Adaptive control of nonlinear mechanical systems with a novel dual-ducted UAV as the case study is considered in this thesis. At the outset, the equations governing the dynamics of the UAV are extracted. Various dynamical traits of the UAV are scrutinized and mathematically modeled. The dynamics of the UAV is highly nonlinear and not originally in the control-affine form. Therefore, a change of variables is proposed to transform the dynamic equations into the control-affine form. It is assumed that the system is subject to unknown disturbances. Furthermore, a nonlinear controller is designed to enable the UAV to follow desired trajectories. Since unknown time-varying disturbances are imposed on the system, the designed controller must be robust to handle such disturbances. Thus, the controller is modified in order to reject any unknown but bounded disturbance. Unlike previous nonlinear robust methods applied to this UAV, the asymptotic stability of the controller in the presence of unknown disturbance is analytically demonstrated. The controller enables the UAV to follow any desired translational and rotational trajectory and also accounts for a range of unknown disturbances. Finally, several computerized simulations are conducted to verify the analytical results.