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...

The Advisory Council of Aviation Research has set the future goals of sustainable development in aviation by reducing pollutant emissions of CO2 by 75% and 90% in NOx emissions per passenger kilometre and noise emission by 65% of the perceived noise level compared with the measurements in 2000. This demand has led to the development of new aircraft concepts with alternative propulsive systems. The development of the all-electric and hybrid-electric aircraft concepts has regenerated the interest in distributed propulsion systems with propellers. Distributed-propeller systems are propulsive configurations with multiple propellers located along the wing or/and aircraft's airframe. Among the benefits of these systems is their electrical connectivity with the power generating sources or energy sources, like batteries, that make them attractive for vehicles performing electrical vertical take-off and landing for urban air mobility.
Previous studies' investigation of multi-rotor systems has been primarily focused on the aerodynamic performance changes in hover conditions. The thrust decrease of these propellers during their operation at a close distance is accompanied by oscillations associated with the flow structures interactions. This results in a noise emission increase attributed to the thrust fluctuations. The investigation, therefore, of the distributed-propeller system in forward flight will contribute to the in-depth analysis of the performance of these systems and illustrate the root of potential changes, assessing the changes in thrust as well as the unsteady loading of the blades. Also, the present analysis will study the mechanisms that lead to noise emission increase and the potential benefits of the relative phase angle variation between adjacent propellers. The effect of the relative phase angle has shown a positive impact on noise emission of multiple-propeller systems without considering the interference between adjacent propellers. Thus, the aim of the present computational study is to investigate the aerodynamic interference and aeroacoustic signature of a distributed-propeller configuration in forward flight, including the effects of the relative phase angle.
The comparison of the distributed-propeller model with the isolated propeller case showed minor differences in the time-averaged performance. The mean thrust coefficient increases slightly by 0.0004, whereas the time evolution plot reveals the thrust oscillation with amplitude equal to 5.3% of the mean value. The trailing vortex systems of adjacent propellers induce velocity components that alter the propellers' flow field upstream and downstream. These induced velocities result in axial velocity increase upstream of the propeller and in non-zero values of the tangential velocity, resulting in thrust oscillations. This results in a change of thrust as the propeller rotates. During the motion of a blade, its thrust is reduced when it approaches the adjacent propeller and subsequently increases during its retreat, resulting in an unsteady loading on the blades. The slipstream of the systems also presents differences from the respective one of an isolated propeller. The symmetric and circular shape of the wake flow behind the isolated propeller is broken and turned into a deformed wake flow behind the distributed-propeller system, as a result of the interference of the tip vortices. The tip vortices emitted by adjacent blades stay close with each other during their downstream motion, with their interaction results in change of trajectory, shape deformation and fast dissipation.
The noise emission of the distributed-propeller system shows a noise level increase in front of the propellers while along the propeller plane, this increase is smaller. The comparison of the middle propeller of the system with the isolated one reveals an increase of 13.5 dB along the propellers axis, while the increase along the plane of rotation is 1.8 dB. The increase along the propeller axis is associated with enhanced tonal components at frequencies up to the 5th blade passing frequency. The directivity of the noise emission and the augmentation of tonal components imply that the unsteady loading is the reason for the noise levels increase. The increase of the broadband component (3.2 dB) at oblique angles with respect to the propeller axis is attributed to the interaction of adjacent flow structures.
The variation of the relative phase angle between adjacent propellers results in blades passing from the region between adjacent propellers at different times. This has positively impacted the unsteady loading as it is decreased compared to the case without relative phase angle variation. The oscillatory behaviour of the thrust is reduced as the standard deviation of the thrust coefficient drops from 0.001 to 4.5e-04. On the contrary, the time-averaged performance of the propellers remains unaffected. The reduction of the unsteady loading results in noise emission reduction in the upstream direction by 5 dB, while at oblique angles in the downstream direction, there is a 1.8 dB increase. This increase is attributed to a different tonal component distribution than in the case without relative phase angle variation. Thus, the impact of the relative phase angle variation could be beneficial on noise emission angles normal to rotor plane due to the reduced unsteady loading.

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.

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.

With the goal of decreasing the environmental impact of aircraft, ducted propellers emerge as an efficient propulsive alternative. Ducts are able to increase the thrust to power ratio of a propeller system by both producing thrust and/or lowering tip losses of propellers. In this thesis, RANS CFD simulations were used to analyse the possible impact of modifying a propeller duct shape from a circular to a square geometry. Initially, the two duct designs and the propeller were studied as isolated cases, in order to characterise their aerodynamic performance. In the installed simulations, the propeller was first modelled as an actuator disk, and afterwards with a full blade model. The results indicate two main disadvantages of square ducts. Square duct corners were found to be prone to separation, and to contribute towards the generation of strong vortices. This research can be important for the future study of unconventional ducted propellers, for example with applications to distributed propulsion concepts.

The research objective is to analyze the aerodynamic interaction effects and quantify the performance of over-the-wing propeller systems. To achieve this objective, two wind tunnel experiments have been performed to characterize the most important aerodynamic interaction effects of a single propeller installed over-the-wing. Results feature wing and wake plane pressure distributions and forces on the propeller. Based on the findings, a low-fidelity numerical model is developed. The model couples a panel method for the wing, a blade element method accepting non-uniform inflow for the propeller and a vortex lattice method for the propeller slipstream. The model has a low computational cost and is used to perform sensitivity analyses. Results indicate that the propeller is positioned optimally near the wing trailing edge. The tool is shown to be adequately accurate for the aircraft conceptual design phase, making it invaluable to explore new aircraft configurations.