The law of combined control with nonlinear stabilizing component in the form of sigma-function is designed for the control system of the center of mass of an unmanned aerial vehicle under the action of external uncontrolled disturbances. This law provides invariance to external disturbances and tracking with a given accuracy of generalized coordinates to reference signals. The original method of synthesis of the lowered order observer for an estimation of the generalized velocities on measurements of the generalized coordinates with piecewise linear corrective actions is developed.