This thesis studies the aerodynamic interaction effects in an over-the-wing (OTW) propeller system to contribute to the development of OTW propeller systems intending at reducing emissions. Previous research shows that an OTW propeller can affect the pressure distribution of the wing such that lift increases and pressure drag decreases. However, studies on the influence of the blade tip vortices on flow separation on a flap due to their interaction with the wing boundary-layer are missing. The results obtained from unsteady RANS simulations in this thesis show that the blade tip vortices significantly contribute to flow separation by causing an additional adverse pressure gradient. To prevent flow separation, the propeller can be inclined with the flap such that the slipstream increases the momentum in the flap boundary-layer. Furthermore, an increase in propulsive efficiency compared to a decoupled propeller-wing system can be achieved if a reduction in propeller thrust is prevented.