Geometric Control of Multiple Quadrotor UAVs Transporting a   Cable-Suspended Rigid Body
  This paper investigates tracking controls for an arbitrary number of cooperating quadrotor unmanned aerial vehicles with a suspended rigid body payload. Assuming that a rigid body is connected to multiple quadrotors by rigid massless links, control systems for quadrotors are constructed such that the rigid body asymptotically follows a given desired position and attitude trajectory. These are developed in a coordinate-free fashion to avoid singularities and complexities associated with local parameterizations. 
Taeyoung Lee
math.OC