Experimental Investigation of the Performance and Flow Field of a Propeller in both Propulsive and Regenerative Conditions

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.