## 5 Experiments  
Experiments were carried out to observe the UAV take-off from sloped surfaces. The UAV is placed on a sloped rigid surface and controlled to take-off. In the first trial, as soon as the UAV's propellers spun up, the UAV slid down the slope, fell off and crashed as shown in Figure 8. The sliding occurred because the spinning propellers created a small thrust, which reduced the friction of the UAV's ground contact. Another trial was performed similar to the first case. This time as soon as the UAV started sliding when the propellers spun up, the thrust increased to avoid sliding by the remote operator, to take off from the surface quickly. However, since the UAV attitude was not leveled, this trial resulted in crashing into a close-by wall as shown in Figure 9.  
We performed experiments to test the adaptive leveling system designed in this work using a three-manipulator system. The manipulator system is designed to carry out heterogeneous tasks with high efficiency. Various experiments using a three-arm aerial manipulator system for landing, grasping and obstacle avoidance were discussed in our previous work [12]. In this work, we only discuss the experiment on maintaining stable posture of the airframe landed on an unstable surface. To perform the experiment, we setup a platform with a large rigid sheet horizontally on top of a small half-spherical base at its center such that its slope can be manually tilted in its centered roll and pitch axes. By default, the rigid sheet stays sloped at an angle such that one of its sides touches the ground under it. The UAV is then manually controlled by a remote operator to adaptive land on the platform, using the same approach of our work in [13]. After landing, the rotors are shut off completely to make sure the UAV is standing on its own with its weight distributed between the three manipulators. After the UAV is landed and is stable, the platform is moved manually by an assisting person in both its roll and pitch axes. The change in the slope of the platform disturbs the stable posture of the airframe, therefore tends to change its angle accordingly. An experiment was performed without the adaptive posture control. The resulting graph plotted for roll and pitch angle of the airframe is shown in Figure 10. The angle of the airframe changes with the change in the slope of the platform, because of the fixed posture of the manipulators. Without adaptive posture control, it was often not possible to take off from a sloped angle of the airframe since the fight controller indicated an erroneous take-off position in most of the cases. Of the cases where the UAV was able to take off, the take-off was not perfectly vertical from the ground and may therefore crash into wall or other objects if a wide empty space around UAV is unavailable. Figure 8: Conventional UAV take-off from a sloped surface resulting in sliding and falling down. Figure 9: Conventional UAV take-off from a sloped surface resulting in sliding and crashing into a wall. The same experiment was repeated with the adaptive posture control. By checking the direction of the load from torque sensors in the joint motors of the last link, it can ensured if that manipulator is touching a surface or not. The adaptive control is activated when all the manipulators are touching to a surface. By activating the adaptive posture control, the manipulators adjusts itself using the flight controllers IMU sensor data feedback to keep the airframe in a stable posture parallel to the earth's horizon. This results in the UAV being able to take off at any instant, even during rapid change of the landing surface angle during take off. The resulting experimental data for roll and pitch angle of the UAV is seen in the graph in Figure 11. Even though the surface shifts to about \(10[^{\circ}]\) (a 17.6% incline), the UAV is seen to be able to adjust within \(2[^{\circ}]\) error.