<p>The trailing edge flap is an effective airfoil flow control technique having many forms, including the external airfoil flap (EAF) which lacks deep studies. In this paper, the calculated results of computational fluid dynamics are first verified through experiments, and the impact of the EAF parameters on the aerodynamic performance of the S809 airfoil is then studied. The obtained results show that the EAF can significantly improve the lift coefficient and stall angle of attack (AOA). Furthermore, the standard L₉(3⁴) orthogonal range analysis is conducted, demonstrating the parameter impact ranking. More precisely, for the improvement of the lift coefficient, the order is normal distance &gt; flap angle &gt; tangential distance. For that of the lift-drag ratio, the order is flap angle &gt; tangential distance &gt; normal distance. For improving the stall and optimal AOA, the order is flap angle &gt; normal distance &gt; tangential distance. The results obtained in this study provide a basis for the blade design of rotating mechanism with EAF.</p>