In the last few years, a new aircraft configuration has been developed: the electric Vertical Take-off/Landing aircraft (eVTOL). To improve aircraft efficiency, structures are lighter and more flexible. Due to the structure's flexible nature and the use of propellers for propulsion, eVTOL are prone to whirl flutter, which consists of a dynamic instability produced by the propeller aerodynamics and structure interaction.
This thesis aims to assess whirl flutter in a multirotor VTOL considering propellers in thrusting conditions. A semi-analytical model was developed and then implemented on MATLAB for two types of configurations: a conventional propeller and two counter-rotating propellers at the tip of a cantilever beam structure.
The propeller aerodynamics in thrust conditions were obtained through blade element momentum theory (BEMT) and extended to counter-rotating propellers. Then, the BEMT flow characteristics were coupled to a quasi-steady aerodynamics perturbation model using strip theory to obtain the forces produced by the propeller whirl motion. Additionally, the beam structure was modeled using a space frame finite element model, in combination with a two-degree of freedom system model for the propeller-pylon structure. Then, the structural system was coupled to the unsteady aerodynamic forces produced by the whirl motion obtaining the aeroelastic model from which the stability analysis was performed.
The model was used to analyze whirl flutter on a multirotor aircraft designed by the company Betronka SPA. It was found that the multirotor does not suffer from whirl flutter for the designed flight conditions, considering the conventional and counter-rotating configurations. Afterward, to study the thrust influence on whirl flutter, flow conditions and propeller angular speeds outside the designed limit were studied. The main findings are: the counter-rotating propeller configuration is more stable compared to the default configuration, increasing the whirl flutter speeds; thrust conditions stabilize the default configuration, increasing the propeller angular speed to encounter whirl flutter by 4%; thrust conditions are negligible for counter-rotating propeller configuration considering constant angular velocity.

Supersonic panel flutter is a self-excited vibration, where energy is transferred into thin flexible panels, such as skin panels in supersonic vehicles, from the (supersonic) flow around them, which can lead to their rapid fatigue failure. Relevant examples of this include the failure of skin panels in the X-15 vehicle, or violent oscillations in non-adaptable nozzles during start-up, such as the space shuttle main booster. With the contemporary trends of decreasing weight and increasing operational speeds in the design of next generation aircraft and launch vehicles, such structures will become more common, and flutter needs to be inherently accounted for in the design.

In recent years, advancements in numerical techniques, such as Computational Fluid Dynamics (CFD) and the Finite Element Method (FEM), have led to numerous studies that address the effect of non-linearities, such as flutter behaviour in the transonic and hypersonic regime. In spite of that, experimental data needed to validate these studies is limited, based on coarse pointwise measurement techniques, and only focused on the structural response of the panel rather than on the entire Fluid Structure Interaction (FSI). The use of simultaneous, full-field and non-intrusive measurements can be used to address these issues. To this extent, panel designs were developed and an experimental campaign was proposed where Digital Image Correlation (DIC) and Schlieren are simultaneously used by means of high speed cameras to study classical panel flutter at Mach 2 inside the ST-15 wind tunnel at the Delft University of Technology. Additionally, a Laser Doppler Vibrometer (LDV) was used to validate the DIC measurements. The experiment allowed for a comparison with well understood theory, such that the setup can be proven to be robust, and can therefore later be used to assess the effect of panel flutter non-linearities or other supersonic FSIs.

Synchronisation between DIC and schlieren was demonstrated through a matching frequency spectrum, and maximum correlation inside corresponding pixel windows between panel displacement and schlieren grey-scale fluctuations with a zero time-lag. One finding was that during the panel down-stroke phase, a travelling wave character was observed which has previously been found only under transonic conditions. Furthermore, at moderate dynamic flutter pressures, a typical second bending panel flutter mode shape was observed, which grew into a single bending mode again with increasing higher dynamic flutter pressures. From the spectral analysis it was found that all configurations fluttered at the same frequency of 770 [Hz], independent of dynamic flutter pressure or edge conditions. This was higher than expected from analysis. These combined observations led to the conclusion that, in the current facility, a frequency lock-in occurs due to merging of the flutter phenomenon with a wind tunnel resonance vibration.

These observations provide an insight into combining these experimental techniques to study supersonic FSI behaviour, and particularly into certain unexpected behaviour encountered at the TU Delft ST-15 wind tunnel. This can be used to further tune future experiments and guide researchers to obtain a better understanding in the non-linearities found in panel flutter.

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.

Quantum computing is a new form of computational technology, which can potentially be used to solve certain problems faster than is possible using classical computers. For this reason, there is an industry drive to develop early quantum computing applications. In this thesis, an overview of quantum computing technologies is provided, along with a practical discussion of the Traveling Salesman Problem, making use of the D-Wave quantum annealer. Subsequently, the main objective of the thesis can be investigated, which is to
explore how quantum computing can be used to aid in solving structural optimization problems. Two methods are developed with which simple 2-dimensional truss systems can be optimized using the D-Wave quantum annealer. The methods aim to find the most lightweight choices for the truss cross-sectional areas while complying with material limit stress constraints. The first method directly casts such an optimization problem into a QUBO format. However, due to difficulties with formulating the stress constraint, this method was found to produce a trivial optimization problem. The second method attempts to symbolically solve a truss finite-element problem, using the resulting symbolic expressions to set up an optimization objective function. Although these objective functions are confirmed to work via classical brute-force analysis, the quantum annealer is shown to have difficulty finding the global optimum solution for truss systems with three or more elements. These results indicate that it is not currently beneficial to use quantum annealing for these structural optimization problems. Nevertheless, some improvements to the method for setting up the objective functions are suggested. The next generation of quantum annealers is expected to perform better for these practical applications, potentially becoming a useful tool in the engineering toolbox.

This study investigates the wing deformation of the flapping-wing micro air vehicle (MAV) DelFly II in various flight configurations. Experiments were carried out with the MAV tethered in a windtunnel test section. To determine the best suited measurement approach, a trade-off study was carried out which showed that a point tracking approach with background illumination is most suitable. The therefore used high-speed camera pair and illumination were mounted on the same rotating frame with the DelFly, which allowed adequate viewing axes of the wings at for all pitch angles. Processing was done a purpose-build algorithm, allowing 136 points per wing to be measured simultaneously with an average lost point ratio of 3.4 % and an estimated accuracy of 0.25 mm. Results of hovering flight show some previously unnoticed behaviors. First, it was noted that the upper and lower wing on each side do not deform purely symmetric but show some considerable asymmetric behavior like heave and camber production. Furthermore, the upper wing shows a torsional wave and recoil behavior at faster flapping frequencies, which was shown to be beneficial in insect flight. Lastly, it was found that an air-buffer remains present between the wing surfaces at all times of the clap-and-peel motion (apart from the root trailing edge). This air-buffer increases once freestream velocity is added, which is investigated during the climbing flight study. Here, the reduced angle of attack of the wing is assumed to reduce the wing loading at faster climb, resulting in lower deformations outside the clap-and-peel motion. The isolated effect of a body pitch angle is also studied. Here, the asymmetrical freestream direction results in larger asymmetries such as wing alignment with the freestream direction and reduced camber and even camber reversal during the upstroke. In forward flight the pitch angle is changed simultaneously with the flapping frequency and freestream velocity. Due to the non-linear properties the wing deforms not directly as a superposition of the individual effects. Deviations are mostly present in increased asymmetry in incidence angle, while the camber behaves more linear and the clap-and-peel motion also remains relatively unchanged. The torsional wave and recoil are here however reduced. Descending flight was also tested. Velocities below 1m/s result in relatively minor deformation changes, while faster descent leads to large flapping frequency fluctuations, making interpretation of the results impossible.

To enable weight reduction of skin stiffened structures in aerospace applications they can be loaded in the post-buckling regime during service. However, to do this safely the fatigue delamination growth behaviour of post-buckled skin stiffened structures must be well understood. To this end, skin-stiffener separation of single stringer compression specimens with an initial delamination loaded in post-buckling has been investigated using the virtual crack closure technique. Both quasi-static and fatigue delamination propagation were studied using Abaqus FEA 2017. Two recently introduced fatigue delamination growth methods, the Constant Amplitude Fatigue method and the Simplified Fatigue method, were investigated. The methods were verified at the simple specimen level using double cantilevered beam and mixed mode bending specimens before they were used to predict delamination behaviour in single stringer compression specimens For the single stringer compression specimen a clear correlation has been observed between the local buckling of the delaminated stiffener flange and sudden delamination extensions, both under quasi-static and under fatigue loading.

Future wind turbines require larger rotor blade radius, leading to an increase of rotor swept area. The larger rotor swept area results in more energy production of the wind turbine. Further increase is however limited as a result of the structural weight, which induces a large bending moment and could result in buckling of the rotor blade. An innovative design solution resulting in a lower weight blade design could be acquired by introducing a grid stiffened structure as a substitution of the conventional sandwich structure. Design and analysis showed an increase in specific stiffness of the grid stiffened structure for higher loading conditions. In an attempt to validate the obtained results, one sandwich and one orthogrid panel has been manufactured and tested.

Offshore wind turbines are used more and more for the production of our electricity. The wind turbines are located in a remote and harsh environment, and are subject to heavy cyclic loading, which may cause fatigue in the support structures. Periodic inspections are required to assess the structural health of the wind turbines. Acoustic Emission Monitoring is the technique of acquiring and processing of the high-frequency sound waves emitted during crack growth. Accurate processing of these signals can lead to detection and localization of fatigue cracks, and further reduction of the need for costly periodic inspections. The factors that determine the coverage area of acoustic emission sensor nodes are source signal strength, attenuation of signal in the medium, and onsite noise level. Together with a cost benefits analysis, this leads to insight into the feasibility of this technique for offshore wind turbines. Investigation of the attenuation is done using a higher-order Spectral Element Method with the input acquired from experiments. The signal that is detected at the sensor has to have sufficiently higher intensity than the surrounding noise. The most severe sources of noise are rain drops hitting the water surrounding the turbine, for which a setup is used in the laboratory, and noise from rotating equipment, for which a measurement has been performed at an on shore wind turbine. The localization accuracy of 5 – 10 percent was shown to be achievable in a laboratory setup. With the coverage area determined, a monitoring strategy for a single wind turbine is proposed, as well as how an acoustic emission monitoring system can be efficiently implemented at a larger scale for a wind farm. It is concluded that acoustic emission monitoring is generally feasible for this application, yet further testing is required in order to decrease the uncertainties and to demonstrate the capabilities to potential operators.