  This paper proposes a novel control law for accurate tracking of agile
trajectories using a tailsitter flying wing unmanned aerial vehicle (UAV) that
transitions between vertical take-off and landing (VTOL) and forward flight.
The global control formulation enables maneuvering throughout the flight
envelope, including uncoordinated flight with sideslip. Differential flatness
of the nonlinear tailsitter dynamics with a simplified aerodynamics model is
shown. Using the flatness transform, the proposed controller incorporates
tracking of the position reference along with its derivatives velocity,
acceleration and jerk, as well as the yaw reference and yaw rate. The inclusion
of jerk and yaw rate references through an angular velocity feedforward term
improves tracking of trajectories with fast-changing accelerations. The
controller does not depend on extensive aerodynamic modeling but instead uses
incremental nonlinear dynamic inversion (INDI) to compute control updates based
on only a local input-output relation, resulting in robustness against
discrepancies in the simplified aerodynamics equations. Exact inversion of the
nonlinear input-output relation is achieved through the derived flatness
transform. The resulting control algorithm is extensively evaluated in flight
tests, where it demonstrates accurate trajectory tracking and challenging agile
maneuvers, such as sideways flight and aggressive transitions while turning.
