Downwash-Aware Design of a Long-Reach Aerial Manipulator for Multirotor UAVs

Aerial manipulation tasks performed by multirotor unmanned aerial vehicles (UAVs) are often constrained by rotor-induced downwash, which generates a concentrated high-momentum axial core capable of destabilizing lightweight manipulators and payloads. This paper proposes a downwash-aware design framework for a long-reach, three-degree-of-freedom (3-DOF) aerial manipulator explicitly optimized to mitigate aerodynamic disturbances. The framework integrates CFD-based characterization of the rotor downwash, forward-kinematic modeling, workspace reconstruction, and experimental validation under controlled and real-flight conditions to ensure that the end-effector operates outside the strong-disturbance zone. Link lengths of 0.40 m and 0.80 m were selected to balance operational reach, aerodynamic safety, and platform stability. A controlled measurement setup was established for validation of the CFD numerical model, where the UAV was laterally fixed on a rigid support frame to eliminate flight-induced motion. The experimental results on the velocities at targeted locations show a good agreement with the CFD-predicted velocity profiles, confirming the reliability of the flow-field prediction model. A prototype integrated with a six-rotor UAV was experimentally validated under real flight conditions, demonstrating stable takeoff, manipulator deployment, and retraction, with a visually observable reduction in end-effector oscillation tendency. Several representative grasping configurations where the airflow velocity at the end-effector remained below the threshold of 1 m/s, or a weak-disturbance region, were identified and achieved via manipulation of the UAV system. We envision promising applications of the downwash-aware design of multirotor UAVs with aerial manipulator in high-altitude sampling, precision harvesting, and other contact-intensive aerial manipulation tasks.