This study deals with the comparison of different numerical fidelity levels to predict the noise of an isolated propeller in positive and negative thrust conditions. For this purpose, unsteady Reynolds-Averaged Navier-Stokes (URANS) and Improved Delayed Detached-Eddy Simulations (IDDES) were carried out and compared with Lattice Boltzmann (LBM) very large eddy simulations and wind tunnel data measured at TU Delft. It was found that the aerodynamic behavior with respect to the propeller loads, flow field in the slipstream, and surface pressure was well predicted by all methods. In the subsequent acoustic propagation, according to the Ffowcs Williams - Hawkings analogy, it was found that the noise directivity differed when using different CFD methods for the case of negative thrust due to increased broadband noise, while all CFD methods showed a similar noise directivity at the first blade passing frequency in the positive thrust condition. In the negative regime, the URANS simulation did not take into account the broadband fluctuations, which led to a significant underestimation of about 40 dB in streamwise directions in the overall noise, while IDDES and LBM showed similar trends, but still deviated about 5 dB in the prediction of the overall noise. Finally, a study was conducted with a low-fidelity acoustic evaluation based on Hanson’s model, which enabled a direct comparison of noise generated only by propeller loads, where a good agreement in tonal noise was achieved compared to the FW-H formulation with all CFD methods exhibit a similar noise directivity in both operation conditions.

Distributed propulsion systems, characterized by multiple propellers, represent a promising approach for full-electric aircrafts, offering several advantages but also introducing technical challenges. The main objective of this paper is to quantify how the propeller performance and noise emissions of an eight-propeller full-electric aircraft configuration compare to that of a conventional fuel-based turboprop. In both cases, the key parameters driving the trade-off between noise emissions and aerodynamic performances are analysed as well as the benefits of each configuration. The propeller noise emissions are analysed in terms of the perceived noise emissions at the three certification points: approach, take-off, and flyover. Optimizations are performed as a function of blade count to investigate the performance and noise trends for different propeller configurations. The results show a promising performance for the battery-electric aircraft with distributed propulsion, achieving a propeller efficiency between 83% and 88% in cruise without incurring a major noise penalty compared to the reference turboprop aircraft, despite the large increase in aircraft size and weight.

The far-field acoustic emissions of an isolated scale-model propeller ingesting turbulent inflow are experimentally investigated in wind-tunnel measurements. Phase-averaging and phase-shifting signal processing techniques are used to separate the deterministic (tonal) and random (broadband) components of the recorded sound signals. It is reported that a combination of the two techniques prevents tones unrelated to the shaft rotational frequency from contaminating the estimate of the broadband part of the signal. Scaling and directivity analyses of the obtained broadband component highlight the presence of two noise generation regimes depending on the considered frequency range. In particular, a quasi-omnidirectional directivity pattern is observed for frequencies for which the Helmholtz number is much lower than unity when turbulence is ingested. On the other hand, a dipole-like pattern with minima close to the propeller’s rotational plane gradually appears for higher frequencies. A time- and frequency-domain analysis through the Continuous Wavelet Transform (CWT) method shows how the increase in broadband noise is due to a large number of short-duration pulses linked to the ingestion of turbulence.

Accurately determining experimental blade loading distributions is crucial for analyzing rotor performance but challenging due to the limitations of conventional measurement techniques. This paper presents a so-called wake-informed lifting line model that estimates blade loading distributions from phase-locked velocity measurements in the slipstream, eliminating the need for blade instrumentation. The model is evaluated against computational fluid dynamics (CFD) simulations under both attached and separated flow conditions. For the attached flow condition, the model achieves excellent agreement with CFD, with errors in the peak value of thrust distribution below 1%. In the separated flow condition, the model captures radial gradients and the shape of the thrust distribution but exhibits discrepancies in absolute values, with a 10% error in the peak value. These differences arise from the inherent limitations of the potential flow model, the increased significance of drag, and the heightened influence of the spinner’s presence in separated flows. Incorporating profile drag through external polar data improves the model prediction, reducing the error to 4%. The model cannot reliably predict power distributions without external polar data for both attached and separated flows due to the crucial role of drag in the torque direction. The application of the model to experimental flowfield data shows a performance similar to that of the validation case. Therefore, the wake-informed lifting line model offers a promising approach for obtaining experimental blade loading distributions, overcoming the limitations of traditional methods.

A 3D unsteady RANS simulation utilizing the Spalart-Allmaras (SA) turbulence model was conducted to investigate aerodynamic interactions within a propeller-wing-flap system. The research specifically examines the complex flow field around a slotted flap, highlighting the interaction between the propeller slipstream and the main wing and flap during powered high-lift conditions. Operating conditions include a chord-based Reynolds number of 2 million, with thrust and flap settings reflecting take-off conditions (J = 0.765, Tc = 1.267, 5/= 15°) at an angle of attack a = 8.3°. Chordwise pressure distributions and surface shear stress contours show strong agreement with previous experimental measurements and oil flow visualizations of the same geometry. Findings indicate that a portion of the propeller slipstream transfers from the pressure side of the main wing to the upper side of the flap through the cove, dominating the flap flow field. Although the upper side of the main wing experiences fluctuating flow originating from the propeller slipstream, this flow does not induce unsteadiness or penetrate the flap upper side boundary layer along the wing span. Furthermore, it is shown that the shedding of vortices from the propeller root, along with the resulting vortices on the lower side of the geometry, weakens the flap boundary layer as this flow is transferred through the cove area, consequently inducing flap flow separation. Overall, the findings provide valuable insights into propeller-wing-flap interactions, which had not been visualized before in this detail, yet emphasizing the need for further research to confirm and expand on these results.

Conventional propellers operating at negative thrust conditions, even at 0 deg angle of attack, are characterized by flow separation and significantly different noise emissions than at positive thrust conditions. Operating the propeller at nonzero angles of attack at negative thrust conditions can further impact aerodynamic performance and far-field noise emission. This paper studies these effects using lattice-Boltzmann very large eddy simulations coupled with the Ffowcs Williams and Hawkings analogy. At positive thrust, operation at 10 deg angle of attack increases thrust along the freestream direction by approximately 3% compared to operation at 0 deg angle of attack, while efficiency remains constant. Conversely, the negative thrust condition shows approximately a 7% decrease in thrust magnitude and a 10% reduction in regenerated power. In this condition, the positively cambered blade sections exhibit dynamic stall, resulting in broadband fluctuations of up to 10% of the mean loading near the blade tip. The nonzero angle of attack induces opposite variations in absolute blade loading between positive and negative thrust conditions, resulting in opposite changes in the noise directivity. At positive thrust, noise increases in the region from which the propeller is tilted away (i.e., below the propeller at a positive angle of attack), while the opposite occurs at negative thrust. The varying blade loading over the azimuth results in destructive interference between loading and thickness noise for the negative thrust case at the 10 deg angle of attack. These findings highlight the crucial role of considering nonzero angles of attack in propeller design and optimization analyses.

This paper presents an improved approach for fast numerical modeling of the mutual aerodynamic interactions between a wing and tractor propellers for preliminary design purposes. Vortex methods are used to model the propeller and wing aerodynamic performance. The blade element momentum (BEM) method, which is used to model the propeller performance, is extended to allow for a nonuniform inflow field, such that the upstream effects of the wing can be included in the propeller performance modeling. The circulation distribution over the propeller blades is then used in the slipstream tube model (STM) to determine the time-averaged propeller slipstream velocities. Finally, an improved vortex lattice method (VLM) is used to model the wing’s spanwise lift distribution. The method includes an often overlooked correction for the finite slipstream dimensions experienced by the wing segments, in both vertical extent and spanwise extent. This physics-based correction, based on the image vortex technique, partially offsets the large discrepancies in the lift augmentation found in previous low-order numerical analyses of propeller–wing interactions, while keeping the analysis routine computationally very cheap. The BEM and STM are validated against experimental data, while the VLM and complete propeller–wing system are validated against high-fidelity numerical data, confirming the accuracy of the used models. Discrepancies are found in regions or operating conditions where viscosity becomes more relevant, such as separated flows. The numerical model derived in this paper can be used for a quick and accurate first-order estimation of the aerodynamic performance of new conceptual distributed propeller aircraft.

The electrification of aircraft is strongly coupled with the use of propellers as a propulsion system because of their high efficiency and their convenient integration with electric motors. Due to the operational flexibility of electric motors, the propeller can also be used in alternative operations, such as negative thrust and power mode. By operating the propeller at negative inflow angles at the blade segments, the torque and thrust are in the opposite direction compared to the conventional positive thrust conditions. This can be useful for control purposes or for energy harvesting. An experimental investigation was carried out to explain the physics behind the aerodynamic performance of a propeller at both positive thrust and energy-harvesting operation. Next to the measured integrated forces and moments on the propeller, stereoscopic particle image velocimetry was used to analyze the flowfield around the blades as well as the slipstream behind the propeller disk to identify the dominating flow phenomena that drive the energy-harvesting operation. The highly cambered blade sections for this typical aircraft propeller do not operate efficiently in energy-harvesting mode due to the associated negative angles of attack. The thin tip blade sections experience separated flow in these conditions, reducing the useful output power, compared to wind turbines, which feature opposite camber. To maximize the output power in the energy-harvesting conditions, a low pitch setting is required in combination with a relatively high advance ratio. However, this also comes at a cost of large negative thrust (drag) values.

The application of an acoustically absorbent material (melamine foam) is investigated for the treatment of turbulence grids in an anechoic open-jet wind tunnel facility featuring an axisymmetric contraction. A comparative study of both the generated turbulence and the grids’ self-noise is performed. It is found that the application of melamine foam on the downstream side of the grids marginally affects the produced turbulence, while providing an efficient suppression of tonal peaks in the grids’ self-noise spectrum. Broadband noise levels instead show opposing trends depending on the frequency range considered. On the one hand, a general decrease due to the acoustic treatment is observed for Strouhal numbers lower than unity. On the other, an increase, in the form of broad peaks, is seen to occur over certain higher frequency ranges.

To reduce climate impact of aviation, it is imperative to consider to introduce aircraft based on electrical engines. These electrical aircraft replace jet engines by propeller-driven propulsion systems, making the propeller the dominant noise source. A quieter and more efficient propeller blade design may generate a different noise signature, justifying a perception study to assess overall noise impact. In this study, a novel designed propeller “S2PROP” is compared with a baseline propeller “XPROP”. Both blades were measured in an aeroacoustic wind-tunnel, and wind-tunnel measurements of tonal and broadband noise were used as an input to generate fly-over sound samples of an aircraft equipped with these propellers. Atmospheric absorption, the secondary ground reflection path and Doppler effect were considered in creating a synthesized flyover sound. A noise simulator with virtual reality glasses and headphones was used to simulate both a visual and audible flyover procedure for participants of the perception study. Although a noise reduction is attained at the highest sound level around 600Hz for the S2PROP, it also generates higher broadband sounds at higher frequencies, resulting in finding no significant differences in perceived loudness or annoyance in the study between the two propeller designs.

Using propellers in negative thrust conditions can potentially result in many benefits, such as a steeper descent, a reduced landing run, reduced community noise, energy regeneration, etc. However, the aerodynamics and aeroacoustics of propellers in this regime are not well understood. This paper presents an aeroacoustic analysis of an isolated propeller operating in both positive and negative thrust conditions, using scale-resolved lattice-Boltzmann very large eddy simulations and the Ffowcs Williams & Hawkings analogy. The propeller was operated at a constant tip Mach number so that any differences in tonal noise between positive and negative thrust conditions were due to changes in blade loading. Results showed that the flow separation around the blades in the negative thrust case led to a 2 to 6 times higher standard deviation in integrated thrust compared to the positive thrust case. The blade loading in the negative thrust case shows the amplitude of fluctuations up to 18% for inboard sections and up to 30% near the blade tip compared to the time-averaged loads. The noise in the propeller plane is 10 dB higher in the positive thrust regime than in the negative thrust regime at a given absolute thrust level of |T_C = 0.08|. The lower noise at negative thrust is caused by two factors: the lower magnitude of the negative torque compared to the positive torque at a given thrust level and the shift of the blade loading inboard in the negative thrust condition due to the stall of the blade tip. Along the propeller axis, the negative thrust regime has 13-15 dB higher noise because of the increased broadband noise generated by the flow separation. In the negative thrust case, the noise along the propeller axis (89 dB) and propeller plane (92 dB) are comparable. However, this is not the case for the propulsive case. The comparison of noise in the vicinity of the propeller plane showed that using the propellers in negative thrust conditions allows for a steeper and quieter descent compared to a conventional descent; as long as the magnitude of the negative torque produced is equal to or less than the torque required to operate the propeller in a conventional landing.

Hybrid- or fully-electric propeller-based propulsion systems have recently gained interest as an option to reduce greenhouse gas emissions within the rapidly expanding aerospace industry. The electrification of aircraft enables the possibility for energy harvesting during flight phases where no power input is required. The use of aircraft propellers to harvest energy was first suggested by Glauert [1] in 1926, although there was no feasible technology at the time of his research to implement the idea. Seventy years later, MacCready [2] and Barnes [3]–[5], revisited the concept in a battery electric and self-launching sailplane, which could operate its propellers as energy harvesters during descending flight. Both MacCready and Barnes found that the optimal propeller geometries for energy-harvesting and propulsive operation significantly differ from each other. Recently, Erzen et al. [6] were able to obtain a 19% decrease in energy consumption during the ascend/descend flight pattern with a rigid propeller designed specifically for propulsive and energy-harvesting operation in comparison to a conventional propeller. The observed performance improvement, however, heavily depends on the selected flight pattern. The proposed paper will investigate the potential for aeroelastic tailoring of composite blades to improve the performance of propellers operating in propulsive and energy-harvesting modes, considering a realistic mission profile of a typical general aviation type of aircraft. To assess the effect of the mission profile on the propeller design, the mission profile was varied by changing the length of the cruise phase relative to the ascend and descent flight phases. To obtain an optimal propeller design featuring tailored composites, an aeroelastic model [7] was assembled by closely coupling a nonlinear Timoshenko beam model with BEM theory. The model was embedded in an optimisation routine considering lamination parameters and pitch setting as design variables, while the propeller geometry in terms of spanwise chord and twist distribution was kept constant. In addition, both fixed- and variable-pitch propellers were considered during optimization studies involving the full mission, and optimal blade designs corresponding to each individual mission segment were also obtained. The collected results confirm that composite tailoring can noticeably improve the performance of dual-role propellers, especially for mission profiles featuring relatively short cruise flight phases through the reduction of energy consumption by up to 2.0% with respect to the baseline rigid propeller. As the cruise distance is increased, maximum decreases in energy consumption are reduced to 1.5%. Lastly, it is interesting to observe that composite tailoring dominated by cruise distance has a positive effect on the performance during the ascend flight phase as well.

The goal of this study is to determine the aero-propulsive performance of an over-the-wing distributed propulsion (OTWDP) system, and to understand how it depends on various operating conditions. For this, a windtunnel test is performed with a simplified OTWDP geometry consisting of three unducted propellers placed side-by-side above a rectangular wing. A numerical model combining 2D panel methods, a slipstream vortex model, and a lower-order method for propeller performance in non-uniform inflow is then used to analyze additional operating conditions. A comparison to experimental data shows that the numerical method captures the changes in wing and propeller performance due to aerodynamic interaction in cruise conditions, though it is inaccurate if flow separation occurs on the wing surface beneath the propeller. For a setup with propellers of diameter-to-chord ratio 0.6 placed above the wing at 80% chord, the sectional lift-to-drag ratio of the wing is found to increase by 40% – 70% for typical cruise lift and thrust coefficients, while the propeller efficiency is decreased by 10% – 15%, compared to the two components in isolation. Parameter sweeps demonstrate that the combined aero-propulsive performance improves with a variable-pitch propeller and at higher lift coefficients, thrust settings, or Reynolds numbers.

Operating a conventional propeller at negative thrust results in the operation of positively cambered blade sections at negative angles of attack, leading to flow separation. Consequently, accurately simulating the aerodynamics of propellers operating at negative thrust poses a greater challenge than at positive thrust. This study offers a comprehensive assessment of the aerodynamic modeling capabilities of numerical methods, spanning low to high fidelity, for computing propeller performance across both positive and negative thrust regimes. Low-fidelity methods, namely, blade-element momentum and lifting line theories, effectively predict propeller performance trends at positive thrust. However, they fail to capture trends at negative thrust beyond the maximum power output point due to the neglect of three-dimensional flow effects. Both steady and unsteady Reynolds-averaged Navier–Stokes (RANS) simulations with y ⁺ < 1 perform well across both positive and negative thrust conditions, with errors below 2%for both thrust and power magnitudes near the maximum power output point. Lattice-Boltzmann very-large-eddy simulations (LB-VLESs) with y ⁺ ≤ 10 exhibit excellent agreement with experimental data with less than 1% error near the maximum power output point but with significant computational costs. Conversely, LB-VLESs with y ⁺ ≥ 15 offer a more economical approach to capture general trends with the computational cost of the same order as unsteady RANS. However, wall models introduce errors in modeling flow separation, leading to a 16% overestimation of power magnitude near the maximum power output point. The results highlight the necessity of using tools with increased fidelity levels when considering propeller operation at negative thrust compared to the conventional positive thrust regime.

Propellers have gained traction in recent years and are subsequently being integrated into unconventional airframe configurations as part of conceptual designs for developing sustainable aircraft. Close coupling of the propeller and airframe enhances the non-uniformity of the inflow to the propeller. This non-uniform inflow introduces unsteady loading, affecting the propeller's performance. This paper investigates the impact of accounting for installation effects during propeller blade design within an optimization toolchain. This toolchain is applied to a propeller mounted at the rear of a fuselage utilizing Boundary Layer Ingestion (BLI). The proposed design methodology utilizes existing analysis methods to assess the installed aerodynamic and structural performance of the propeller. A gradient-based optimizer is coupled to these analysis methods to design two distinct blades for a multi-segment mission with the objective to maximize the aerodynamic performance. The geometry of the first propeller blade is optimized for the uniform inflow and consequently optimized to operate in the non-uniform inflow. The second blade is from the outset designed for the non-uniform inflow condition. The implementation of the proposed design methodology captures the sensitivity of the inflow field within the design loop, resulting in two distinct blade designs. Upon comparing these blades in BLI inflow conditions to meet identical performance requirements, it was observed that the blade designed to account for installation effects featured a higher inboard chord distribution and consumed 1.58% less energy compared to the blade optimized for uniform inflow for the considered mission profile.

The integration of distributed electric propeller systems on aircraft wings presents complex aerodynamic interactions that are not yet fully understood. This study investigates the slipstream deformation in a distributed propeller configuration and compares it with a single propeller setup, visualised by experimental measurements of total pressure in the wake. Furthermore, we investigate the effects of relative blade phase angle on the resulting deformation. Overall, we identify several phenomena in distributed propeller-wing aerodynamic interaction that warrant attention in future research.

This study investigates the acoustic characteristics of a distributed propeller-wing system in a tractor configuration, focusing on the potential for noise reduction using the synchrophasing technique. The experimental setup features three propellers with nacelles mounted side-by-side on the leading edge of a wing, resulting in aerodynamic interference between adjacent propellers due to a tip clearance of 5% of the radius. The study investigates the acoustic impact of varying relative blade-phase angles (synchrophasing) and further explores the implications of changing the angle of attack. The findings show that synchrophasing influences noise emissions at the locations investigated, potentially amplifying or reducing noise relative to a random configuration. Moreover, these effects persist when investigating higher angles of attack.

Propeller–wing–flap systems are subject to complex aerodynamic interactions between each part of the system. Although the propeller–wing interaction in cruise conditions is well defined, the high-lift condition is relatively unexplored. Effective analysis of the complex aerodynamic relationship between propeller, wing, and flap is being impeded by a lack of understanding of the underlying mechanisms. In this paper, we therefore investigate the effects of a 2D jet impinging on a multisection airfoil. We quantify which factors that define a jet–wing–flap configuration dominate lift, drag, and moment responses. We further investigate interactions between these factors and discuss how they affect the flow. We find that the jet velocity ratio is by far the dominant factor in lift, drag, and moment responses, but it does not have strong interactions with other factors. The sensitivities of the multi-element airfoil do not change significantly when impinged upon by a jet, except when critical Mach numbers are exceeded. This strongly affects the aerodynamic response and dominant sensitivities. We furthermore conclude that the immersion of the flap is a key aspect when it comes to augmenting the lift by increasing the dynamic pressure in the flowfield. The conclusions from this paper can provide key insights for propeller–wing–flap flows.

A wind-tunnel experiment was performed at the DNW Low-Speed Tunnel with a powered propeller-wing model to prove the concept of energy-harvesting with propellers and assess its impact on the wing performance. By separating the contributions of the propeller and wing to the overall system forces, both for positive and negative thrust settings improved understanding was obtained of the propeller-wing interaction. A tip-mounted propeller configuration was simulated. At positive thrust settings, the operation of the propeller increased the lift gradient and improved the aerodynamic efficiency of the wing (L/D) by 10-35% compared to the propeller-off configuration. At CL = 0.5 and net zero force in streamwise direction the benefit was 12%, while at CL = 1.0 and a net force in streamwise direction of approximately three times the wing drag the benefit was 32%. At negative thrust, the propeller operation decreased the lift gradient, but the wing aerodynamic efficiency was still higher than that of the propeller-off configuration. This was an unexpected result, which was explained by the reduction in friction drag on the wing immersed in the propeller slipstream due to the lower dynamic pressure, and a possible reduction in wing induced drag due to downwash on the outboard part of the wing. The aileron effectiveness was decreased by about 10% when switching from positive to negative thrust operation. However, for angles of attack up to approximately 14 degrees even at negative thrust, the aileron effectiveness was still higher than for the clean wing.

This paper discusses the early-stage development of a fast propeller design tool using low-fidelity methods. Aerodynamics, aeroacoustics, and structural behavior of the propeller have been incorporated into an optimization framework to generate more efficient and quieter propeller designs. A first optimization process has successfully provided a set of more efficient and/or quieter designs among which one specific geometry has been manufactured. CFD validation has confirmed its aerodynamic performances and reasonable agreements have been observed with experimental results, with some discrepancies, however. Additional parametric studies are also discussed.