Aeroelastic tailoring of dual-role propellers

Abstract

Hybrid- or fully-electric propeller-based propulsion systems have recently gained interest as an option to reduce greenhouse gas emissions within the rapidly expanding aerospace industry. The electrification of aircraft enables the possibility for energy harvesting during flight phases where no power input is required. The use of aircraft propellers to harvest energy was first suggested by Glauert [1] in 1926, although there was no feasible technology at the time of his research to implement the idea. Seventy years later, MacCready [2] and Barnes [3]–[5], revisited the concept in a battery electric and self-launching sailplane, which could operate its propellers as energy harvesters during descending flight. Both MacCready and Barnes found that the optimal propeller geometries for energy-harvesting and propulsive operation significantly differ from each other. Recently, Erzen et al. [6] were able to obtain a 19% decrease in energy consumption during the ascend/descend flight pattern with a rigid propeller designed specifically for propulsive and energy-harvesting operation in comparison to a conventional propeller. The observed performance improvement, however, heavily depends on the selected flight pattern. The proposed paper will investigate the potential for aeroelastic tailoring of composite blades to improve the performance of propellers operating in propulsive and energy-harvesting modes, considering a realistic mission profile of a typical general aviation type of aircraft. To assess the effect of the mission profile on the propeller design, the mission profile was varied by changing the length of the cruise phase relative to the ascend and descent flight phases. To obtain an optimal propeller design featuring tailored composites, an aeroelastic model [7] was assembled by closely coupling a nonlinear Timoshenko beam model with BEM theory. The model was embedded in an optimisation routine considering lamination parameters and pitch setting as design variables, while the propeller geometry in terms of spanwise chord and twist distribution was kept constant. In addition, both fixed- and variable-pitch propellers were considered during optimization studies involving the full mission, and optimal blade designs corresponding to each individual mission segment were also obtained. The collected results confirm that composite tailoring can noticeably improve the performance of dual-role propellers, especially for mission profiles featuring relatively short cruise flight phases through the reduction of energy consumption by up to 2.0% with respect to the baseline rigid propeller. As the cruise distance is increased, maximum decreases in energy consumption are reduced to 1.5%. Lastly, it is interesting to observe that composite tailoring dominated by cruise distance has a positive effect on the performance during the ascend flight phase as well.