This paper addresses the complex aerodynamic interaction between a rectangular, unswept wing-segment and an over-the-wing mounted ultra-high bypass ratio (UHBR) engine. In the past, various studies have revealed a beneficial interference effect with respect to installation drag due to over-wing-nacelles (OWN). Previous investigations claimed, that the overall drag of an OWN configuration might be favorable compared to an under-wing-nacelle (UWN) in case of using UHBR engines. Further investigations within the Collaborative Research Center 880 on a short-range aircraft configuration with UHBR OWN engines at M=0.78 confirmed a positive installation effect on an OWN. However, the physical mechanism and cause leading to this interference effect could not be identified. Thus, the present investigation is based on a simple test case, allowing for distinct parameter variations to identify the driving mechanism leading to drag reduction on a nacelle located above the wing trailing edge. For that reason, a parameterized rectangular wing was combined with an UHBR engine, taken from the AVACON project. Based on the parameterized wing, several airfoils were investigated ranging from NACA 4-digit series to the NASA SC(2) series. Especially the NACA 4-digit series enables an explicit parameter variation, like airfoil thickness and camber. In addition, flow velocities ranging from M=0.5 up to M=0.8 were covered to assess transonic effects. Finally, the vertical distance between wing trailing edge and nacelle leading edge was investigated. The evaluation revealed a correlation between lift, induced by the wing, and nacelle drag.