The importance of designing a more proficient airfoil with an effective flap and slat to increase lift force, reduce stall formation, or delay flow separation has become one of the main concerns of aerodynamicists. Considering this fact, a multi-element airfoil, NACA-2415, with a NACA-22 leading-edge slat at a specific 25˚ angle and a plain flap with three angle deflections (0˚, 15˚, 30˚), have been studied in this work. The outcome of varied deflections of the plain flap with various Angles of Attack (AOA) has been scrutinised. This paper also specifies the critical AOA and CLmax for the multi-element airfoil's flap deflection. The entire analysis involves 2D modelling and computational fluid dynamics (CFD) simulations conducted at a high Reynolds number.