  This paper highlights the significance of the rotor dynamics in control
design for small-scale aerobatic helicopters, and proposes two singularity free
robust attitude tracking controllers based on the available states for
feedback. 1. The first, employs the angular velocity and the flap angle states
(a variable that is not easy to measure) and uses a backstepping technique to
design a robust compensator (BRC) to \textbf{\textit{actively}} suppress the
disturbance induced tracking error. 2. The second exploits the inherent damping
present in the helicopter dynamics leading to a structure preserving,
\textbf{\textit{passively}} robust controller (SPR), which is free of angular
velocity and flap angle feedback. The BRC controller is designed to be robust
in the presence of two types of uncertainties: structured and unstructured. The
structured disturbance is due to uncertainty in the rotor parameters, and the
unstructured perturbation is modeled as an exogenous torque acting on the
fuselage. The performance of the controller is demonstrated in the presence of
both types of disturbances through numerical simulations. In contrast, the SPR
tracking controller is derived such that the tracking error dynamics inherits
the natural damping characteristic of the helicopter. The SPR controller is
shown to be almost globally asymptotically stable and its performance is
evaluated experimentally by performing aggressive flip maneuvers. Throughout
the study, a nonlinear coupled rotor-fuselage helicopter model with first order
flap dynamics is used.
