soaring UAV designing scheme [19]. However, neither of the dynamic soaring UAV designs were tested. In 2015, [PERSON] carried out a preliminary test flight of dynamic soaring using a glider [20]. However, this flight test was based on a simplified simulation program, which differs from the actual situation. In 2016, [PERSON] used a UAV to verify the dynamic soaring trajectory calculated by the optimal control [21], limited by the wind field and site; the flight result was different compared with the optimal trajectory. Recently, [PERSON] used a 6-DOF flight simulation program to verify one type of dynamic soaring: closed-loop trajectory [22]. The research carried out by [PERSON] was closer to reality; the only disadvantage was that it included only one trajectory. Normally, the albatross uses open-loop trajectory dynamic soaring to achieve migration. Regardless of the previous studies having significant contributions to the dynamic soaring research, more detailed work is needed.