  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.
