Mechanical design of a propeller for a High-Altitude Pseudo-Satellite (HAPS) using 3D printing of thermoplastics

The objective of the study is to provide a novel approach for the structural design, evaluation, and production of 3D-printed thermoplastic propeller blades. The blades can work at high altitudes (16 km) and can reach a length of 100 cm. The diverse parts are produced using Fused Deposition Modelling (FDM), which enables the production of complex, aerodynamically optimized blade forms.

The method is predicated on carefully selecting a certain set of printing parameters that lessen the anisotropy of the 3D-printed pieces. This enables the use of simpler linear isotropic numerical models tuned to experimental data. Tensile tests on 3D-printed samples in the XYZ directions at both ISA sea-level and cruising altitude temperatures (-60°C) are the first step in the research for the experimental characterisation of the 3Dprinted materials. The numerical models derived from these observations are utilized to perform the structural simulations. Liquid nitrogen is used for low-temperature experiments, and an infrared camera is used to monitor how the temperature changes throughout the course of the test. The behaviour of 3D-printed substitute and twisted blades under tensile and bending loads, as well as a modal analysis test, is then simulated using the developed numerical models. For validation, the numerical predictions are compared with experimental results. For the tensile and bending behaviour of 3D-printed blades, the comparison is made using force-displacement curves, and for the modal analysis, it is done using natural frequencies. The deformations and natural frequencies of the 3D-printed blades are numerically predicted with good precision using the material numerical models. This demonstrates that the numerical simulations of the 3D-printed materials can be relied upon and numerically applied for the structural evaluation, stress analysis, and prediction of the impact of deformation on the aerodynamic performance of the blades using fluid-structure interaction of full-scale propellers.

Applying the proposed method to hollow 3D-printed blades demonstrates its applicability to more complicated forms. The hollow blades are produced by a two-objective structural optimization using genetic algorithms. By creating internal holes and spars along the longitudinal direction of the blade (spanwise direction), it is possible to reduce its mass and deflection while it is in use. The dimensions of the holes and spars are considered as optimization variables. Four (04) candidate blades (optimized hollow blades) have been chosen from the set of optimal solutions to be printed and statically tested in bending and vibration. The four 3D-printed blades’ experimental tests have been numerically reproduced, and the results have been compared in terms of force-displacement curves and natural frequencies. The behaviour of these optimized hollow blades is predicted with good accuracy proving the validity of the current methodology in predicting the behaviour of 3D-printed blades. It can then be applied in the industrial development of high-altitude propeller blades.