The Impact of Geometric Nonlinearities on the Behaviour of Floating Wingtips

Abstract

Recent studies have considered using flared folding wingtips (FFWTs) to enable aircraft designs with higher aspect ratios, lowering the induced drag whilst reducing gust loading and meeting airport operational requirements. However, most studies have relied on the linear assumptions inherent in preliminary aircraft design despite the presence of large wingtip rotations. Such large deformations are analogous to those seen in the modelling of highly flexible wings and can introduce geometric and aerodynamic nonlinearities that significantly affect the system’s overall behaviour.

This thesis uses low-order nonlinear numerical models to investigate how geometric nonlinearities (due to large wingtip rotations) affect the static and dynamic behaviour of aeroelastic systems incorporating FFWTs.
It is shown that the geometrically nonlinear deformation of the wingtip at its equilibrium position can significantly alter the system’s aerodynamic stiffness and flutter speed when compared to purely linear analysis techniques.
It is also shown how using an additional control surface to change a wingtip’s equilibrium position in flight can augment the flutter speed of the aircraft. These findings are validated using a series of wind tunnel experiments.

Subsequent numerical investigations reveal that geometric nonlinearities can trigger supercritical limit-cycle oscillations beyond the linear flutter boundary, limit an aircraft’s sideslip angle by introducing bistable regions, and alter an aircraft’s roll authority. In all these studies, a strong correlation is seen between further experimental investigations and results obtained from nonlinear low-order numerical models, ensuring the validity of the resulting conclusions, which provide novel insights into the physical mechanisms driving the observed behaviour.

This thesis reveals that for aircraft incorporating FFWTs, the large rotation of the wingtips introduces geometric nonlinearities that significantly impact the aircraft’s aeroelastic response. Therefore, it is crucial to consider these geometric nonlinearities early in the design process to accurately predict an aircraft’s aeroelastic behaviour.

Date of Award	23 Jan 2024
Original language	English
Awarding Institution	University of Bristol
Supervisor	Jonathan E Cooper (Supervisor) & Djamel Rezgui (Supervisor)