This article describes an experimental investigation of the aerodynamic interaction that occurs between distributed propellers in forward flight. To this end, three propellers were installed in close proximity in a wind tunnel, and the changes in their performance, flow-field characteristics, and noise production were quantified using internal force sensors, total-pressure probes, particle-image velocimetry (PIV), and microphones recessed in the wind-tunnel wall. At the thrust setting corresponding to maximum efficiency, the efficiency of the middle propeller is found to drop by 1.5% due to the interaction with the adjacent propellers, for a tip clearance equal to 4% of the propeller radius. For a given blade-pitch angle, this performance penalty increases with angle of attack, decreasing thrust setting, or a more upstream propeller position, while being insensitive to the rotation direction and relative blade phase angle. Furthermore, the velocities induced by the adjacent propeller slipstreams lead to local loading variations on the propeller disk of 5% – 10% of the average disk loading. Exploratory noise measurements show that the interaction leads to different tonal noise waveforms of the system when compared to the superposition of isolated propellers. Moreover, the results confirm that an active control of the relative blade phase angles between propellers can effectively modify the directivity pattern of the system.

Advances in aerodynamic and propulsive efficiency of future aircraft can be achieved by strategic installation of propellers near the airframe. This paper presents a robust and computationally efficient engineering method to estimate the load distribution of a propeller operating in arbitrary nonuniform flow that is induced by the airframe and by different flight conditions. The time-resolved loading distribution is computed by determining the local blade section advance ratio and using the sensitivity distribution along the blade, which is a property of the propeller in isolated conditions. The method is applied to four representative validation cases by comparing to full-blade computational fluid dynamics (CFD) simulations and experimental data. For the evaluated cases, it is shown that the changes in the propeller loads due to the nonuniform inflow are predicted with errors ranging from 0.5 up to 12% compared to the validation data. By extending the quasi-steady approach with a correction to account for unsteady effects, the time-resolved blade loading is also well approximated, without adding computational cost. The proposed method provided a time-resolved solution within several central processing unit seconds, which is seven orders of magnitude faster compared to full-blade CFD computations.

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.

This paper presents an experimental and numerical study of the aerodynamic in-plane and out-of-plane loads of a propeller which are induced by the wake of an upstream wing impinging on the lower half of the propeller disk. A propeller was installed behind a wing model in a low-speed wind-tunnel and measurements were taken with an external balance and a rotating shaft balance to determine the aerodynamic characteristics of the wing and propeller. The installation of the wing shows negligible changes in propeller thrust coefficient at low advance ratios, while at medium thrust conditions (C _T ≈ 0.3), the wing shows a small increase in propeller thrust in the order of 1%. The installation of the propeller aft of the wing shows a change on propeller efficiency ranging from ∆η _p =–0.01 to +0.04. The location of the wake impingement at the propeller plane is shown to play an important factor for the time averaged and unsteady propeller loads. The radial location where the largest change in load occurs due to wake impingement, coincides with the location of highest propeller loading. A simplified and computationally efficient method is presented for estimation of these unsteady propeller loads in non-uniform inflow. The method shows good agreement for the integral unsteady blade thrust and integral propeller for different wake impingement locations.

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.

This paper presents an experimental and numerical study of the aerodynamic interaction between horizontal tail-mounted propellers and the airframe. A representative aircraft model was installed in a low-speed wind-tunnel and measurements were taken with an external balance to determine the effect of propeller installation on integral forces and moments. Total pressure measurements were performed downstream of the model for qualitative analysis of the propeller–airframe interaction. The experimental data were complemented by full blade CFD analyses, which correlate excellently to the experimental data. Balance measurements indicate that the propeller installation results in an offset and a change in the slope of the pitching moment curve over the complete range of angles of attack. The extent to which the propellers contribute to the longitudinal control and stability was shown to be dependent on the angle of attack of the aircraft and the rotation direction of the propellers. The flowfield and computed propeller loads show that an inboard-up rotating propeller results in a neutral contribution to longitudinal stability towards higher angles of attack, while an outboard-up rotation enhances the stability for all positive angles of attack. The non-uniform inflow to the propeller induced by the airframe leads to a lateral shift of the thrust which influences the trim condition.

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.

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.