The flared folding wingtip (FFWT) is an emerging concept that offers potential benefits in reducing airframe loads during maneuvers and gust encounters, while also enhancing aircraft handling qualities. FFWT performance is strongly influenced by the orientation of the hinge line relative to the incoming flow, making the system particularly sensitive to nonzero sideslip angles, such as those encountered during crosswind landings. This paper investigates the static and dynamic behavior of FFWTs under sideslip conditions using a custom-designed wind tunnel model capable of large wingtip deflections, alongside a geometrically nonlinear numerical model. Results demonstrate the existence of stable equilibrium configurations up to, and beyond, a 90 deg fold angle, even when the effective flare angle becomes zero or reverses sign. Assuming that wingtip contact with the inner wing bounds the safe flight envelope, the maximum achievable sideslip angle is shown to be approximately equal to the flare angle but is also dependent on angle of attack and wingtip twist (or equivalent aerodynamic camber). These effects introduce additional unstable equilibrium branches at sideslip angles up to 5 deg below the flare angle, narrowing the viable operational envelope. Finally, it is shown that accurate prediction of frequency variations with sideslip requires accounting for additional geometric effects, such as changes in wingtip sweep, which reduce flutter speed and may further constrain the flight envelope.

This study investigates the aerodynamic, aeroacoustic, and psychoacoustic behaviour of a side-by-side twin-propeller Unmanned Aerial Vehicle (UAV) system operating under both static and forward-flight conditions, with particular focus on the effects of asynchronous rotational speeds. Experiments were conducted using two identical five-bladed constant pitch propellers with a diameter of 9 in (228.6 mm) and a pitch to diameter ratio of 1. Rotational speed differences between 0 and 300 rpm were examined in 50 rpm increments at inflow velocities of 0 m/s, 14 m/s and 24 m/s. The results show that variations in rotational speed have a significant influence on both acoustic levels and perceived annoyance. Asynchronous operation causes the dominant tonal peak at the blade passing frequency to split into two components, reducing tonal reinforcement. This produces noise level reductions of approximately 2 dB in static and high advance ratio conditions, increasing to about 5 dB reduction at low advance ratios. Psychoacoustic metrics show greater sensitivity to tonal structure than to overall sound pressure level, with annoyance reductions of about 5% in static conditions and up to 15% at low advance ratios. A modest aerodynamic penalty of about 5% at ΔN=50 rpm is observed, increasing with larger speed mismatches.

Distributed Electric Propulsion (DEP) systems offer strong potential for sustainable aviation, yet unconventional rotor installations introduce new aerodynamic and acoustic challenges. This study experimentally examines how rotor-rotor separation influences the aerodynamic and acoustic performance of twin-isolated and wing-mounted DEP configurations using 9 '', five-bladed P/D = 1 propellers. The presence of the wing provides a small but repeatable installation benefit, resulting in an approximately 4% improvement in propulsive efficiency for advance ratios J > 0.4. For the twin-isolated configuration, separations up to 3% of propeller diameter produce noise levels around 2 dB higher than those at larger spacings, while noise characteristics stabilise for 4% of the propeller diameter. At the smallest spacing 1% of the propeller diameter, enhanced wake interaction leads to elevated velocity fluctuations and increased noise. In contrast, the DEP configuration shows only minor sensitivity to separation, providing acoustic shielding that reduces noise above the propulsion system, while increasing side-field radiation. Under static thrust conditions (M-T approximate to 0.210 to 0.315), the twin-isolated and DEP configurations show nearly identical noise levels, indicating that tip speed dominates the acoustic response. Under inflow, the twin-isolated configuration produces stronger broadband and tonal noise, with increases of up to 4 dB and 3 to 5 dB respectively, while the DEP configuration consistently remains quieter.

While Distributed Electric Propulsion (DEP) architectures are critical for emerging eVTOL aircraft, the complex rotor-structure interactions they introduce remain difficult to predict and experimentally validate. However, standard aeroelastic models often oversimplify the propulsion unit as a rigid mass, neglecting the internal structural dynamics that govern rotor-structure coupling. Furthermore, limited experimental data exists to validate how these gyroscopic and inertial mechanisms manifest on a representative flexible wing. To address this gap, this study presents the development and validation of a dynamically scaled Multirotor Test Rig. Inspired by the Maxwell X-57, the rig’s structural design was sized using a nonlinear Sequential Quadratic Programming optimisation algorithm to replicate the aircraft’s specific modal frequency ratios and mode shape sequences. Although designed for multi-rotor applications, this specific investigation isolates a single tip-rotor configuration, referred to as the Flexible Rotor-Wing Demonstrator, to establish a fundamental baseline for gyroscopic and structural coupling effects. Experimental Modal Analysis was conducted in non-rotating and rotating states and correlated with two MSC NASTRAN models: a high-fidelity 3D solid model and a computationally efficient 1D beam element model. The results successfully captured critical dynamic phenomena, including resonance interactions, frequency splitting into forward and backward whirling modes, and modal veering. These findings validate the numerical framework, providing a verified baseline for future investigations into full multi-rotor dynamics.

The development of distributed electric propulsion systems aims to advance sustainable aviation; however, increased noise levels, particularly with unconventional rotor placements, pose new challenges. This study experimentally investigates the acoustic performance of overlapping propellers, focusing on the effects of lateral and axial separations under varying inflow conditions and rotational speeds. Experiments were conducted in an aeroacoustic wind tunnel using a twin side-by-side configuration with five-bladed propellers. Noise analysis reveals that tonal noise is the primary contributor to the increased overall sound pressure level, exhibiting significant sensitivity to the lateral distance across all advance ratios and directivity angles. At a lateral distance of approximately 40% of the propeller diameter, tonal noise levels are found to be significantly elevated—by approximately 14–18 dB—compared to the non-overlapping case. Results indicate that lateral separation distance is the dominant factor influencing acoustic behaviour due to blade-vortex and blade-wake interactions, while axial separation distance plays a secondary role.

Galloping instability is considered to be one of the most important wind-induced effects in overhead transmission lines which can cause severe damage or forced outages. The improved understanding of this phenomenon and the associated predictive capabilities are at the centre of ongoing research. The use of minimal dynamic models and high level of modelling abstraction allows to narrow the analysis on the critical factors influencing this phenomenon. Among these, owing to its complex physics, the characterisation of aerodynamic loading associated with the rotational activity of a multi-conductor bundle in cross-flow has traditionally relied on crude assumptions regarding the location of the aerodynamic centre. This work aims to determine and assess the location of this point under the angular reorientation of a four-conductor bundle within a newly developed identification framework, leveraging two established mathematical models and experimental data. The damping matrices corresponding to the two chosen models are correlated and the least square optimisation problem is formulated to identify the location of the aerodynamic centre. Under the gradually increasing bundle angular reorientation, it is found that, whilst the aerodynamic centre maintains a direct upstream azimuthal orientation, the distance from the geometric centre of the bundle fluctuates significantly. The resulting updated model, which uses the new configuration-dependent aerodynamic centre specification, is then successfully validated against the reference model and experimental data with the aim to be eventually transferred to real-world scenarios in industry. Based on the obtained characteristics, it is suggested that the configurational dependency of the aerodynamic centre is caused by the combined influence of the cyclic symmetry and aerodynamic shadowing between the upstream and downstream cables inherent to the multi-conductor bundles.

This study provides an in-depth experimental investigation into the aeroacoustic characteristics of Distributed Electric Propulsion (DEP) systems, emphasizing the influence of relative phase angles between adjacent propellers. Phase synchronization is identified as a highly effective noise reduction strategy, leveraging destructive interference within the coherent acoustic source field generated by the propellers. Experiments were conducted at a constant propeller rotation rate of 5000 rpm across various advance ratios to examine the impact of operational conditions on noise emissions. Results reveal a clear relationship between relative phase angles and far-field noise levels, with the most significant noise attenuation observed at a phase angle of Delta Psi = 90 degrees . At this configuration, reductions in the first blade passing frequency (BPF) amplitude reached up to 24 dB under inflow conditions (J > 0), compared to Delta Psi= 0 degrees, highlighting the potential of phase synchronization in mitigating tonal noise. This reduction is attributed to destructive wave interference driven by the angular positioning of the propeller blades. The results confirm that phase control can be a powerful tool in reducing perceived loudness and psychoacoustic annoyance, especially under inflow conditions (J > 0). To implement and evaluate precise phase control, a LabVIEW-based dual-loop feedback controller was developed and tested on a twin-rotor system in non-axial flight. The controller maintained phase deviations within 1.5% (approximately 5 degrees) across various tip Mach numbers and inflow velocities, demonstrating robust and consistent performance. SPL directivity measurements revealed that while Delta Psi = 90 degrees effectively reduced noise in axial flight, its benefits in non-axial configurations were more localized and direction-dependent.

This study aims to experimentally and numerically investigate and compare the aerodynamic performance of two triple-propulsor leading-edge mounted Distributed Electric Propulsion (DEP) configuration. The experiments are conducted in the Wind Tunnel Facilities at the University of Bristol. Propeller performance is examined experimentally against their positions on the wing at different inflow speeds and wing angles of attack (AoA). Numerical Vortex Particle Method (VPM) simulations are carried out to compare with the experimental aerodynamic results, and provide an exploratory noise comparison between the configurations. The DEP configurations involved in the current work are three 12-inch (the 12-12-12 configuration) and three 15-inch (the 15-15-15 configuration) propellers. Aerodynamic measurements for the propellers and the overall wing are examined and analysed at inflow speeds of 10 and 20m/s. The 15-15-15 configuration is operating at lower propeller rotational speeds to match overall thrust to be the same as of the 12-12-12 configuration. The aerodynamic measurements indicate that mid propeller produces less thrust than adjacent propellers while having comparable torque, and this difference becomes more significant as propeller rotational speeds and wing AoA increase. For the 12-12-12 configuration, the propeller performance is less sensitive to the change of angle of attacks at 10m/s, whereas a noticeable increment in thrust is captured for the inboard propeller for the 15-15- 15 configuration as AoA increases. Wing performance comparison indicates that the 15-15-15 wing has slight lift-to-drag ratio improvements at low angles of attack, whereas the 12-12-12 configuration is marginally more favourable at high angles of attack. Numerical simulations show good agreement on propeller thrust predictions, whereas torque predictions significantly vary across different positions on the wing. The noise results indicate that the 15-15-15 configuration has a lower tonal noise emission than the 12-12-12 configuration at 1(st) Blade Passing Frequency (BPF) when producing the same thrust on the wing.

Advanced air mobility represents efficient and sustainable transportation through the use of electric vertical takeoff and landing (eVTOL) aircraft, which will see their propellers operating in edgewise flow for a significant portion of their mission profile, giving rise to challenging aerodynamic and aeroacoustic features, different from the conventional helicopter rotors. The present experimental study examines the effect of the blade tip Mach number on the acoustic signature emitted by a propeller operating in edgewise flight conditions. Both aeroacoustic and aerodynamic measurements were performed for a 12-in., two bladed propeller over a wide range of tip Mach numbers (MT) from 0.12 to 0.56, and inflow velocities from 0 to 24 m/s. The results confirm the highly sensitive nature of the propeller noise to the tip Mach number. The overall sound pressure level (OASPL) in the plane-parallel to the propeller's plane of rotation, scales with MT3 to MT4 at low tip Mach numbers (MT<0.3) and with MT6 at MT>0.3, for the no inflow velocity case. Similar trends are observed for the plane perpendicular to the plane of rotation, but with noise scaling up to MT8. At the highest inflow velocity case, the OASPL scales with MT3 to MT7, depending on the observer location and MT, highlighting the increased complexity and sensitivity of noise directivity at higher inflow velocities. Tonal noise is the dominant source of noise at lower inflow velocities, with an increased contribution of broadband noise at higher inflow, possibly leading to the larger deviations of the tip Mach number scaling observed. These findings offer valuable insights into the acoustic behaviour of propellers during edgewise flight conditions, which can be leveraged to design more efficient, low-noise propellers in an eVTOL application.

This study aims to investigate the effects of phase synchronisation on tonal noise reduction in a multi-rotor UAV using an electronic phase-locking system. Experiments at the University of Bristol explored the impact of relative phase angle, propeller spacing, and blade geometry on acoustic performance, including psychoacoustic annoyance. Results show that increasing the phase angle consistently reduces the sound pressure level (SPL) due to destructive interference. For the two-bladed configuration, the highest noise reduction occurred at relative phase angle Δψ=90∘, with a 19 dB decrease at the first blade-passing frequency (BPF). Propeller spacing had minimal impact when phase synchronisation was applied. The pitch-to-diameter (P/D) ratio also influenced results: for P/D=0.55, reductions ranged from 13–18 dB; and for P/D=1.0, reductions ranged from 10–20 dB. Maximum psychoacoustic annoyance was observed when propellers were in phase (Δψ=0∘), while annoyance decreased with increasing phase angle, confirming the effectiveness of phase control for noise mitigation. For the five-bladed configuration, the highest reduction of 15 dB occurred at Δψ=36∘, with annoyance levels also decreasing with phase offset.

This study presents a numerical investigation into the aerodynamic and aeroacoustic characteristics of a 5-bladed propeller installed upstream of a symmetric wing in forward flight. The research aims to assess near-field flowinteractions and predict far-field noise levels, using two different approaches: a Lattice-Boltzmann-Method based solver implemented in the software PowerFLOW (R), in conjunction with the Ffowcs-Williams and Hawkings acoustic analogy, and a recently developed mid-fidelity fast and non-empirical approach, realized as a hybrid Reynolds Averaged Navier Stokes - Computational Aeroacoustics method, based on rotating line-distributed propeller sources. The computational setup emulates a typical operational condition, enabling detailed examination of how the upstream propeller's wake impacts the aeroacoustic profile of the wing-propeller configuration. This work provides insights into tonal and broadband noise contributions and highlights tonal noise amplification at blade passing frequencies due to the propeller-wing interaction. Further analysis will focus on the flow field characteristics, which are anticipated to reveal interactions between wake turbulence and wing surface, leading to complex noise patterns that could be mitigated in future design optimizations. Upon completion, this study will extend the understanding of installation effects on noise generation in propeller-wing configurations, with the ultimate aim of guiding more efficient aeroacoustic designs for advanced applications.

This study experimentally examines how turbulent inflow affects the noise emitted by a single propeller-wing configuration and compares it with that of an isolated propeller in clean flow, using a five-bladed propeller. Tests were conducted at free-stream velocities from 8 to 30 ms(-1), corresponding to advance ratios of J = 0.26-1.05. Aerodynamic measurements show that the wing improves propeller performance, increasing thrust and propulsive efficiency across the entire advance-ratio range relative to an isolated propeller. Spectral analysis of baseline SPL reveals that the overall reduction in noise with increasing.. is driven mainly by a pronounced decline in broadband energy between the first and third blade-passing frequencies. The propeller-wing system was exposed to both clean flow and three turbulent inflows generated by grids with progressively larger integral length scales and intensities. Turbulence-flow reduces the aerodynamic force coefficients yet preserves the general trend with advance ratio observed in clean flow. Far-field data demonstrate that turbulence-flow markedly increases the overall sound-pressure level (OASPL) at high J. For 0.26 < J < 0.46, the smallest-scale grid produces noise levels indistinguishable from those in clean flow, whereas the two higher-intensity grids introduce only mild broadband increases. Above J approximate to 0.46, all turbulent cases diverge from the baseline, with the highest-intensity grid adding up to 10 dB downstream. These results confirm that low-intensity, small-scale turbulence has little acoustic impact at high advance ratios, whereas larger, more energetic eddies significantly amplify radiated noise.