This thesis investigates the aerodynamic performance of various Distributed Electric Propulsion (DEP) configurations using low-fidelity modeling tools. The study is motivated by the growing interest in sustainable aviation and the potential of DEP systems to offer improved aerodynamic efficiency, reduced noise, and greater design flexibility. Conducted in collaboration with Pipistrel Vertical Solutions, the research focuses on evaluating and optimizing different DEP layouts that combine minimum induced loss (MIL) propellers, dedicated for efficient cruise, and lift-augmenting (LA) propellers, designed to enhance lift during take-off and landing.

A key objective of the work is to develop a computationally efficient optimization framework suitable for early-stage design, capable of assessing multiple DEP configurations. To that end, the study integrates propeller design methodologies, slipstream modeling, and a DEP-specific lifting line solver into a single low-fidelity analysis tool. This framework is then coupled with multi-objective optimization algorithms, such as NSGA-II and SMPSO, to explore a wide design space and identify configurations that minimize take-off distance and cruise power requirements.

The thesis compares three main DEP arrangements, varying the number, size, and spanwise placement of MIL and LA propellers. The results show how propeller positioning and interaction effects influence overall aerodynamic performance, and they highlight the value and limitations of low-fidelity models in capturing these phenomena. The insights gained provide a foundation for future development of DEP systems and for refining low-fidelity tools for preliminary aircraft design.

With a symmetric rotor, tip vortex helices develop regularly before interacting, following the leapfrogging instability. The latter can occur earlier when the helices are placed at an initial radial offset, which is realized by considering blades of different lengths. This study investigates the spatio-temporal development of near-wake behavior for rotors with a significant blade length difference. Large eddy simulations and actuator line method on a modified NREL 5MW turbine under various inflow conditions were conducted to evaluate the impact of blade length differences ranging from 5 to 30%. The development of tip vortex helices, the onset of leapfrogging, vortex merging, and ultimately their three-dimensional breakdown were monitored. The analysis is corroborated by using a simplified (two-dimensional vortex-filament model).
The results show that the leapfrogging process begins immediately downstream of the vortex release when blades of different lengths are considered. The instability growth rate obtained from the 2D vortex model agrees with the LES data. Although the rotor asymmetry accelerates the leapfrogging and, in some conditions, the vortex merging process alone, it proves insufficient to cause a large-scale breakdown of the helix system and enhance wake recovery. Under laminar and 5% turbulence inflow conditions, the rotor asymmetry has minimal effect on momentum deficit recovery. In other conditions, no appreciable variations of wake recovery can be observed.

Turbofan engines are still one of the loudest noise sources of modern commercial aircraft. Noise radiation from the engine is highly directional and it is, therefore, important to obtain this directivity pattern from microphone-array data from static engine noise tests. Recently, Sijtsma has shown that the deconvolution method CLEAN-SC is able to break down engine noise sources and that it is capable of determining the directivity of these sources in the far field using measurements of a DGEN380 static engine test. This study further investigates this capability of CLEAN-SC by performing acoustic experiments under controlled conditions in the anechoic chamber at the faculty of applied sciences at Delft University of Technology. The experiments consist of a variety of tests with 2 different speakers placed inside an aluminum cylindrical pipe simulating an engine. In these experiments, the location and directivity of the individual noise sources can be measured separately, so that the CLEAN-SC results can be compared to the ground-truth reference. Moreover, the results are also compared with other acoustic imaging methods, such as DAMAS and conventional beamforming.

The aviation industry requires continuous reductions in greenhouse gas and noise emissions, which is resulting in a shift towards hybrid and fully electric aircraft. The propeller is an attractive form of propulsion system for electric aircraft as it can achieve high efficiencies and has excellent scalability potential. However, a major concern is the high community and cabin noise levels. It is possible to mitigate the propeller noise levels by reducing the blade loading and shifting the loading inboard, though this will often adversely impact the aerodynamic efficiency. Another method of reducing noise is by introducing blade sweep. This can result in destructive interference of sound waves. The downside is that sweep may impose significant structural deformations when the blade is loaded. These structural deformations may alter both the aerodynamic and acoustic performance. The objective of this thesis is to identify how these structural deformations will impact the aerodynamic and acoustic performance of low-noise, swept propeller blades. A parametric study was performed where the blade sweep was parametrised. The aerodynamic, acoustic and structural performances
of these propeller configurations were then analysed to identify the relations between these three disciplines.

A multidisciplinary propeller framework was used where aerodynamic, acoustic and structural tools could be coupled. For the aerodynamic analyses, a Vortex Lattice tool was developed, which can perform inviscid, incompressible simulations of any propeller geometry. The acoustics were calculated using Hanson’s Helicoidal Surface Theory, which models the blade sweep and dihedral as phase lag effects to model the interference between
sound waves. The structural tool used the Euler-Bernoulli beam theory for bending deformations and Saint-Venant’s torsion theory for twisting deflections. These three tools were coupled such that the final deformed shape of a loaded propeller could be obtained and evaluated. Each tool was validated individually using high-fidelity and experimental data, confirming that accurate results may be obtained within the tool’s applicable limitations.

The elastic blades were compared to their rigid counterparts to analyse how much the blade deformed, and how these deformations impacted the aerodynamic and acoustic performance. It was found that bending deformations are caused by the centrifugal forces, opposing any bending moments caused by the aerodynamic forces. The torsional deformations, mainly caused by the aerodynamic forces, result in a wash-out. Both the
bending deformations and wash-out of the blade reduce the local angle of attack, reducing the aerodynamic loads acting on the propeller blades. Since these structural deformations are concentrated near the blade tip, the local loading distributions are only impacted near the tip. This effectively shifts the loading inboard. The loss in performance can be significant if substantial sweep is applied, with losses of up to 20% of the thrust and torque. The efficiency, on the other hand, does not significantly change due to elasticity. The acoustic performance was mainly altered due to the changes in aerodynamic performance, with any changes in blade geometry having negligible effects. The overall sound pressure levels reduce by up to 2.5 dB, correlating with the reduction in thrust and torque. By eliminating the noise’s dependency on the overall thrust level, a clear correlation between the shifting of the local loading and the noise emissions was also found. These inboard shifting loads reduced the thrust-specific sound pressure by up to 1 dB.

It is possible to reduce the effect of elasticity by letting the structural forces impose a twisting moment, reducing the wash-out of the propeller blades. High bending deformations, or the addition of dihedral, allow the moment arm of the centrifugal loads to grow enough such that the twisting moments of the aerodynamic forces can be overcome. This can greatly diminish the aerodynamic performance loss. On the other hand, the associated increase of the peakiness of the local aerodynamic loading distributions may increase the noise
emissions even when a net loss in aerodynamic performance is still present.
Clear relations between the aerodynamic, acoustic, and structural performance of elastic, swept blades were identified. These changes in aerodynamics and acoustics are too great to be ignored for highly swept blades. Analyses of such flexible, highly swept blades clearly require the inclusion of structural calculations to accurately predict the aerodynamic and acoustic performance.

Minimisation of the climate impact of flights in the short term may be achieved by contrail avoidance. This involves avoiding ice supersaturated regions (ISSRs), which cause contrails to persist. ISSRs occur when the relative humidity over ice is above 1, but numerical weather prediction (NWP) models struggle to predict such regions. Below 235.15 K, solely ice supersaturation can be considered. This study validates the prediction of ISSRs in the ERA5 reanalysis using IAGOS in-situ measurements from a pressure level (350 to 175 hPa), seasonal, yearly (2011 to 2022) and regional perspective. Validation of ERA5’s temperature showed a cold bias in all extratropic regions and at low altitudes in the tropics, which can result in ERA5 predicting ISSRs when none are observed. Increasing temperature deviations between IAGOS and ERA5 were observed between the years 2020 and 2022. In terms of relative humidity over ice, ERA5 and IAGOS showed good agreement below ice supersaturation, but ERA5 struggled in simulating values close to or above a value of 1. For the latter conditions, ERA5 showed a dry bias in the extratropics below the tropopause. A moist bias was found in some regions and seasons, including South Asia, in June, July, and August. This may be due to inconsistencies in the modelling of the atmospheric ice content in ERA5. Years 2013 and 2014 also showed a possibility of a moist bias in ERA5 in some regions, but this was most likely the result of regional changes in sampling by IAGOS. The moist bias caused false detection of ISSRs in ERA5. Meanwhile, the dry bias led to an underestimation of the occurrence of ISSRs, with 25 to 50% remaining undetected. This causes an overestimation of no persistent contrail regions and an underestimation of persistent contrail and reservoir regions. When ERA5 overestimates the ISSR occurrence, it leads to an overestimation of persistent contrail and reservoir regions but an underestimation of no persistent contrail regions. Hence, improvements are necessary to successfully avoid or ensure unnecessary avoidance of ISSRs, where the latter could cause an increase in the climate impact of flights. Improvements should be done before implementation in contrail avoidance strategies, otherwise the benefit of such strategies may be defeated.

Boundary Layer Ingestion (BLI) is a promising technology for reducing the impact of aviation on the environment. By placing a propulsor on an aircraft such that it ingests the slower moving air within the boundary layer, a decrease in power consumption can be achieved. Evaluating the performance of BLI configurations remains a challenge though. Many conventional methods for computing the thrust and drag of an aircraft cannot be used due to the interference between the airframe and the propulsor. As a result, researchers have to rely on either low fidelity methods with limited accuracy, or on expensive CFD simulations. In this work, an attempt was made to create a computationally inexpensive method that can evaluate the drag, thrust and propulsive power of an axisymmetric fuselage with a BLI propulsor. The method proposed relies on potential flow modeling and makes use of a boundary layer model and several corrections.

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.

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.

Due to the increasing interest in modelling floating offshore wind turbines, simulation tools need to be adapted from fixed-base applications to surge applications. In this study, an open-source inviscid 3D panel method named Vortexje was adapted for surge motion and its ability to capture the physics of severe surge motion (when the rotor velocity approaches or exceeds the incoming wind velocity) was investigated. A simple test case first found instabilities with direct surface translation, likely a result of attached wake panel placement, justifying the use of equivalent dynamic inflow conditions for subsequent simulations.

It was then found that despite validation for a fixed-base rotor, inaccuracies and numerical instabilities remain when integrating the panel method pressure values directly. Estimating the thrust through the bound circulation provided a more accurate solution, comparable to existing viscous CFD results. It was found that the rotor could, in severe surge cases, briefly operate in propeller mode. Inflow angles were also estimated using existing CFD-related techniques, which provided reasonable results for fixed-base applications but inconsistencies with moderate and severe surge. It was found that the variations in induction factor did not approach those required to induce vortex ring state.

A propeller has a high propulsive efficiency, yet inherently leaves the air it propels swirling. The efficiency of a propeller can be enhanced by placing it in the tip vortex of a wing. Such a tip-mounted propeller configuration lets the vortex created by the wing and the vortex created by the propeller counteract each other, enhancing the propulsive efficiency of the system. Even though the benefits of such a configuration are clear, the concept is not commercially viable. Among other benefits, electric engines are scalable, light and can be placed in advantageous positions. This allows the engine to be placed at the tip of a wing. The batteries for electric propulsion are heavy, so synergistic effects are needed to improve the efficiency to such an extent that electric propulsion will be commercially viable. Another benefit can be defined by tip-mounted propulsion. As the propeller is mounted at the tip of the wing, the moment arm is large. This could result in a large directional control moment. On the other hand, if one engine is inoperative, the resulting moment is so large that it cannot be compensated by a rudder deflection. Therefore, if a one-engine inoperative situation arises, to still produce thrust both propellers have to be interlinked resulting in a heavy system, or both engines should be switched off. If tip-mounted propellers are used to enhance directional stability and control, the vertical tailplane size can be reduced, resulting in a reduction of mass, drag and hence overall power consumption. In this thesis the contribution of tip-mounted propellers to static stability, dynamic stability, and control is researched, for both positive and negative input power. Negative input power denotes that the propellers are recuperating energy fromthe airflow, and is a state that is applicable when one engine is inoperable. The forces on a tip-mounted pusher propeller are obtained by linking a lifting line wing model and a combined blade-element momentum vortex propeller model. The lifting line model is used to calculate wing-induced velocities on the pusher propeller disk. These induced velocities are used as input for the propeller model, and a resulting thrust and power is obtained. These are corrected for an angle of attack on the propeller disk by empirical relations. The thrust, normal force and power are saved to a 7D dataset. A non-linear flight mechanics model of the Piper Seneca III uses the dataset as lookup table and implements the tip-mounted propeller as forces at the tip locations. Thrust variations in the 7D dataset are parametrically visualised, showing the expected trends. Resulting forces are compared with a rudder deflection, to estimate a potential rudder size reduction. When the aircraft is flying slow, tip-mounted propellers can match the maximum moment produced by the conventional rudder. When flying fast, around half the reference yawing moment can be produced by tip-mounted propellers. The static stability contribution of tip-mounted propellers is visualised parametrically for propeller diameter, advance ratio, blade pitch and a toe-in angle. Negative toe-in angles, hence toe-out angles, prove to greatly enhance the directional static stability. High thrust values greatly enhance longitudinal static stability as the reference aircraft’s wing tips are positioned above the centre of gravity. In the same parametric fashion as the static stability evaluation, the contribution of tip-mounted propellers to dynamic stability is evaluated. The damping factor and natural frequency are obtained for first order motions Dutch roll and the Phugoid by fitting an exponential curve to the time-response of the flight mechanicsmodel. This method succeeds in capturing the motion characteristics, and indicates clear trends: the frequency of the Dutch roll increases with toe-out angles and thrust, and the frequency increases with thrust. The Phugoid’s natural frequency increases with thrust, and damping factor decreases with thrust. An attempt is done to use the curve-fitting method for the second order motion Short period as well, yet proves too inaccurate. A typical tip-mounted propeller design is evaluated to summarise the effects of using tip-mounted propellers for directional stability and control. For this non-optimised design, the static stability is enhanced by 37%. The dynamic stability has increased significantly as well. Maximum blade pitch deflection and maximum rudder deflection result in similar time-history sideslip responses when the aircraft is flying slow, when flying fast the tip-mounted propeller design’s sideslip response is around half the reference rudder deflection response. This conclusion also applies to a one engine inoperative simulation. In this thesis, a significant first step is made toward enhancing directional stability and control with tip-mounted propellers by defining the contribution of tip-mounted propellers in a parametric fashion. The results are directly applicable to an aircraft that has been designed with tip-mounted propellers, yet does not rely on tip-mounted propellers for directional stability and control. A typical design is evaluated in cruise, recuperative and one-engine inoperative cases, proving that from a stability and control point of view the vertical tailplane can be reduced in size. Before tip-mounted propellers can be used for directional stability and control behaviour analysis in the stalled regime, more research has to be conducted. This is left as recommendation.