The Taguchi optimization of an external airfoil flap was performed numerically in the current study. ANSYS-Fluent software was used for two-dimensional analyses. Six different turbulence models were used for experimental validation, and the Spalart-Allmaras turbulence model was found to be the most precise. The study found that when the angle of attack is 10°, the NACA 4412 airfoil should be used for both external and base airfoils to maximise aerodynamic performance. Furthermore, the CL/CD ratio was maximised when the external airfoil angle was equal to 10° and length of external airfoil was 0.15*chord length. While the maximum CL/CD ratio for NACA 0018 was found to be 31,36 and 42,54 for NACA 4412, it was calculated 44,69 for the optimised design.