## 2 UAV Dynamics Model
### Analysis of Downward Performance and Design of Steep Descent Control Strategy
(2) Analysis of the characteristics of drone glide envelopes  
Find the boundary points for the stable glide of the drone, and draw the velocity glide angle envelope, velocity pitch angle envelope, and pitch angle throttle envelope of the drone glide process below a flight speed of 15 m/s, as shown in Figure 22.  
As the flight speed increases, both the glide angle range and pitch angle range of the drone first increase and then decrease. If the drone is required to meet the task of steep descent with a large pitch angle, its glide angle should not be less than 10\({}^{\circ}\), and the drone's glide flight speed should be considered within the range of 6.3 m/s to 10.3 m/s. As the pitch angle of the drone increases, the available range of its throttle first increases and then decreases; when the pitch angle exceeds 40\({}^{\circ}\), the available range of the throttle sharply decreases. The larger pitch angle corresponds to a smaller flight speed with the corresponding glide state. Based on the pitch angle velocity envelope mentioned earlier, it is recommended that the descent speed of the drone should not be less than 7.5 m/s. In summary, based on the comparative analysis of the velocity glide angle envelope, velocity pitch angle envelope, and pitch angle throttle envelope during the descent process of unmanned aerial vehicles, it is recommended that the range of descent velocity for unmanned aerial vehicles be between 7.5 m/s and 10.3 m/s. (3) Design of Control Strategy for Unmanned Aerial Vehicle Downward Landing  
In order to meet the mission scenario requirements of drones, it is necessary to design landing trajectories with low energy consumption and short forward flight distance. The drone mission package includes takeoff, climb, cruise, descent, and steep descent. Assuming that the drone completes the takeoff climb task, it performs cruise flight at an altitude of 500 m and a speed of 15 m/s. When receiving the landing command, the drone needs to slow down from the current altitude until the landing task is completed. Below is an analysis of the descent rate, power, and process energy consumption of unmanned aerial  
Figure 22: Stable sliding balance envelope. vehicles at different climb angles and speeds, in order to seek the optimal landing trajectory, as shown in Figure 23.  
From the analysis of the above figures, it can be seen that the maximum sinking rate of the drone can reach around 6.5 m/s. As the sliding angle and flight speed decrease, the sinking rate of the drone decreases accordingly. Based on the performance and mission scenario requirements of the unmanned aerial vehicle in this article, the designed sinking rate during approach landing should not exceed 3 m/s. The output power of the drone motor and the total energy output of the descent stage motor both decrease correspondingly with increasing speed and descent angle. Based on the above simulation results, this paper proposes an optimal steep descent strategy, which divides the UAV steep descent process into two stages:  
1 Low-power rapid decline stage. The main task of drones in this stage is to achieve the shortest distance altitude reduction with minimal energy consumption, that is, to seek the maximum sinking rate and minimum energy consumption decline state of drones within the allowable speed range. Assuming the starting altitude of the drone is 500 m and the landing target altitude in this stage is 50 m, the drone is designed to steadily glide at a \(-40^{\circ}\) glide angle, a flight speed of 8.5 m/s, and a sinking rate of 5.46 m/s. The state point has a balanced pitch angle of 18.8\({}^{\circ}\), a balanced angle of attack of 58.8\({}^{\circ}\), a balanced elevator deflection angle of \(-32.8^{\circ}\), and a balanced power of 662.6 W.  
2 High-power and low-sinking-rate landing phase. The main task of the drone in this stage is to achieve a minimum ground speed landing, which requires the drone to gradually reduce its flight speed with decreasing altitude and land at a sinking rate of nearly 0 m/s.