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.@en

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 (propeller advance ration of 0.765, thrust coefficient of 1.267, flap deflection angle of 15 deg) at an angle of attack of 8.3 deg. 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.@en

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.@en

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.

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A tightly coupled aeroelastic design code for composite propeller blades was developed, verified, and used to perform design sensitivity studies. The design code features a structural model that accounts for geometric nonlinearities through the application of a corotational framework, nonlinear responses to loads, and a cross-sectional modelling approach to accurately represent the detailed 3D blade structure as a reduced-order Timoshenko beam element model. Blade element momentum (BEM) theory was used to evaluate aerodynamic loads, which are mapped onto the structural mesh. The nonlinear aeroelastic analysis routine uses Newton’s method to converge on a solution, with analytical derivatives for all applied loads. Excellent agreement with other analysis methods was shown during verification studies for all developed models. During validation, performance trends obtained from BEM were consistent with experimental results, with a maximum error of 20% at operating conditions under consideration during this research. The use of either symmetric-unbalanced or symmetric-balanced laminates was considered during sensitivity studies. Small variations in performance compared to the rigid propeller were observed from blades constructed out of symmetric-balanced laminates, as the minimal amount of bend-twist and extension-shear coupling resulted in small twist deformations. Conversely, propellers constructed out of symmetric-unbalanced laminates were shown to yield a noticeable variation in thrust and power compared to the rigid propeller due to the presence of bend-twist and extension-shear coupling, which results in coupling between twist and blade axis deformations. The presence of an aerodynamic wash-out effect was also found to alleviate blade loads, resulting in a lower power requirement at a given thrust setting, and an opposite trend was observed in the presence of a wash-in effect. The proposed analysis framework may be applied towards more extensive design studies or optimization in future projects.

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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.

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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. @en

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.@en

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.

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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.@en

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. @en

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.@en

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.@en

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.@en

Novel propulsion systems have been studied in literature to reduce aircraft emissions with hydrogen or other electrical energy sources. Hybrid Electric Propulsion (HEP) system consists of electric machines as an alternative way to provide power for propulsion resulting in the reduction of aircraft fuel consumption. While reduction of emission is the main driver of new HEP designs, aircraft noise reduction and performance improvement will also need to be investigated. Much quieter electrified aircraft than conventional aircraft is explored with considering the benefits of coupled design between the propeller and electric machines. In this study, several electric machine designs have been explored and coupled with the propeller design to study the trade-off between the aerodynamic and acoustic performance of the propeller. Aerodynamic optimization is used as a baseline to minimize the energy consumption to find the aerodynamics optimum subject to constraints on the thrust levels during the mission. The propeller aerodynamic optimizer considers the electric machine efficiency map, which is a function of propeller torque and rotational speed, to find the optimum combination of propeller and electric machine designs. The objective function of the acoustic optimizations is to reduce the cumulative noise level over the entire mission. It is shown that a wider envelope of peak motor efficiency in the efficiency map provides acoustics and aerodynamic performance benefits. The trade-offs between reducing noise or increasing aerodynamic efficiency to reduce energy consumption are demonstrated.@en

With the rise of distributed propeller concepts, there is an increased interest in quantifying the interaction between propeller slipstream and wing. It is known from literature that the local upwash induced by the propeller swirl can lead to a reduction of the wing-induced drag, a phenomenon often referred to as swirl recovery. However, at the same time, the distortion of the lift distribution due to the slipstream interaction cancels part of the swirl recovery benefit. These two separate mechanisms are often grouped together, but their relative contribution to the change in induced drag of a propeller-wing system is unknown. The goal of this paper is to separate the two mechanisms and to quantify their relative importance in terms of the induced drag of a wing immersed in a propeller slipstream. To this end, an improved approach for fast low-order modeling of the interaction between propeller and wing was implemented. The propeller performance is calculated using a BEM model, after which the induced velocities in the slipstream are modeled using a slipstream tube model. The propeller-induced velocities then have been implemented into a vortex-lattice analysis of the wing, including an often overlooked correction for the finite slipstream height experienced by the wing sections immersed in the propeller slipstream. It was found that the tip-mounted configuration with an inboard-up rotating propeller showed the largest reduction in total induced drag, even though the spanwise lift distribution was disturbed the most compared to other spanwise propeller positions. The swirl-induced drag mechanism outweighs the trailing vortex-induced drag mechanism. This is also true when the propeller is rotating in the other direction, when the largest performance degradation is obtained for the tip-mounted configuration.@en

A low-order numerical tool for estimating noise emissions from distributed propeller configurations is presented. The paper describes the tool’s computational framework, which uses Hanson’s near-field theory to calculate the thickness and loading noise components. The formulation assumes steady blade loading, but an unsteady case can be handled numerically by redefining the pressure distribution over the blade at each new time step. Two represent ativecases are analyzed to validate the tool: an isolated propeller operating in uniform flow and an array of three propellers in a side-by-side configuration under aerodynamic inter ference caused by adjacent propellers. The results obtained from the low-fidelity tool are compared to high-fidelity data to evaluate the accuracy and differences in predicting the noise of a distributed propeller system. The low-fidelity tool provides accurate results for both cases, with less than a1.5 dB difference up to the fifth blade-passage frequency (BPF) when comparing tonal noise predictions at an observer located 10 diameters away and at the propeller plane. When analysing the source directivity at the first BPF, there is a difference of approximately 0.5 dB at the propeller plane. However, this difference increases to 6 dB as the observer moves toward the inflow direction. This difference is due to the dominance of broadband noise near the propelleraxis. The paper concludes with a noise analysis of the distributed propeller system, examining the relative importance of aerodynamic interference in the noise emitted by a propeller. In this case, the unsteady blade loading generated a tonal component of 40 dB at the first BPF in the propeller axis, while it had an insignificant effect at the rotor plane.

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This paper studies the effect of operation at non-zero angles of attack on the aerodynamic performance and far-field noise emissions of an isolated propeller operating at positive and negative thrust conditions. To achieve this, scale-resolved lattice-Boltzmann very large eddy simulations coupled with the Ffowcs Williams & Hawkings analogy have been used. The results show that when the propeller operates with a 10◦ angle of attack at the positive thrust condition, the blade loading increases on the advancing side and decreases on the retreating side, leading to a 9.6% increase in integrated thrust (when computed along the propeller axis) and a negligible increase (0.1%) in propeller efficiency. Conversely, at the negative thrust condition, the operation at 10 deg angle of attack results in a 7.9% decrease in thrust magnitude and an 11.1% reduction in energy-harvesting efficiency. In this condition, the positively cambered blade sections exhibit dynamic stall at the 10◦ angle of attack, resulting in broadband fluctuations of up to 10% of the mean loading. As a result of the opposite change in absolute blade loading in the negative thrust condition compared to the positive thrust condition at the 10◦ angle of attack, the change in the noise directivity is also the opposite. Whereas in the positive thrust case, the noise increases in the region from which the propeller is tilted away (i.e., below the propeller at a positive angle of attack), in the negative thrust case, it is the other way around. This study highlights the need to account for non-zero angles of attack in propeller design and optimization analyses.@en

The use of electric motors enables a more flexible operation of propellers compared to conventional combustion engines. One possible benefit is the easier operation at negative thrust, which could be beneficial for energy recovery, control purposes, and lower noise through steeper descents. By changing the pitch of the propellers and actively braking them, the torque and thrust are in the opposite direction compared to the conventional positive thrust conditions. The aerodynamic off-design operations at the blade section in this operational regime impact the blade loading. An experimental investigation was carried out to analyze the aerodynamic performance of a three-bladed propeller in both positive and negative thrust and power conditions. Next to the integral propeller forces, the slipstream was analyzed to obtain a better understanding of the physical phenomena that determine the performance in the negative thrust regime. Both stereoscopic PIV and a 5-hole probe were used to obtain the local velocity and total pressure distributions inside the slipstream for three different thrust settings. The results show that the negative thrust operation is dominated by stall on a large part of the blades, limiting the negative shaft power. The maximum energy-harvesting efficiency is obtained for a low pitch setting and was found to be about 11%. However, the energy-harvesting at low pitch settings comes at a cost of high negative thrust. For low negative thrust values, the propeller can actually harvest more energy at a higher pitch setting. The slipstream analyses showed an almost flat blade radial loading distribution in the negative thrust regime indicating sub-optimal blade performance and possible separation on the blade sections. The velocity field in the propeller slipstream showed a reduction in axial velocity and an opposite swirl direction compared to the positive thrust mode when the propeller was used to harvest energy.

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Novel aircraft designs with (distributed) propellers often feature a close coupling between propellers and air¬frame, leading to unsteady blade loading which impacts propeller efficiency, noise emissions, and vibrations. The goal of this paper is to study the impact of such installation effects on propeller design optimization. A combination of existing, rapid analysis models is used to compute the installed aerodynamic and acoustic propeller performance. These analysis models are coupled to a gradient-based optimization scheme for the design studies. Comparisons are made between optimizations performed with and without taking installation effects into account, analyzing a 5-deg angle-of-attack case and a boundary-layer-ingestion case. The results show that increasing the blade count improves aerodynamic and acoustic performance both for isolated and installed configurations. Furthermore, the acoustic performance is improved significantly by decreasing the blade tip Mach number, albeit with associated efficiency penalty. For the nonuniform inflow fields considered, accounting for installation effects inside the optimization procedure did not lead to large benefits in terms of aerodynamic and acoustic performance compared to the isolated design. For the 5-deg angle-of-attack case, the installed design was similar to the design for symmetric inflow, with negligible change in propeller efficiency and at most 0.5 dB noise reduction. For the boundary-layer-ingestion case, the installed design featured in¬creased solidity and decreased twist, resulting in 0.4% lower energy consumption compared to the design for symmetric inflow. Acoustic performance was not evaluated for this case. Future work will focus on the sensi¬tivity of the results to the blade tip Mach number, the impact of sweep and airfoil design on installed propeller performance, and acoustic optimization of installed configurations with more complex nonuniform inflow fields.

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