Biomimetic propeller design inspired by maple seed leaf for turboprop engines

Since turboprop engines provide high thrust and low fuel consumption at low speeds, they are among the most preferred propulsion systems in many applications in today's aviation industry, from UAVs to passenger aircraft. The thrust efficiency of turboprop engines largely depends on the aerodynamic performance of the propellers, which are one of the most important parts of the propulsion system. Therefore, optimizing the aerodynamic performance of turboprop engine propellers emerges as an important research topic. Simply, a propeller geometry can be described as a twisted 3D model consisting of a combination of 2D geometries with a specific cross-sectional shape arranged at different angles of attack along the axis. Therefore, it is possible to say that the first stage of propeller design is to optimize the cross-section geometry in terms of aerodynamic performance. NACA 44 series (e.g., NACA 4412 and 4415) section geometries, which are known to provide reasonable lift-to-drag ratio at low speeds, are generally used in practice. However, there is no comprehensive study in the literature investigating the most suitable propeller cross-section geometry for turboprop engines. The main purpose of this study is to propose an alternative propeller cross-section geometry for turboprop engines with CFD and FSI analyses. In this direction, the effects of design parameters related to the cross-section geometry on aerodynamic loads under critical conditions such as positive angle of attack (5°) and low flight speed (30 m/s) were examined in detail with CFD analyses. In the NACA four-digit series, the section geometry has three basic design parameters: maximum hump (𝑚), maximum hump point (𝑝) and thickness ratio (𝑡). In this study, aerodynamic loads were estimated by analysing the 2-dimensional incompressible turbulent flow situation on 120 different cross-sectional geometries with the same chord length (140 mm) and different 𝑚, 𝑝 and 𝑡 values with CFD under steady flow condition. Spalart-Allmaras was used as the turbulence model, and a detailed network independence study was carried out on the reference section geometry (NACA 4412) and a validation study was carried out with experimental and numerical results provided by NASA. Using CFD results, mathematical models were created between the propeller cross-section geometry design parameters and aerodynamic loads using the response surface method. Then, with the help of these models, possible propeller cross-section geometries that would maximize the lift force and minimize the drag force were estimated through a multi-objective optimization process. As a result of the optimization study, it was determined that the section geometry with 𝑚 = 7.8926, 𝑝 = 3.1536 and 𝑡 = 6.9928 provides approximately 40.95% better lift-drag ratio compared to NACA 4412. In the second stage of the propeller design, the design parameters and levels of the wing planform were determined by using the optimized cross-section geometry and examining the wing structures of maple seeds collected from nature in detail, and because of optimization methods and CFD analysis, a 3D solid model of the final turboprop propeller was created. In the last stage, in order to obtain the most suitable geometry in terms of aerodynamics and structure, biomimetic propeller geometries inspired by the maple seed leaf were determined, and the structural loads that may occur on the propeller were predicted with bi-directional FSI analyses under high-speed conditions of 2000 𝑑/𝑑𝑘 and 70 𝑚/𝑠. As a result of FSI analyses, it was concluded that the maximum stress value was below the yield limit of the aluminium alloy selected as the propeller material, the maximum deformation occurred at millimeter levels, and the life calculation of the propeller under dynamic loads was determined and the designed propeller geometry was applicable. Keywords: Turboprop engine, CFD, Fluid solid interaction, Biomimetic, Response surface method.