###### Abstract
A DARPA funded research in [11] developedadaptable landing gears for a helicopter to quickly and safely land on an irregular surface. In prior work, we designed a UAV with a three-arm aerial manipulator system (TAMS) [12] to execute a variety of tasks, including vision-based adaptive landing, which can be very useful in disaster events. A VTOL UAV requires less room for landing (theoretically, only the diameter of its airframe plus the propeller span). When considering a traditional VTOL-type UAV, however, it is always required to ensure that the airframe is aligned with the horizon. Failure to fulfill this requirement will often result in unexpected UAV movements and a crash. To minimize unexpected movements, some current off-the-shelf flight controllers will not let the propellers start spinning if the airframe is not parallel to the horizon. Previously, in [13], we accomplished adaptive landing with a UAV employing three manipulators to safely land on uneven surfaces and avoid rolling accidents. However, unlike a stable surface, the landed surface may alter after landing due to disturbances. A rocking ship deck is one of the excellent examples for an unstable surface targeted by this approach. An adaptive landing system could aid in landing and would operate until the UAV lands on a moving ship deck; but because the ship deck is not stable, the UAV would not be able to take off safely from this surface. As a result, the UAV would not be able to return to its base station, resulting in a failed mission. A mechanism for airframe stabilization is necessary to keep the airframe parallel to the horizon at all times, even after landing, for safe take-off in this scenario. This work focuses on two fundamental ideas. The first consideration is the manipulator's design. We suggest a novel manipulator system with a lesser number of actuation when compared to other research using general-purpose manipulator systems [3; 14] for a particular task. This is especially important in the case of aerial manipulation to decrease power consumption because the battery size is restricted by payload constraints. The second step is to demonstrate the stability of the UAV's airframe after landing by employing manipulators against changing ground elevation as seen in Figure 1. We show that with our system, a UAV can land and take-off on uneven ground even if the ground slope varies. In Section 2, the concept and design requirements for a manipulator system is discussed with the objective to attain various heterogeneous aerial manipulator tasks using TAMS. Section 3 provides the hardware description of the manipulator system, kinematics and the airframe size required to extend the workspace of the manipulators. The control system for the airframe stabilization against surface slope variation is explained in Section 4. The experiments conducted and the results obtained are shown in Section 5, for the airframe stability task. In Section 6, discussion about the experimental results and scope for future development is provided, followed by a conclusion in Section 7. Figure 1: The UAV with three-arm aerial manipulator system landed and ready to take off from an unstable platform with changing slope.