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.

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 propellerwing 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).

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The subcaptions of Fig. 4 are the wrong way around. Figure 4a depicts the pusher configuration, while Fig. 4b depicts the tractor configuration.