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

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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The aim of the present manuscript is to investigate the noise footprint of an isolated propeller in different flight configurations for the propulsion of a hybrid-electric aircraft. Experimental tests were performed at the Low-Turbulence Tunnel located at Delft University of Technology with a powered propeller model and flush-mounted microphones in the tunnel floor. The propeller was investigated at different advance ratios in order to study the noise impact in propulsive and energy harvesting configurations. For brevity, this work only reports the results at the conditions of maximum efficiency in both propulsive and energy harvesting regimes, for a fixed blade pitch setting. Comparing these two configurations, a frequency-domain analysis reveals a significant modification in the nature of the noise source. In the propulsive configuration, most of the energy is related to the tonal noise component, as expected for an isolated propeller; however, in energy harvesting configuration, the broadband noise component increases significantly compared to the propulsive mode. A more detailed analysis requires separation of the two noise components and, for this purpose, an innovative decomposition strategy based on proper orthogonal decomposition (POD) has been defined. This novel technique shows promising results; both in the time and in the Fourier domains the two reconstructed components perfectly describe the original signal and no phase delays or other mathematical artifices are introduced. In this sense, it can represent a very powerful tool to identify noise sources and, at the same time, to define a proper control strategy aimed at noise mitigation.

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The distributed propeller concepts and associated electrification of aviation bring new opportunities to use the propellers in an efficiency-enhancing manner. Similar to regenerative braking in (hybrid-)electric cars, propellers can be used to recover part of the potential and kinetic energy during flight phases where no energy input is needed. The use of propellers as an energy recuperation system will result in a completely different flow field around the propeller blades, where the slipstream velocities will be markedly different than the propulsive case. Furthermore, the operation in off-design conditions will have a negative effect on the blade loading, since the positively cambered airfoils will be prone to separation at the negative angles of attack associated with the regenerative operation. The flow separation on the blades becomes very significant at the higher regenerative conditions and hence limits the regenerative capabilities of the propeller. To characterize the change in performance, blade loading and slipstream flow field when using the propeller in regenerative conditions, an isolated propeller experiment was performed in the Low-Speed Low-Turbulence Tunnel at Delft University of Technology. The experiment was done using a three-bladed version of an original six-bladed propeller which is representative of a turboprop aircraft. Three blades were removed from the original propeller to limit power requirements in propulsive and regenerative regimes while keeping a representative blade loading condition. The loads of the propeller were measured using an internal load cell and an external balance, to be able to separate the interaction effects between propeller slipstream and support structure. For the analysis of the flow field, stereoscopic PIV and a 5-hole pressure probe were used. Figure 1 displays a photograph of the test setup.@en

Wingtip-mounted propellers are a promising solution for advanced propulsion integration on future (hybrid-)electric aircraft. Previous work has confirmed the favorable aerodynamic interactions between the propeller and the wing that occur for wingtip-mounted propellers in both tractor and pusher configuration. However, a direct comparison of the performance effects for the tractor and pusher configurations is unavailable in open literature. Moreover, the separate contributions of the propeller and wing forces to the overall system performance have not been sufficiently separated in previous studies. This paper presents the results of a wind-tunnel experiment performed at Delft University of Technology with a modular propeller-wing setup that addressed these knowledge gaps. A powered propeller model with a nacelle was installed at the tip of a cambered wing model. The nacelle could be reversed in order to change from tractor to pusher configuration. Measurements with an external balance quantified the system loading, while an internal balance provided a separate measurement of the propeller loading. The results highlight the differences between the interaction mechanisms for the tractor and pusher configurations. An assessment of the system performance showed that the pusher configuration required the lowest propeller shaft power to achieve a given system lift coefficient and net force coefficient in the flight direction. Power reductions of up to 9% were achieved compared to the tractor configuration for lift coefficients between 0.0 and 1.0 and net axial force coefficients between 0.00 (force balance in flight direction) and +0.08 (net positive force in flight direction).@en