  In this paper, we prove that the dynamical model of a quadrotor subject to
linear rotor drag effects is differentially flat in its position and heading.
We use this property to compute feed-forward control terms directly from a
reference trajectory to be tracked. The obtained feed-forward terms are then
used in a cascaded, nonlinear feedback control law that enables accurate agile
flight with quadrotors. Compared to state-of-the-art control methods, which
treat the rotor drag as an unknown disturbance, our method reduces the
trajectory tracking error significantly. Finally, we present a method based on
a gradient-free optimization to identify the rotor drag coefficients, which are
required to compute the feed-forward control terms. The new theoretical results
are thoroughly validated trough extensive comparative experiments.
