Flexible Composite Propeller Design for Optimized Performance in Propulsive and Regenerative Operating Conditions

Abstract

The ever-growing aerospace industry's urgent need to reduce greenhouse gas emissions has ignited a surge of interest in hybrid or fully electric propulsion systems. The electrification of aircraft introduces the possibility for energy to be recovered during phases of flight where no power input is required, and previous research has demonstrated the potential for small energy balance enhancements using dual role propellers. The design and operation of dual-role propellers involves considering two opposing load cases: positive thrust and torque during propulsive operation, and negative thrust and torque in regenerative operation. Thus, the ideal blade shape for maximizing performance in each opposing operating condition will be very different. Structural tailoring of the composite blades may be an effective approach for reducing energy consumption during operation in both regimes. Accordingly, the primary objective of this research is to determine the extent of dual-role propeller performance improvements that may be obtained through the application of aeroelastic tailoring. Sensitivity studies were conducted to convey an understanding of how dual-role propeller performance is affected through variations in structural designs (ply orientations and thicknesses). Optimization studies were subsequently performed to identify the extent of performance enhancements yielded solely through aeroelastic tailoring for flexible constant- and variable-pitch propellers of fixed geometry, assuming installation on a reference aircraft, and evaluated over multiple cruise distances for a climb-cruise-descent mission.

The thesis objectives were achieved through the development and application of an aeroelastic analysis and optimization framework. Blade element momentum (BEM) theory was used for the aerodynamic model with engineering corrections for compressibility, effects of rotation, root- and tip-losses, and the turbulent wake state. Excellent agreement was obtained from comparisons with a previous BEM code, and reasonable agreement in performance trends was observed through comparisons with experimental data during verification and validation. A modified version of PROTEUS, an aeroelastic tailoring code that was developed at the TU Delft, was used for the structural model. The aerodynamic analysis routine of PROTEUS was modified to instead use the developed BEM code for the evaluation of loads, sensitivities, and performance. The structural model of PROTEUS was modified to feature centrifugal forces and different input structures that are more conducive to the analysis of rotor blades. The structural model implemented during this project accounts for geometric nonlinearities, as well as nonlinear loads through the application of a corotational framework, and it is capable of accurately representing the detailed 3D blade as a reduced-order Timoshenko beam element mesh through its use of a cross-sectional modeller. Finally, a tightly coupled aeroelastic analysis procedure was developed and applied, which ensures convergence through the minimization of a residual vector using Newton's method, and analytical sensitivities for all loads were included in the analysis. Excellent agreement was obtained during verification studies for both the structural and aeroelastic analyses.

Results from the optimization and sensitivity studies indicate that the flexible blades constructed out of symmetric-unbalanced laminates yield a significant variation in thrust and power through the presence of bend-twist and extension-shear coupling, which results in an increasing change in twist distribution with increasing deflection or elongation. Only small variations in performance were observed from symmetric-balanced laminates, as the minimal amount of coupling resulted in negligible twist deformations, which confirms that the presence of bend-twist and extension-shear coupling drives variations in performance obtained through aeroelastic tailoring. Furthermore, it was found that the presence of an aerodynamic wash-out effect augments the range of advance ratio values corresponding to high-efficiency operation during both propulsive and regenerative modes. An opposite trend was observed in the presence of a wash-in effect. Lastly, due to the significantly decreased loading in descent, combined with its small contribution towards the total mission energy consumption, effects of aeroelastic tailoring are significantly greater in propulsive conditions (climb and cruise) in comparison to regenerative conditions.

From optimization, it was found that the flexible constant-pitch propeller features an energy consumption that is between 0.7% and 1.5% lower than the energy consumption of the rigid propellers. Despite this consistent decrease in energy consumption, all optimal flexible constant-pitch propellers were found to regenerate between 3% and 25% less energy than the rigid variable-pitch propeller, and between 3% and 10% less energy than the rigid constant-pitch propeller. This further suggests that the application of aeroelastic tailoring is most-suitable for improving performance in propulsive mode, as the enhanced performance yielded in propulsive mode outweighs the degraded performance in descent by a significant enough margin to enable the flexible constant-pitch propeller to outperform all rigid propellers. Moreover, the flexible variable-pitch propeller naturally yielded an even better performance than the constant-pitch propeller, with a total mission energy consumption that is between 1.5% and 2.0% less than the energy consumption of the rigid propellers. It has thus been shown that aeroelastic tailoring can yield noticeable improvements in propeller performance, at least for the fixed mission profile and reference aircraft that was studied.