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Nowadays unmanned rotorcraft are designed to operate with greater agility, rapid maneuvering, and are capable of work in degraded environments such as wind gusts etc. The control of this rotorcraft is a subject of research especially in applications such as rescue, surveillance, inspection, mapping etc. For these applications, the ability of the rotorcraft to maneuver sharply and hover precisely is important (Koo and Sastry 1998). Rotorcraft control as in these applications often requires holding a particular trimmed state; generally hover, as well as making changes of velocity and acceleration in a desired way (Gavrilets, Mettler, and Feron 2003). Similar to aircraft control, rotorcraft control too involves controlling the pitch, yaw, and roll motion. But the main difference is that, due to the unique body structure of rotorcraft (as well as the rotor dynamics and other rotating elements) the pitch, yaw and roll dynamics are strongly coupled. Therefore, it is difficult to design a decoupled control law of sound structure that stabilizes the faster and slower dynamics simultaneously. On the contrary, for a fixed wing aircraft it is relatively easy to design decoupled standard control laws with intuitively comprehensible structure and guaranteed performance (Stevens and F. L. Lewis 2003). There are many different approaches available for rotorcraft control such as (Altug, Ostrowski, and Mahony 2002; Bijnens et al. 2005; T. Madani and Benallegue 2006; Mistler, Benallegue, and M'Sirdi 2001; Mokhtari, Benallegue, and Orlov 2006) etc. Popular methods include input-output linearization and back-stepping. The 6-DOF airframe dynamics of a typical quadrotor involves the typical translational and rotational dynamical equations as in (Gavrilets, Mettler, and Feron 2003; Castillo, Lozano, and Dzul 2005; Castillo, Dzul, and Lozano 2004). The dynamics of a quadrotor is essentially a simplified form of helicopter dynamics that exhibits the basic problems including underactuation, strong coupling, multi-input/multi-output, and unknown nonlinearities. The quadrotor is classified as a rotorcraft where lift is derived from the four rotors. Most often they are classified as helicopters as its movements are characterized by the resultant force and moments of the four rotors. Therefore the control algorithms designed for a quadrotor could be applied to a helicopter with relatively straightforward modifications. Most of the papers (B. Bijnens et al. 2005; T. Madani and Benallegue 2006; Mokhtari, Benallegue, and Orlov 2006) etc. deal with either input-output linearization for decoupling pitch yaw roll or back-stepping to deal with the under-actuation problem. The problem of coupling in the

Sliding Mode Approach to Control Quadrotor Using Dynamic Inversion