This thesis investigates the aerodynamics of a propeller-wing-flap system, spurred by the growing interest in propeller-driven aircraft and the potential introduction of distributed electric propulsion. The research focuses on the complex flowfield involving the interaction of the propeller slipstream with the main wing and flap at high-lift conditions. Using unsteady RANS simulations at a chord based Reynolds number of 2 million, the study explores the impact of a propeller slipstream on flap time-dependent boundary layer variations. The computational results are validated against experimental data, showing good agreement. The analysis reveals intricate details of the flow field, including propeller slipstream shifts and complex vortex systems contributing to flap stall mechanics. The study highlights the influence of these phenomena on the spanwise lift distribution and flap flow separation areas. The findings contribute valuable insights into the understanding of propeller-wing-flap interactions, yet emphasizing the need for further research to confirm and expand upon the current discoveries.