## 5 Outdoor Experiment of the Quadrotor with Soft Landing Gears  
To further test the proposed control approach performance in the outdoor experimental situation. The system shown in Figure 9 is proposed to meet the requirements of non-flat surface landing tasks. The onboard devices include three units: The Flight Unit (FU), the Computing Unit (CU), and the Landing Gear (LG). \begin{table}
\begin{tabular}{c c} \hline RMSE Horizontal \(X\),\(Y\) [m] & 0.03 \\ \hline RMSE Vertical \(Z\) [m] & 0.02 \\ \hline \end{tabular}
\end{table} 
Table 4: Simulation landing position error. Figure 8: Simulation curves of the quadrotor states in hovering stable tests. The time-consuming complex tasks, such as path planning, GPS waypoint receiving, and manual switch engaging, are performed on the CU, which has an onboard processor. The position and altitude control are made on the FU. The FU receives speed values, hovering requirements, and landing tasks, sent from either the CU or from a human pilot equipped with remote controller (RC). The integrated systems of the quadrotor helicopter are listed as follows:  
* Flight System **Autopilot devices**: The onboard flight assistant device is a Pixhack-V5 flight autopilot board. The board is based on the Pixhawk open hardware design; it runs PX4 on the NuttX OS and is fully compatible with the PX4 firmware [38,39]. **Airframe**: The structure of the quadrotor cross-shaped frame was built using polyvinyl chloride (PVC). The quadrotor UAV is equipped with four brushless DC motors and four Electronic Speed Controllers (ESCs). **Attitude and altitude control**: The 3D position is estimated by jointly using IMU and GPS. **Communication Protocol**: The communication link used onboard is the Mavlink protocol. The IMU and the onboard processor use this protocol to send command data [40]. The UAV and GCS are connected through a universal asynchronous receiver/transmitter (UART). * Landing System **Control devices**: A PDMS-based four-finger soft landing gear mounted on the bottom the quadrotor body. To evaluate the soft landing gear performance, twenty real field experiments were carried out in an open-space test field, as shown in Figure 10. The flight tests were performed under different weather conditions. The external wind speed was below 5 m/s. To date, the quadrotor helicopter with soft landing gear has successfully achieved stable landing on non-flat surfaces by using the proposed control approach. During the tests in open space, there was always one pilot inspecting the state of the drone and one ground crew member monitoring the flight testing field. The ground crew was in charge of the  
Figure 9: System structure of the quadrotor helicopter and the soft landing gear for the outdoor experiments. GCS. A human pilot took control over autopilot unit using the RC transmitter. Both personnel haD the option to send a full shutdown command to switch off all rotors immediately. During the experiments, the quadrotor helicopter attempted to land on a non-flat surface, as shown in Figure 10, which is common in real-world situations. In each of these 20 experiments, the propsosed quadrotor helicopter successfully landed on a non-flat surface without rolling over. Figure 11 displays the velocity, attitude estimation, and altitude over time in the landing process. Table 5 shows the RMSE of the touchdown position related to the desired position, where the vertical errors were generated by the quadrotor speeds decreasing to zero before solid contact with the surface. Impact absorbing tests were implemented by controlling the quadrotor landing with different descent speeds (0.5 m/s, 1 m/s, 1.5 m/s, and 2 m/s). The relative landing accelerations were obtained by the Pixhawk accelerometer. The peak accelerations during the landing impacts were 21 m/s\({}^{2}\), 47 m/s\({}^{2}\), 96 m/s\({}^{2}\), and 194 m/s\({}^{2}\), respectively. Therefore, the impact forces that the soft landing gear encountered were 42 N, 94 N, 192 N, and 388 N, respectively.