The use of small unmanned aerial vehicles (UAV) has extensively increased during the last decade. Their most common applications are in sport events’ photography, search-and-rescue missions and military reconnaissance. Additional area with potentially rapid growth is the autonomous deliveries. Using small multi-rotor drones to deliver door-to-door has many benefits, ranging from reducing shipping time and costs to high reliability and convenience. The drawback is a relatively small lifting capacity of currently operational multi-rotor UAVs. It can be mitigated, to some extent, by employing a formation of UAVs to the same task.
Designing a control algorithm for a formation of multi-rotor-UAVs carrying a sling load is a challenging task, not only because of the complexity of the formation’s dynamics, but also because of the complexity of the rotors’ aerodynamics. Normally, a simplified aerodynamic model is adopted, in which the forces and moments generated by a rotor are determined solely by its rotational speed. In this study the simplified aerodynamic model is replaced by a realistic one. It is demonstrated that the commonly used model is inadequate in windy environment, especially in urban areas where wind gradients are large. By exposing a formation of four quad-rotor UAV’s carrying a sling load to turbulent winds, it is shown that a ‘standard’ trajectory tracking guidance with force controller, designed using the simplified model, can crash. An update to the original controller scheme is suggested, to improve the system performance.
