A commercial CFD solver is used to simulate the unsteady aerodynamics performance of rigid and flexible wing airfoils for a high-performance jet trainer aircraft. The configuration used in the computational analysis is NACA 64012. In the numerical simulations the turbulence is modeled by enhancing Spalart-Allmaras turbulence model. To simulate the fluttering motion of the upper surface, an algorithm written in C computer language is integrated with the Fluent. The program controls the oscillation of the upper suction surface to specific defined displacement and the mesh dynamic that adjacent to the moving surface of the airfoil. The numerical experiments for both, rigid and flexible airfoils are carried out at flight speed of 85 m/s and angle of attack from zero to 18 degree. In order to verify the results of numerical simulation, the solver is validated against prior experiment of a lift coefficient. In comparison between rigid and flexible airfoils, the aerodynamic forces produced by a flexible airfoil shows that the lift coefficient is increased by 10% for angle of attack ranging from the incidence degree to 10 degree and then decreased slightly till the stall angle located at 16 degree. The flow separation in rigid airfoil is predicted at 7.5% of airfoil chord, whereas in the flexible airfoil it is at 59% of the airfoil chord
