With the resurgence of interest in propeller-powered aircraft for short-haul and regional missions, understanding the aerodynamic interaction between propellers and nearby surfaces has become increasingly crucial. While propellers offer superior propulsive efficiency and sustainability benefits, their integration introduces complex unsteady flow phenomena that remain insufficiently explored. Additionally, concepts related to regenerative braking and the negative thrust regime aim to harness the full potential of propellers in the pursuit of sustainable aviation. However, the interactions are more complex in the negative thrust regime and need to be accounted for.

This study investigates the unsteady aerodynamic effects of a pusher-propeller operating upstream of a downstream airfoil, with a focus on both positive and negative thrust regimes. A flexible PCB embedded with microphones and pressure sensors was used to capture unsteady surface pressure fluctuations across different operating conditions. The motivation stems from the limited understanding of unsteady surface pressure fluctuations in negative thrust conditions.

The experimental campaign was conducted in two wind tunnel labs: the M-Tunnel and the Small Low-Turbulence Tunnel in the Low-Speed Wind Tunnel Laboratory of Delft University of Technology, Netherlands. The research was carried out in two experimental phases. In the first phase, the device was validated to verify the response of the microphones and the pressure sensors. A known tonal excitation case, along with a case involving an upstream cylinder, was conducted to assess whether the results aligned with theory and expected trends from the literature. These validation experiments confirmed the device’s capability to capture unsteady flow behaviour. However, certain limitations, such as overshoots in measured pressure data and restricted chordwise and spanwise resolution, were encountered. The second phase was conducted in the Small Low-Turbulence Tunnel featuring a fixed airfoil section downstream of a rotating propeller. The device, comprising the microphones and BMP390 pressure sensors, was wrapped around the leading edge of the airfoil to measure pressure fluctuations across the airfoil surface. Initial validation confirmed the reliability of the device in measuring the flow in the propeller slipstream, with good agreement with results from the literature. Comparative analysis was performed across multiple cases, including nacelle-only baselines and propeller-on conditions at two different advance ratios. In the positive thrust regime, the propeller generated a strong tip vortex trace, which significantly influenced the laminar separation bubble and led to elevated pressure fluctuations and peaks at the tonal harmonic of the blade passage frequency. In contrast, the negative thrust regime featured a weaker tip vortex trace and a broadband-dominated spectrum, with reduced suction observed on the upper surface due to lower dynamic pressure in the slipstream.

The study also highlights that the influence of the propeller slipstream extends well beyond its boundary across the span of the airfoil model. Key limitations included discrepancies in the data measured by two rows of microphones due to surface mounting issues.

Overall, the device proved to be a valuable measurement tool for investigating the unsteady surface pressure fluctuations associated with propeller–wing interaction. The insights gained contribute to a better understanding of surface pressure fluctuations on a body immersed in a propeller slipstream, particularly in energy-harvesting operating regimes. Recommendations for future work include improving sensor mounting fidelity, increasing chordwise resolution, and incorporating time-resolved flow visualisation techniques to complement the surface pressure measurements and provide additional insight into the spatial and temporal evolution of the flow field.