Experimental Investigation of the Aerodynamic Performance of a Propeller at Positive and Negative Thrust and Power

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.