Many electric vertical takeoff and landing concepts are characterized by nontraditional vehicle layouts with distributed propellers. Two propeller interaction types were distinguished in this Paper, which investigates how propeller interaction in side-by-side and one-after-another configuration affects performance, in terms of thrust, power, in-plane forces, and out-of-plane moments, and how those performance effects depend on axial and lateral propeller spacing. A wind-tunnel experiment was performed with two propeller units, one instrumented with a force/torque sensor and the other introducing the aerodynamic interaction. Total pressure and planar particle-image velocimetry measurements were taken to investigate slipstream characteristics. A strong dependency of interaction effects on the geometric layout was found. For side-by-side interaction characteristic of vertical takeoff and transition, interaction effects varied from weak at small angle of attack to strong at larger angles. A drop in rear propeller thrust of up to 30% was found at constant advance ratio. Keeping thrust constant resulted in power penalties up to 13% for the two propellers combined. For one-after-another interaction, characteristic of cruise, a maximum reduction of thrust of up to 80% was observed. Thrust compensation led to power penalties up to 30% for the rear propeller alone. An extended blade element momentum model captured most interaction effects with sufficient accuracy.@en

An experimental dataset is presented of propeller performance in static condition and at low subsonic airspeeds for various angles of attack up to 90 deg. Numerical investigation through a Reynolds-averaged Navier–Stokes computational fluid dynamics model revealed the mechanisms behind performance changes with advance ratio, angle of attack, and configuration changes. The experimental dataset was found to be free of major errors and is very suitable for validation of propeller models. Furthermore, aerodynamic interaction with an upstream wing was tested with the propeller and wing normal to the flow, to represent the interaction occurring with a time-averaged main rotor slipstream on a compound helicopter. From numerical investigation it was concluded that the results are qualitatively representative of this interaction. The experimental data showed that addition of the wing results in a net reduction of all propeller performance quantities, with thrust reducing up to 20%. Thrust decreasing and thrust increasing mechanisms were found numerically. For most tested operating conditions, the wing resulted in a small decrease of propeller thrust-over-power ratio. Decreasing propeller advance ratio, increasing wing distance, and increasing flap deflection generally decreased the effect of the wing on thrust and power; however, the influence of flap deflection was found to be small.@en

Auxiliary lift and/or thrust on a compound helicopter can introduce complex aerodynamic interactions between the auxiliary lift and thrust components and the main rotor. In this study high-fidelity computational fluid dynamics analyses were performed to capture the various aerodynamic interactions which are occurring for the Airbus RACER compound helicopter, featuring a box-wing design for auxiliary lift in cruise and wingtip-mounted lateral rotors in pusher configuration for auxiliary thrust in cruise and counter-torque in hover. Although the study was limited to a specific geometry, the effects and phenomena are expected to be to some extent applicable in general for compound helicopters and wingtip-mounted rotors in pusher configuration. A quantitative indication of the aerodynamic interaction effects could be established by leaving away different airframe components in the simulations. The downwash of the main rotor was found to cause a small negative angle of attack in cruise for the wings and lateral rotors and impinged directly on the lateral rotors in hover, resulting in moderate to very significant sinusoidally varying blade loading. The wing increased lateral rotor propulsive efficiency in cruise through its wingtip rotational flowfield and to a lesser extent through its wake. An upstream effect of the lateral rotors on the wing loading was also found. In hover the wing caused a net increase in left lateral rotor thrust as the deflection of the main rotor flow towards the rotor resulted in a local thrust decrease and the low momentum inflow to the rotor from the wake of the wing resulted in a local thrust increase. A small thrust decrease for the right lateral rotor was found due to the wing disturbing its slipstream as this rotor produced reversed thrust. In general, very significant aerodynamic interaction effects can be expected when a main rotor, lateral rotors and wing are in proximity to each other.

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This research is on the aerodynamics of propellers in interaction. As a propeller is dependent on the flowfield it encounters, any disturbance in that inflow results in changes in loading on the propeller. This can result in propulsive efficiency changes and affect stability through generation of in-plane forces and out-of-plane moments. The loading changes can also impact noise through loading fluctuations and cause vibrations in the structure leading to fatigue. The objective of the research in this dissertation is to get a fundamental understanding of the role of aerodynamic interaction on the loading and performance of primarily the propeller and secondarily the interacting object(s) in typical configurations where interaction dominates the flowfield. For the propeller this refers to situations where the inflow is disturbed by such an amount that the disturbance is a dominating factor in the propeller loading.@en

The wingtip-mounted pusher propeller, which experiences a performance benefit from the interaction with the wingtip flowfield, is an interesting concept for distributed propulsion. This paper examines a propeller design framework and provides verification with RANS CFD simulations by analysing the wing of a 9-passenger commuter airplane with a wingtip-mounted propeller in pusher configuration. In the taken approach, a wingtip flowfield is extracted from a CFD simulation, circumferentially averaged and provided to a lower order propeller analysis and optimisation routine. Possible propulsive efficiency gains for the propeller due to installation are significant, up to 16% increase at low thrust levels, decreasing to approximately 7.5% at the highest thrust level, for a range of thrust from 5% up to 100% of the wing drag. These gains are found to be independent of propeller radius for thrust levels larger than 30% of the wing drag. Effectively, the propeller geometry is optimized for the required thrust and to a lesser degree for the non-uniformity in the flowfield. Propeller blade optimization and installation result in higher profile efficiency in the blade root sections and a more inboard thrust distribution.

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Wingtip-mounted propellers installed in a tractor configuration can decrease the wing induced drag by attenuating the wingtip vortex by the propeller slipstream. This paper presents an aerodynamic analysis of the propeller-wing interaction effects for the wingtip-mounted propeller configuration, including a comparison with a conventional configuration with the propeller mounted on the inboard part of the wing. Measurements were taken in a low-speed wind tunnel at Delft University of Technology, with two wing models and a low-speed propeller. Particle-image-velocimetry measurements downstream of a symmetric wing with integrated flap highlighted the swirl reductions characteristic of the wingtip-mounted propeller due to wingtip-vortex attenuation and swirl recovery. External-balance and surface-pressure measurements confirmed that this led to an induced-drag reduction with inboard-up propeller rotation. In a direct comparison with a conventional propeller-wing layout, the wingtip-mounted configuration showed a drag reduction of around 15% at a lift coefficient of 0.5 and a thrust coefficient of 0.12. This aerodynamic benefit increased upon increasing the wing lift coefficient and propeller thrust setting. An analysis of the wing performance showed that the aerodynamic benefit of the wingtip-mounted propeller was due to an increase of the wing's effective span-efficiency parameter.

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Wingtip-mounted tractor propellers enhance aerodynamic performance by attenuating the wingtip vortex with the propeller slipstream and inducing a favorable upwash on the wing. However, the close coupling between propeller and wing means that wing performance may be degraded when the propeller produces negative thrust. This paper analyzes the aerodynamic interaction effects due to the wingtip-mounted propeller under such conditions, that occur when the propeller is windmilling or used for energy harvesting. Experiments in a low-speed wind tunnel and simulations with a RANS solver highlighted the drop in wing performance at negative thrust for the case with inboard-up rotation. The interaction phenomena are reversed compared to the beneficial propulsive case, since the inflow velocity and angle of attack are now reduced on the part of the wing washed by the slipstream. Because of the reversal of the swirl in the slipstream at negative thrust, the interaction is then favorable with outboard-up rotation. For the considered propeller, that was not optimized for operation at negative thrust, the energy-harvesting efficiency was about 10%. This can be improved for future designs by optimizing the blade geometry and pitch setting of the propeller.@en

The potentially favorable interaction of a propeller slipstream with a wingtip is a complex problem, because of the importance of vortex interaction and viscous effects. This paper examines the capability of different propeller modeling methods in a Reynolds-averaged Navier–Stokes (RANS) solver for the simulation of wingtip-mounted propellers. The applicability of actuator-disk and actuator-line models to reduce the cost of propeller modeling is investigated in its most accurate form, by applying propeller-blade-loading results extracted from simulations, in which the blades are fully resolved. The numerical results are validated by comparison with measurement data from in-house wind-tunnel experiments. It is concluded that the aerodynamic interactions for the wingtip-mounted propeller in tractor configuration can be predicted by RANS simulations with a simple one-equation turbulence model (Spalart–Allmaras), provided that the uncertainty due to numerical diffusion is accounted for by a griddependency study, or reduced by local grid refinement. The actuator-line model reduced the computational time by 17% without introducing errors into the time-accurate and time-averaged wing loading. The actuator-disk model reduces the computational cost by 85% by removing time dependency, with a small penalty in the accuracy of the time-averaged flowfield and a 3.9% overprediction of the wing lift. @en

Installation effects of the lateral rotors for a compound helicopter were investigated by means of unsteady CFD simulations. The helicopter featured a box-wing design for additional lift in cruise and wingtip-mounted lateral rotors in pusher configuration for additional thrust in cruise and counter-torque in hover. It was found that propeller performance installation effects for a compound helicopter are highly beneficial in cruise, while some penalties exist in extreme and specific cases in hover and autorotation. In cruise the main interaction was between the wing and lateral rotors, resulting in a propulsive efficiency increase up to 10.6% due to wingtip vortex energy recovery. In hover the main rotor slipstream resulted in a near perpendicular inflow to the lateral rotors, with a disturbance from the wings due to the deflection of the main rotor slipstream. For higher than nominal lateral rotor thrust settings, this resulted in a performance penalty. In autorotation a small power installation penalty was present due to inflow disturbances.@en

This paper addresses the effects of propeller installation on the aerodynamic performance of a tailplane featuring tip-mounted propellers. A model of a low aspect ratio tailplane equipped with an elevator and a tip-mounted propeller was installed in a low-speed wind-tunnel. Measurements were taken with an external balance and surface pressure taps to determine the aerodynamic characteristics of the tailplane, while the flowfield in the wake of the model was investigated using particle-image velocimetry. The experimental data are supported by CFD analyses, involving both transient simulations of the full-blade configuration and steady-state simulations the propeller replaced by an actuator-disk model. The upstream effects on the propeller time-average and time-accurate thrust and normal-forces are found to be limited for different tailplane operating conditions. It is shown that for a given propeller rotation direction, the load distribution on the tailplane is highly dependent on the direction of elevator deflection. The rotation direction of the tailplane tip-vortex relative to the propeller swirl therefore significantly affects the integral loads on the tailplane, resulting in differences in the normal-force gradient and elevator effectiveness.@en

This paper examines the capability of a commercial RANS solver for the simulation of wingtip-mounted propellers. The applicability of actuator-disk and actuator-line models to reduce the cost of propeller modeling is investigated in its most accurate form, by extracting and applying propeller blade loading from full-blade simulations. The results obtained from all numerical simulations are validated based on measurement data from an in-house wind-tunnel experiment. An extensive grid dependency study is presented for the isolated propeller and the wing to distinguish discretization errors from model errors. It is concluded that RANS CFD with a simple one-equation turbulence model (Spalart–Allmaras) is capable of modeling the aerodynamic interactions for the wingtip-mounted propeller in tractor configuration, provided that numerical diffusion is accounted for by a grid dependency study or prevented by local grid refinement. The actuator-line model is fully able to replace propeller blade modeling in the simulation, and agreement with the full-blade simulations is found in time-accurate and time-average wing loading. The actuator-disk model further reduces the cost of the simulation by removing time dependency, at the cost of a small penalty in the accuracy of the time-averaged flowfield and lift distribution on the wing.@en

Swirl-recovery vanes (SRVs) enhance propulsive efficiency by converting the rotational kinetic energy in a propeller slipstream into additional thrust. This paper discusses the aerodynamic and aeroacoustic impact of the installation of a set of SRVs downstream of a single-rotating propeller. Experiments were carried out in a large low-speed wind tunnel, while simulations were performed by solving the Reynolds-averaged Navier–Stokes equations. Favorable comparisons between the experimental and numerical slipstream data validated the simulations, which predicted a maximum propulsive-efficiency increase of 0.7% with the current design of the SRVs. This can be improved further by optimizing the SRVs' pitch distribution. The upstream effect of the SRVs on the time-averaged propeller performance was negligible. Yet, small but systematic unsteady propeller loads were measured with a peak-to-peak amplitude of at most 2% of the time-averaged loading, occurring at a frequency corresponding to the five SRV passages during one revolution. The downstream interaction was one order of magnitude stronger, with unsteady loading on the SRVs with a peak-to-peak amplitude of about 20% of the time-averaged load. The interaction mechanisms caused an increase of the tonal noise levels of 3 to 7 dB, with the noise penalty decreasing with increasing propeller thrust setting.

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In this paper we address a preliminary assessment of the performance effects of swirl recovery vanes (SRVs) in a installed and uninstalled tractor propeller arrangement. A numerical analysis was performed on a propeller and a propeller-wing configuration after the SRVs were optimized first in a separate process. The SRVs are essentially designed to recover the swirl in the rotor slipstream, thereby increasing the propulsive efficiency. To confirm the main flow effects of the SRVs, a separate windtunnel study was performed on a single-rotating propeller model in the large low-speed windtunnel. A steady RANS-based numerical analysis showed that the application of SRVs in a propeller-only model may lead to an increase of propulsive efficiency in the order of 2 %. In case of the SRVs in a representative tractor propeller-wing configuration, the positive effects are reduced due to the swirl recovery effect of the wing and the upwash effect in the SRV plane. The latter requires an optimization per SRV blade which was not performed in this particular study. In de experimental study performance data of the propeller were acquired with a rotating shaft balance. Furthermore, Particle Image Velocimetry PIV measurements in the slipstream of the propeller with and without SRVs substantiated the efficacy of the vanes in reducing the swirl in the propeller slipstream. Reductions of 50% in the swirl kinetic energy was observed at a medium thrust settings.

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