## 6 Flight Test Experiment
### Disturbance Rejection Experiments  
There were three steps in this disturbance rejection experiment. First, a constant attitude command was applied to the control system to keep the helicopter stabilized and hovering. Second, a dropping order was sent to the radio to release the block, which could generate the disturbance. Finally, it waited for a few seconds until the controller corrected the attitude. The procedure is shown in Figure 8.  
Figure 6: The structure diagram of block dropping system.  
Figure 7: The testbed of model helicopter with dropping device.  
Both the proposed Backstepping-ADRC and the PID controller with high integral gain were tested under the same conditions. The recorded attitude tracking data, disturbance observation data and the control signal data are illustrated in Figure 9.  
For the Backstepping-ADRC, the simulation data coincide with the flight test data validating that the algorithm presented above has excellent performance. According to the attitude tracking data of flight test, the helicopter controlled by the Backstepping-ADRC responded immediately to the disturbance and started to pitch for 0.7 s to the minimum value of \(-3^{\circ}\) until the disturbance was rejected. After that, the helicopter started to recover with a negligible overshoot and settled to a hovering state without steady error. The total settling time for the disturbance (\(\theta\) within \(\pm 1^{\circ}\)) was approximately 1.5 s. However, the PID controller with a high integral gain responded with a minimum pitch of \(-6.5^{\circ}\), an overshoot of \(4^{\circ}\) and a total settling time of 5 s. During the whole disturbance rejection experiment, the average tracking error of Backstepping-ADRC is \(0.39^{\circ}\), while it is \(1.68^{\circ}\) for the PID controller, and the standard deviations of the tracking errors are \(0.74^{\circ}\) and \(1.83^{\circ}\), respectively. The attitude tracking data suggests that the proposed control algorithm has 78% improvement of control accuracy and 60% improvement of disturbance rejection capability compared to PID control method, and it is able to successfully reject intense and swift disturbances and precisely track the command.  
Figure 8: The block dropping procedure of the experiment. (**a**) Hovering before dropping the block; (**b**) the moment after dropping the block.  
In addition, its rejection is much more efficient and swifter than that of the PID controller. The disturbance observation data could better explain the difference. The observation of the ESO in the Backstepping-ADRC responded rapidly to the disturbance and achieved stable observation within 2 s. In contrast, the integral term of the PID controller is more efficient than the PID controller. The disturbance observation data could better explain the difference. The observation of the ESO in the Backstepping-ADRC responded rapidly to the disturbance and achieved stable observation within 2 s. In contrast, the integral term of the PID controller is more efficient than the PID controller. The difference between the PID controller and the PID controller is more efficient than the PID controller.  
Figure 9: Flight test data of disturbance rejection (L_PID denotes the integral term of the PID controller). (**a**) Tracking of pitch angle; (**b**) estimation of disturbance; (**c**) longitudinal cyclic pitch angle of main rotor.  
controller responded much slower than the ESO and induced characteristic high magnitude oscillation, resulting in a long settling time and high overshoot of the attitude tracking.