This paper deals with the co-design of a Networked Control System (NCS). The simulation of a 4-rotor helicopter using Matlab/Simulink is used as benchmark. The control law of this system is non linear and its observer uses data provided by an Inertial Measurement Unit. The controller and the rotors are interconnected by a Switched Ethernet network. The objective of this paper is firstly to analyze the effects of perturbations induced by the network on the helicopter control. Secondly, the goal is to tune the levels of Classification of Services (CoS) defined in such a network to minimize the impact of communications on the helicopter behavior. The network is simulated thanks to the TrueTime toolbox. A new TrueTime model enabling to represent the concept of CoS has been developed for achieving this study