Unmanned helicopters pose a challenge to nonlinear control design and development. Unmanned helicopters are inherently unstable without closed loop control,
setting it apart from many other classes of mechanical systems that possess desirable
structural properties such as passivity or dissipativity. In addition, helicopter
dynamics are highly nonlinear and strongly coupled, and therefore disturbances
along a single degree of freedom can easily propagate to the other degrees of
freedom, leading to a loss of performance or even destabilization. Furthermore,
unrestrained helicopter motion is governed by underactuated configuration, that is,
the number of control inputs is less than the number of degrees of freedom to be
stabilized, which makes it difficult to apply the conventional approach (usually in
the field of robotics) for controlling Euler–Lagrange systems. Due to these reasons,
substantial research efforts have focused on control design that guarantee stability
and robustness for unmanned helicopters.