This research focuses on developing a control system for a compound coaxial helicopter (CCH) similar to the SB-1 Defiant, capable of both hovering and high-speed forward flight. The study introduces a 9 degrees of freedom mathematical model with 3 inflow states for the upper rotor, lower rotor, and pusher propeller, accounting for mutual rotor interference. The model undergoes a trim and stability analysis, revealing instability during transitional flight, necessitating a stability and control augmentation system. This system is implemented using an Explicit Model-Following (EMF) method, stabilized by a PID controller and a Weighted Pseudo-Inverse (WPI) control allocation algorithm. The controller effectively manages various flight controls through transition, as tested in a bob-up/bob-down mission task. While it performs well, there is room for improvement in input coupling and jitter behavior. Finally, an objective handling quality assessment based on ADS-33E metrics shows that the control system meets Level 1 handling quality criteria in the transition region for attitude and bandwidth, though some inefficiencies in control allocation during large attitude maneuvers were identified. ... Read more