Aeroacoustic simulation and improvement of a propeller for an electric self-launch sailplane

Abstract

Propeller design for various aviation type applications has always been a challenging task where airframe, engine and propeller properties must match in order to obtain adequate thrust at the required flight speeds. This challenge becomes particularly intricate in applications like electric sailplanes, where propeller geometries face restrictions due to fuselage constraints, limiting the propeller's radius. Consequently, compromises must be made such as high rotational speeds when adequate thrust is needed. These compromises usually lead to secondary adverse effects such as inefficiency and excessive noise. Regulations govern the noise levels of electric-powered sailplanes, and certification of these propulsion systems requires strict adherence to specified standards. Propeller designers must, therefore, incorporate noise levels as a crucial parameter to minimise. Aeroacoustic CFD models have the potential to predict noise levels of propellers, where the pressure fluctuations from the fluid flow simulation are used as sources and coupled with equations to describe the propagation of small perturbations in pressure and velocity that produces sound waves. The study investigates the complexities of propeller design, exploring strategies such as incorporating humpy chords, sweeps, and reducing the tip airfoil thickness of the blades to achieve noise reduction without significantly compromising e fficiency. An electric propulsion system of a sailplane was used as a baseline validation case, where the propeller produced significant noise levels when measured. The research integrates principles of aerodynamics and fluid mechanics to propose systematic geometric modifications, addressing airflow characteristics and vortex formation. The study also considers the unique constraints imposed by sailplanes, such as diameter limitations based on fuselage dimensions and takeoff requirements. The article concludes by highlighting the promising results of a modified propeller, showcasing a 6 dB reduction in noise levels through a combination of reduced RPM and geometric alterations. Leveraging advanced simulation techniques and embracing geometric modifications hold the promise of achieving a balance between t hrust, e fficiency, and noise reduction for electric self-launch sailplanes.