  Aerial Cable Towed Systems (ACTS) are composed of several Unmanned Aerial
Vehicles (UAVs) connected to a payload by cables. Compared to towing objects
from individual aerial vehicles, an ACTS has significant advantages such as
heavier payload capacity, modularity, and full control of the payload pose.
They are however generally large with limited ability to meet geometric
constraints while avoiding collisions between UAVs. This paper presents the
modelling, performance analysis, design, and a proposed controller for a novel
ACTS with variable cable lengths, named Variable Aerial Cable Towed System
(VACTS).Winches are embedded on the UAVs for actuating the cable lengths
similar to a Cable-Driven Parallel Robot to increase the versatility of the
ACTS. The general geometric, kinematic and dynamic models of the VACTS are
derived, followed by the development of a centralized feedback linearization
controller. The design is based on a wrench analysis of the VACTS, without
constraining the cables to pass through the UAV center of mass, as in current
works. Additionally, the performance of the VACTS and ACTS are compared showing
that the added versatility comes at the cost of payload and configuration
flexibility. A prototype confirms the feasibility of the system.
