# Short Landing for Flying-Wing Unmanned Aircraft with Thrust Vector
## 6 Simulation Verification and Discussion  
### Effectiveness of the Short-Landing Strategy  
The proposed method's effectiveness was evaluated through numerical simulations, which only considered events up to the touchdown, and the simulations stopped once the UAV's landing gear was compressed. Table 4 provides a summary of the control parameters, while Figure 21 displays the three-dimensional (3D) trajectory of a short landing. The use of thrust vector control in the short-landing process allows for a reduction in touchdown speed, which, in turn, reduces the required length of the runway. Without the thrust vector and lift fans, the touchdown speed of the UAV can be calculated [27] by using the lift coefficient in Figure 6b, resulting in a speed of approximately \(46\) m/s. However, with the thrust vector control, the touchdown speed can be reduced to \(35\) m/s, which is \(24\%\) lower than the classic speed. This reduction in touchdown speed results in a corresponding \(24\%\) reduction in the length of the runway, as determined by the integral relationship between velocity and position. As a result, for a given run \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Parameter** & **Explanation** & **Value** & **Parameter** & **Explanation** & **Value** \\ \hline \(k_{p}\) & \(p\) control bandwidth & \(6\) & \(k_{\theta}\) & Pitch angle control gain & \(1.2\) \\ \(k_{q}\) & \(q\) control bandwidth & \(10\) & \(k_{\beta}\) & Side slip angle control gain & \(0.8\) \\ \(k_{r}\) & \(r\) control bandwidth & \(6\) & \(k_{sr}\) & Sink rate control gain & \(0.4\) \\ \(\omega_{o,g}\) & \(p\) observation bandwidth & \(7\) & \(\omega_{o,sr}\) & Sink rate observation bandwidth & \(1\) \\ \(\omega_{o,q}\) & \(q\) observation bandwidth & \(5\) & \(\omega^{r}\) & Velocity control bandwidth & \(0.25\) \\ \(\omega_{o,r}\) & \(r\) observation bandwidth & \(10\) & \(\omega_{y}\) & CTE control bandwidth & \(0.15\) \\ \(k_{\phi}\) & Roll angle control gain & \(1.2\) & \(\omega_{as}\) & Course angle control gain & \(0.5\) \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Inner and outer loop control parameters. Figure 20: Sink rate and bank angle command calculation. way, the short-landing technique allows for the recovery of more payload with the same landing speed as the classic landing process. The simulation results are presented in Figure 22, which demonstrates the effectiveness of the ESO-based angular rate control loops during the short-landing process. Specifically, the three-axis decoupled control achieved a side slip angle under 1.2 degrees, resulting in a coordinated turn in Figure 22d,h. The sink rate during the switch to short-landing mode was approximately \(-1\) m/s, as shown in Figure 22e. During Stage I, the nozzle pitch angle was increased to 45 degrees, and the throttle command was set to 0.45, as depicted in Figure 22l,n. As the nozzle angle decreased, the rotation speed increased to 8000 rpm to balance the moment produced by the vectored thrust. In Stage II, which began when the nozzle angle reached 45 degrees, the throttle was fixed, and the nozzle pitch angle was adjusted to reduce the speed. Figure 22l shows that the nozzle pitch angle was increased to 90 degrees in Stage II to reduce the speed from 60 m/s to 35 m/s. During both Stage I and Stage II, the pitch attitude angle controlled the sink rate, but the sink rate was still affected by changes in the bank angle, as shown in Figure 22e. Next, the short-landing process entered Stage III, where the nozzle-pitch angle was used to maintain a constant velocity, the throttle was used to control the sink rate, and a combination of the elevator and the lift fan controlled the angle of attack. The baseline angle-of-attack command value was 6 degrees, and Figure 22b shows that there was no steady-state error in tracking. When the sink-rate control mode was switched to the ESO-based control in Stage III, the sink-rate controller performed well in terms of its tracking performance.