Steady Reynolds-Averaged Navier-Stokes (RANS) simulations utilizing the k-ω SST turbulence model are conducted to investigate the aerodynamic performance of a wing-integrated ram-air duct housing a heat exchanger for propeller-driven aircraft, including its impact on wing-body junction flow. The research is conducted in two stages: first, a 2D aerodynamic analysis employing a Design of Experiment (DoE) methodology to assess the sensitivity of key geometrical parameters–including stagger angle, leading-edge droop, duct gap, and heat exchanger characteristics–on lift, drag, and duct mass flow rate; and second, a 3D investigation of the junction flow behavior in the nacelle/ducted-wing configuration. The heat exchanger pressure drop is modeled as a porous media zone using the Darcy-Forchheimer quadratic drag law. Heat transfer is incorporated through a variable energy source term applied via a userdefined function (UDF) based on the ε-NTU correlation. Findings from the 2D aerodynamic analysis indicate that heat exchanger characteristics, particularly porosity and thickness, have a more pronounced impact on aerodynamic performance than external duct geometry. However, intake stagger angle and leading-edge droop play critical roles in mitigating flow separation and optimizing the wing pressure distribution. In addition, the redistribution of pressure due to flow restriction alters the stagnation point location, inlet-velocity ratio, and static pressure distributions, all of which influence the aerodynamic loading of the ducted wing. The optimal ducted airfoil configuration, featuring a lower-surface outlet aft of the maximum thickness and a thin heat exchanger, minimizes aerodynamic penalties while maximizing duct mass flow rate. However, thermal feasibility assessments reveal that meeting the cooling demands of fuelcell systems necessitates a thicker heat exchanger to accommodate sufficient heat transfer area within the constrained wing volume. This increase in thickness impairs aerodynamic performance through increased pressure drop and resultant drag. Although higher porosity mitigates flow resistance, the required thickness offsets this advantage, reinforcing the inherent trade-off between aero-thermal performance. In 3D, the presence of the heat exchanger inside the duct fundamentally alters the local aerodynamics by modifying boundary layer interactions at the wing-body junction. The flow resistance imposed by the heat exchanger directly affects the strength and topology of secondary flow structures, particularly the horseshoe vortex (HSV), which governs junction flow behavior and whose strength scales with the Reynolds number based on the momentum thickness of the incoming boundary layer. At low porosity levels, the stronger HSV , with an increased vertical extent above the wing, entrains high-momentum freestream flow into the chordwise and spanwise boundary layers, mitigating corner flow separation. Conversely, at high porosity levels, lower flow resistance alters HSV topology, reducing its vertical extent and allowing part of the vortex to enter the duct, inducing a secondary vortex at the lower lip. This weakens the HSV’s ability to stabilize the boundary layer, leading to earlier separation, increased pressure losses, and higher drag. A moderate porosity level provides an optimal balance between HSV strength, vertical positioning, and junction flow stability, reducing corner flow separation and associated pressure losses. Collectively, these findings yield critical insights into integrating ram-air cooling ducts within the wings of propeller-driven aircraft, offering a compelling approach to achieving efficient thermal management systems with minimal aerodynamic penalty. This investigation provides unprecedented detail in visualizing and understanding the intricate coupling between ducted wing aerodynamics and heat exchanger-induced flow interactions, while emphasizing the need for further research to validate and expand upon these findings

As the aviation industry faces growing environmental challenges, there is a critical push for innovations to reduce its climate impact. Advances in propeller technology present a promising, more efficient alternative to traditional turbofans, especially for short to medium-range flights. Propellers offer high propulsive efficiency and flexibility, though they pose challenges like increased noise and performance limitations at higher speeds. Recent developments explore propeller integration into airframes to harness aerodynamic benefits, such as distributed propeller systems that enhance lift during take-off. The design of such integrations requires a comprehensive understanding of the highly complex flow dynamics involved in the interaction between a propeller, wing, and flap.
The objective of this dissertation is therefore to characterise the phenomena and mechanisms that govern the aerodynamic interaction between a propeller, wing, and flap. The propeller-wing-flap aerodynamic interaction can be viewed as a combination of two fields which are already established in literature: propeller-wing aerodynamic interaction and multi-element airfoil aerodynamics. Using these foundations, the present research approaches the problem from two perspectives: the extension of propeller-wing interaction to include a flap, and the influence of a non-uniform flowfield induced by a propeller on the aerodynamics of a multi-element airfoil. Special attention is paid to the slipstream deformation, which is known to be substantial in propeller-wing interaction at high angles of attack and is likely to be very significant for the flow over a deployed flap. Additionally, the explorations of additional components in the system, such as distributed propellers and the role of the nacelle integration, are included to provide a basis for further research in the field....

Flying characteristics of a multi-engined light general aviation (GA) aircraft during and after an engine failure is often a major safety consideration when both designing and operating the aircraft. In the meantime, the propellers, being large rotating masses, can exert considerable gyroscopic effect on the aircraft during flight, itself contributing to a coupling between the pitch and yaw axis, thus affecting flight dynamics. This study presents an investigation on the impact of propeller gyroscopic effects on the flying motion of a representative twin-engine GA aircraft. This is done using a modular flight mechanics toolbox that performs analyses in both frequency domain and time domain. A steady-state windtunnel aerodynamic and control surface model with empirically estimated unsteady aerodynamic coefficients, along with a propeller governor engine system simulation, complements the gyroscopic inertia model in the simulation setup.

Firstly, a modal analysis showed that all modes aside the spiral mode does not get discernibly affected by the rotating inertia typical to the reference aircraft’s propellers. Then, time-domain simulations of various rapid maneuvers show that gyroscopic effect does cause significant change in the angular response of the coupled axis, e.g. sideslip angle response during a pitch input only maneuver, whilst its impact on long term phugoid motion remained inconclusive due to undesired and uncontrolled roll motion. To compensate for this, maneuvers were performed again with a manually tuned simple wing leveler and results showed that pitch input maneuvers does not show much deviation in phugoid motion, whereas yaw input maneuvers such as sudden left engine failure shows discernible difference in airspeed and altitude responses, though the difference in magnitude is still small. Next, comparisons made with different powertrain responsiveness showed that in a power reducing case such as sudden one engine failure, the effect of the powertrain time delay is independent from the influence of gyroscopic effects, whereas for a power increase case, such as going around, the impact of the two is
simultaneous and intertwined. Finally, a sensitivity study on unsteady aerodynamic coefficients showed that their effects on flying motion are generally independent from the gyroscopic effect.

The aerodynamic performance of a regional, propeller-powered strut-braced wing is investigated. The main wing planform and operating conditions are based on an ATR72, to which a strut and jury strut are added. The research focuses on the different drag components in cruise. The first research objective is to quantify the effect the different wing components have on the change in induced drag caused by the propeller. The second objective deals with viscous effects, such as separation and friction. At the strut-wing and strut-jury strut junctions, it is tested whether the propeller affects the size and location of any separated regions. In addition, the influence of the propeller on the skin friction drag is quantified. The final objective deals with interference and compressibility. The close proximity of the strut, jury strut and wing, combined with the increased slipstream velocity of the propeller, and the change in angle of attack caused by swirl recovery might lead to high supervelocities. The regions with the highest supervelocities are localised, and it is verified whether any supersonic flow exists.

Induced drag is calculated with a panel method (Flightstream), which allows simulating different configurations in a relatively short time. By simulating different combinations of wing elements (with and without strut, jury strut and propeller) it is possible to isolate their contributions. The research objectives about viscous effects and interference were investigated using unsteady RANS. The CFD simulation were also used to validate the panel method. In general, it agreed well with CFD. Some discrepancies were caused by the absence of vortex dissipation and an offset in the pressure distribution inside the propeller slipstream.

The propeller reduced induced drag significantly, around 58% for all configurations. The main strut had the largest effect. Both the strut itself and interference of it with the wing lead to an additional reduction of 1% in induced drag. The jury strut had limited effect. Under the influence of a propeller, it had a small induced thrust component. Interference with the main wing cancelled out this benefit. These results were obtained using unoptimised loading distributions, optimising these would increase the gains for both the conventional and strut-braced wings.

The strut-wing junction only showed separation at the strut leading edge. The local flow behaviour was not influenced by the propeller. The strut-jury strut junction also exhibited leading edge separation, in addition to corner separation at the trailing edge of the jury strut, and separation at the trailing edge of the main strut. The size of the corner separation reduced under the influence of the propeller, by favorably changing the pressure gradient on the jury strut. The net effect of the propeller on the separated region at the trailing edge of the strut was to move it inboard, by increasing the pressure gradient there, and moving the location of the horse-shoe vortex system inward. Friction drag increased by roughly 3%, insignificant compared to the reduction in induced drag.

Finally, the region most sensitive to high supervelocities was the strut-jury strut junction. The closely spaced elements, combined with the higher slipstream velocity and increased angle of attack lead to a small supersonic pocket. Due to its limited size, it is expected that using a slightly different airfoil for the jury strut can already eliminate it.

While some attention needs to be payed to junction flows and interference effects, this work has shown the advantage of a propeller-powered strut-braced wing for regional aviation, compared to conventional aircraft.

The advent of distributed electric propulsion aircraft concepts requires novel physics-based analysis methods for the modeling of the complex aerodynamic interactions between the closely coupled rotor, wing, and flap. For this purpose, the analysis tool Open Rotor-Wing-Flap Interaction Solver (ORWFIS) is developed with existing and novel low-fidelity methods. The research aims to answer how well an open rotor-wing-flap configuration can be modeled by the low-fidelity methods.

The method includes a blade element rotor model that accounts for non-uniform inflow conditions, a vortex based slipstream method that models contraction and deflection, and a non-planar VLM for the wing and flap that is coupled to MSES through a novel decamber method.

The developed method is shown to predict good results for the total system lift and system drag at low and higher angles of attack close to stall for the retracted flaps case. The decamber method is shown to successfully account for general thickness and viscous effects if solutions from MSES are available. For wings with deployed flaps, reasonable results are achieved at low and medium angles of attack, but the accuracy of the results is largely sensitive to the quality of the finite slipstream correction method which performs worse for larger off-sets of the flap from the main wing. The analysis is unable to provide reliable results for higher angles of attack for wings with deployed flaps due to convergence issues by MSES.

Propeller technology offers high efficiency and therefore it is increasingly attractive for sustainable aviation concepts. Leading edge mounted propellers blow the wing and thus can augment the wing lift however they also introduce undesirable interaction effects. This study focuses on understanding and addressing the interaction effects introduced by the nacelle through wing leading edge and nacelle modifications to increase the high angle of attack performance. This is achieved through the use of a RANS model that is validated using wind tunnel test data. Two methods are evaluated to improve the performance, leading edge droop and a nacelle wing junction fillet. While the droop was able to improve leading edge alignment and reduce the leading edge pressure peak it was not able to substantially improve performance. The fillet improved the junction flow, resulting in an increase in performance however neither modification alone offers complete a solution to the negative interaction effects.

The Dutch Electric Aviation Centre possesses and uses a Cessna Skymaster 337F as an experimental flying testbed to help gain knowledge to aid the transition towards hybrid/ electric aviation. The interaction between the rear propeller slipstream and the horizontal tail has been likened to the interaction of propeller-wing configurations seen on larger regional aircraft. The DEAC intends to study this interaction through ground-based high-speed taxi-tests before any flight tests. By doing so, tests are conducted in a safer and more accessible environment without the need for re-certification, albeit, with changes to both testing environment and aircraft operational settings. This numerical study focused on evaluating the feasibility of such a testing approach to help gain an insight and better prepare for experiments. RANS CFD studies were performed on a simplified self-designed and developed half-airplane half-propeller model of the aircraft in ground-run configuration and free-flight configuration. In this stead-state simulation, the propellers were treated as actuator disks and had a specified constant pressure rise. Vortices being shed from the fuselage and interacting with the tail were discovered. The uncertainties due to the chosen simplifications, the neglect of twin contra-rotating propeller swirls, and the fact that the formation and evolution of the wake vortices, and their interaction with both the rear propeller and the horizontal tail are inherently unsteady effects, precludes a definitive conclusion. A more detailed numerical model is required to address the situation and evaluate the feasibility of this approach.

It is the need of the hour to focus research and development towards curbing emissions due to the growing aviation industry. Employing boundary layer Ingesting(BLI) propellers for development of propulsive fuselage concept(PFC) aircraft while utilizing hydrogen as an alternative energy source, is the idea behind project APPU(Advanced Propulsion and Power Unit). However, the empennage wake is detrimental to the propeller. This study uses chord-wise wake blowing technique to fill the empennage wake in order to mitigate the detrimental effects of the wake on the inflow of the aft mounted BLI propeller. A near complete wake filling was observed for the blown configuration, with a 99.7% uniformity in the velocity profiles for the filled empennage wake. The addition to the overall aircraft drag due to the wake blowing system was less than 2 drag counts and a mass flow rate of 0.91-1.23Kg/s was found to be required in the blown cases.

Testing novel concepts in the allocation and use of aircraft is commonly done in air traffic simulators. Current day simulators rely on proprietary models and often propose questionable solutions. This paper uses BlueSky as simulator, in cooperation with performance models OpenAP and BADA. BlueSky aims to provide the user with full freedom to simulate any scenario. Improving BlueSky and adding functionality further aids in this goal. By expanding logic of the simulator, the user should attain more functionality without compromising the user-friendliness of BlueSky. This paper treats the implementation of thrust setting and flight path angle in BlueSky. This enables continuous descent operations to become part of the possibilities within BlueSky. The challenge was to prepare a set of ADS-B data using Fuzzy logic to validate the new model. Its robustness was then be demonstrated using a large number of flights.

Aviation is responsible for a large part of the emissions due to transportation, which is why, in recent years, a trend towards implementing sustainability in aviation has emerged. In small aircraft, the concept of Leading-Edge Distributed Propulsion (LEDP) can provide aerodynamic advantages to reduce fuel consumption thanks to the ability to increase lift due to the interaction between the slipstream of the propellers and the wing.

Therefore, a wind tunnel experiment is performed to study the aerodynamic interaction in propeller-wing-flap systems when Leading-Edge Distributed Propulsion is used. The objective of this research is to gain insight into the difference in wing loading distribution between the distributed-propeller and single-propeller configurations in the flap retracted case, about the effect of flap position on the lift enhancement in LEDP and about the effect of flap deflection on the wake downstream of the system...

Recent developments in the field of hybrid-electric propulsion (HEP) have opened the door to a wide range of novel aircraft configurations with improved energy efficiency. These electrically-driven powertrains enable “distributed propulsion” configurations, in which the aerodynamic interaction between the propulsive devices and the airframe is exploited to enhance the aero-propulsive efficiency of the aircraft. In this context, the present research focuses specifically on over-the-wing distributed propulsion (OTWDP) for regional propeller aircraft. Over-the-wing (OTW) propellers are particularly promising because they can significantly enhance the lift-to-drag ratio of the wing, as well as reduce flyover noise due to shielding by the wing.

The objective of this research is therefore to quantify the impact of OTWDP on the energy efficiency of hybrid-electric aircraft. For this, the research is divided into three main parts. First, a sizing method for hybrid-electric distributed-propulsion (HEDP) aircraft is developed, independently of where the propellers are positioned with respect to the airframe. Second, the aerodynamic interaction effects and performance characteristics of OTWDP systems are investigated, independently of the type of powertrain used to drive the propellers. And third, the sizing method and aerodynamic performance estimates of the previous two points are combined to assess the effect of hybrid-electric OTWDP on aircraft-level performance metrics. [...]

The objective of the present dissertation is to show how redundant control surfaces can be exploited to shape an aircraft dynamic behavior and obtain desired flight mechanics performance. This is achieved by introducing novel approaches and methods for flight mechanics and control, mainly revolving around original implementations of traditional formulations of the Control Allocation (CA) problem. Control surfaces and, more in general, control effectors are defined as redundant if they are capable to independently control the same motion axis of the aircraft.

Redundant effectors can be linked together, and to the pilot input, in many ways according to different optimality criteria and/or performance objectives. In particular, the research presented in this dissertation focuses on the possibility to achieve Direct Lift Control (DLC). The latter is intended as the ability to use control effectors to alter the aircraft lift "without, or largely without, significant change in the aircraft incidence, and ideally is meant not to generate pitching moment."

The ability to do so is essentially dependent on the position of the Control Center of Pressure (CCoP), which is the center of pressure of aerodynamic forces solely due to control surface deflections. In case of a single control surface dedicated to DLC, the CCoP coincides with the control surface itself. In case of redundant control surfaces, their deflections can be coordinated to induce the position of the CCoP towards some preferred location, as allowed by the architecture of the aircraft and the available control effectiveness.

The first three chapters of the dissertation are dedicated to establishing the societal, scientific, and technical background underlying the subsequent research studies, including an overview of the CA problem for redundant control effectors. The following four chapters present, in this order: an evaluation of the mission performance of a staggered box-wing aircraft model designed for commercial transonic operations; a comparison of different CA methods on the design of an optimum control surface layout for a box-wing aircraft, with control surface both fore and aft the aircraft center of gravity; a trim problem formulation which employs forces and moments due to the aircraft control surfaces as decision variables, to maximize control authority, minimize aerodynamic drag or obtain a prescribed pitch angle; a CA-based formulation aimed at altering the characteristics of the transient response of an aircraft by exploiting the properties of the CCoP.

The conclusive chapter presents a comprehensive, top-level recap of the main aspects and topics covered within the dissertation. It reflects on the classic meaning of DLC, and what it means to achieve it with redundant control surfaces that are not expressly dedicated to it. With some considerations on the needs of aviation market, it speculates on the practical role of unconventional aircraft configurations in the near future. Lastly, it provides suggestions for improvements and future research studies.

Turbulent drag is the largest source of fuel consumption in aviation and forms a significant contribution to climate warming. Recent numerical studies have suggested the ability of surfaces with streamwise-preferential permeability to reduce turbulent friction, and a theoretical framework behind the working mechanism has been proposed. This work is the first experimental study on this concept in air and in which the permeability requirements for predicted drag reductions are met. Three different physical realisations of streamwise-preferential permeable substrates were manufactured: two seal furs, a substrate with unidirectional fibres, and a 3D-printed structure. They were investigated through wind tunnel experiments using direct force measurements and planar (2D-2C) particle image velocimetry (PIV). Results show an increase in drag for all test specimens and suggest that drag sources other than surface friction are non-negligible. One-point turbulent statistics show an overall increase in turbulence, mainly wall-normal velocity fluctuations enabled by the wall-normal permeability, which results in higher Reynolds stresses. No significant flow modulation in terms of turbulent events or change in coherent structures is observed. The measurements and results are a first of its kind and none of the theoretical framework predictions on drag reduction or flow modulation are found. Based on analytical derivations relating characteristic length scales to permeability, the validity of the framework and its assumptions in experimental settings is questioned. It is thought that there is an inherent mismatch between the pore size assumption and the virtual origin approach taken in the framework. All things considered, it is deemed unlikely that turbulent drag reduction by means of streamwise-preferential permeable surfaces is feasible in experimental settings. Nevertheless, given the relatively small increase in drag for the 3D-printed specimen, streamwise-preferential permeable surfaces might be interesting for other (flow control) purposes involving turbulent boundary layers.

There is renewed interest into propeller-driven aircraft to improve fuel economy. Some novel concepts such as distributed electric propulsion (DEP) use propellers for lift augmentation. Low-order methods are required to quickly determine designs with improved high-lift performance. A 2D tool may be required to “tune” a 3D inviscid model such as a vortex lattice method to compensate for viscous losses in lift. A 2D viscous-inviscid solver (MSES) is modified to analyze high-lift devices blown by slipstreams. The inviscid results for an airfoil in a fully developed slipstream match well with CFD results. It is shown that higher propeller positions may augment lift by means of upper surface blowing. The presence of a slipstream influences boundary layer growth and can both aggravate or postpone wake-bursting effects to improve high-lift performance. The gap and overlap of a blown multi-element airfoil are optimized, illustrating that the tool is useful for parametric studies.

A next step in aviation will include a step in power train hybridization to target a reduction in emissions and to limit the environmental impact of air travel. One of the concepts with potential of increasing aero-propulsive efficiency is the distributed Over-The-Wing (OTW) propulsive concept. To obtain a further understanding of the performance of this concept, a numerical aero-propulsive performance investigation into the wing shape and propeller position of an over-the-wing distributed-propulsion configuration is made. Results show the ability to increase the propulsive efficiency of the system by altering the inflow angle and axial flow velocity to the propeller. Altering the wing geometry below the propeller showed potential in augmenting the beneficial propeller-wing interaction. The research shows that there is a strong sensitivity in changing the propeller axial position, propeller incidence, shroud incidence and wing shape in an OTW configuration.

Technology selection is ubiquitous in all manners of complex systems engineering, design and everyday business operations. Technologies are often complex entities shrouded in uncertainty, assumptions and interpretation. Therefore, quantifying their effect (i.e. outcome) becomes challenging for several reasons. First, as simulations may not be available for novel technologies, experts have to estimate their impacts. Secondly, as the technologies may not be well-defined, different experts have different interpretations and will assign different outcomes. Thirdly, even if analysis methods are available, there is epistemic and model uncertainty in the outcome. Finally, available analysis frameworks are typically application-specific, non-extensible and inflexible. It is the objective of this thesis to pave the way towards a technology selection methodology that offers a structured, repeatable and traceable way to represent technologies and consecutively quantify their impacts on an engineering system. Such a methodology is implemented as a decision support system, i.e. a computer program that assists a decision maker throughout the decision-making process. Three components of said methodology can be identified: technology representation and portfolio generation, technology (portfolio) evaluation and technology (portfolio) selection...

With the focus on the reduction of fossil fuel emissions, aircraft are continuously growing towards higher fuel efficiency. The traditional limits of aircraft performance can be surpassed through the use of composite materials which offer a reduction in aircraft weight. Due to the multidisciplinary nature of aircraft design, integration of different disciplinary analyses is required to arrive at a feasible design. The inclusion of composite design in the preliminary design process, however has become a challenge. This is due to the high computational cost associated with the composite aeroelastic tailoring tools used in the design process. A possible solution is available in the form of surrogate models which can reduce the computational costs. The current work focuses on the development of a methodology that allows the inclusion of surrogate-based model in the optimization for a computationally expensive aeroelastic tool (PROTEUS) developed at TU Delft. The resulting methodology can be expanded to a generic, computationally expensive tool in a multidisciplinary optimization setting. Wing design optimization is carried out based on surrogate modeling methodology and metal based design method. Comparison is made of the final optimized designs based on structural and performance parameters.

In the past decade, MDO researchers have developed capabilities to simultaneously optimise the shape and internal structure of aircraft based on coupled high fidelity Computational Fluid Dynamics and Finite Element Analysis. However, to date the finite element methods used for the structural analysis are typically linear and are known to be inaccurate for structures experiencing large deformations, as modern high aspect ratio wings do. To address this challenge, robust and efficient solution strategies for nonlinear structural analysis and coupled aerostructural analysis have been developed for the high-fidelity MACH MDO framework. Structural and aerostructural analyses and optimisations are then performed to investigate the effect of geometrically nonlinear structural phenomena on the results of high-fidelity aerostructural analyses and, consequently, how these changes effect the optimal design of aircraft wings.

Electrification of aviation is driving new aircraft configurations consisting in slender, lighter wings with several propellers spread across the wing span, including the wing tips. Thinner wings with high aspect ratios tend to be more flexible, and thus, more susceptible to wing flutter, a dynamic aeroelastic instability characterised by diverging oscillatory motions of the wing. As propellers are flexibly attached to the wing, another aeroelastic instability, called precession or whirl flutter, can occur. The nature of this phenomena lies in the additional forces and moments created by rotating blades as the propeller hub describes a “whirling” motion about the static thrust axis due to gyroscopic effects. These dynamic instabilities can happen independently or coupled together causing severe structural damage on the aircraft and when occurred during flight, lead to fatal accidents. Hence, it becomes essential to analyse any new propeller-driven aircraft configuration regarding wing flutter and whirl flutter. A mathematical model has been derived to analyse the dynamic behaviour of a flexible cantilever wing with flexibly attached tractor propeller/s: the wing structural model is a linear dynamic Euler-Bernoulli beam model, the wing aerodynamic model is represented by a combination of strip theory and two-dimensional Theodorsen's unsteady aerodynamic theory, and the propeller aerodynamic model is defined by Houbolt-Reed propeller aerodynamic derivatives. The aeroelastic behaviour of a wing with flexibly mounted propeller/s may be very different from that of a wing with rigidly mounted propeller/s. Reasons lie in the interaction between the additional whirl modes and the wing modes due to gyroscopic coupling. Whereas propeller and wing aerodynamics may drive the (coupled) backward whirl mode unstable, wing aerodynamics alone may drive the (coupled) forward whirl mode unstable.

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.