Influence of more accurate propeller modelling on the whirl flutter stability of a propeller-driven aircraft

Abstract

This thesis demonstrates that using more accurate propeller modelling, especially regarding blade elasticity, has an important impact on whirl flutter stability of a full aircraft configuration. Due to the general goal of reducing climate impact of aircraft, turboprop aircraft developments are currently increasing. Whirl flutter stability is an important certification criterion for these aircraft. This stability is so far verified using analytically derived propeller derivatives in frequency-domain flutter analysis. These derivatives are based on rigid blade assumptions though. Recent studies on an isolated propeller model with a new method using frequency dependent transfer-matrices for the propeller hub loads showed, that including blade elasticity in the analysis increases the whirl flutter stability significantly. However, this effect has not been examined on full aircraft level yet. This work first reproduces these findings on two degrees of freedom level and performs some additional studies regarding eigenfrequency variation and scaling of blade stiffness. Then, the transfer-matrix method is extended to enable its application on complex aircraft models. To decouple the specific propeller model used for transfer-matrix generation from the structural model of the airframe, a few adaption procedures for the transfer-matrices are introduced, like eliminating the propeller mass influence from the transfer-matrices as well as aligning the propeller orientation with the coordinate system definition of the structural model. Furthermore, an interpolation routine for the transfer-matrices is introduced to reduce the number of necessary velocity steps for transfer-matrix generation. Then, frequency-domain flutter analyses of a generic twin-engine turboprop aircraft configuration are performed and the impact of the new propeller modelling approach and the influence of blade elasticity and thrust on stability are evaluated. While investigations on an aircraft model showing bending-torsion flutter expose a very low impact due to these propeller modelling aspects on stability, the influence on whirl flutter is higher. It is successfully demonstrated that the stabilisation due to blade elasticity also occurs on full aircraft level. Even though this stabilising effect has a limit for very low blade elasticity, this investigation reveals an additional flutter stability margin, that should be examined further to take advantage of during future aircraft designs.