  This paper proposes a solution to the attitude tracking problem for a novel
quadrotor tailsitter unmanned aerial vehicle called swiveling biplane
quadrotor. The proposed vehicle design addresses the lack of yaw control
authority in conventional biplane quadrotor tailsitters by proposing a new
design wherein two wings with two attached propellers are joined together with
a rod through a swivel mechanism. The yaw torque is generated by relative
rotation of the thrust vector of each wing. The unique design of this
configuration having two rigid bodies interconnected through a rod with zero
torsional rigidity makes the vehicle underactuated in the attitude
configuration manifold. An output tracking problem is posed which results in a
single equivalent rigid body attitude tracking problem with second-order moment
dynamics. The proposed controller is uniformly valid for all attitudes and is
based on dynamic feedback linearization in a geometric control framework.
Almost-global asymptotic stability of the desired equilibrium of the tracking
error dynamics is shown. The efficacy of the controller is shown with numerical
simulation and flight tests.
