Nonlinear Flexibility Effects on Flight Dynamics of High-Aspect-Ratio Wings

TL;DR

This study examines how geometric nonlinearity and structural flexibility influence the flight dynamics of high-aspect-ratio wings, highlighting the importance of nonlinear analysis for flexible aircraft configurations.

Contribution

A coupled aeroelastic framework integrating structural, aerodynamic, and flight dynamic models for high-aspect-ratio wings, with systematic parametric analysis of flexibility effects.

Findings

Increasing flexibility alters trim conditions and flutter boundaries.

Large static deformations create an effective dihedral, affecting lift and trim angles.

Flutter speed decreases significantly with higher flexibility, especially when pre-stress is considered.

Abstract

This paper investigates the effects of geometric nonlinearity and structural flexibility on the flight dynamics of high-aspect-ratio wings representative of high-altitude long endurance aircraft configurations. A coupled aeroelastic flight dynamic framework is developed, combining a geometrically exact beam formulation for the structure, unsteady two-dimensional strip theory for the aerodynamics, and quaternion-based rigid-body equations for the flight dynamics. The three subsystems are monolithically coupled through consistent load and motion transfer at each time step. A systematic parametric study is conducted by varying the wing stiffness across several orders of magnitude, spanning from nearly rigid to very flexible configurations. The study reveals that increasing flexibility fundamentally alters trim conditions, flutter boundaries, and dynamic gust response. In particular, large static deformations create an effective dihedral that modifies the lift direction and necessitates higher trim angles of attack. The phugoid mode is shown to destabilise at high flexibility levels, consistent with findings in the literature. Flutter speed degradation is quantified as a function of the stiffness parameter, showing significant reductions for very flexible configurations when the pre-stressed equilibrium is correctly accounted for. The framework is validated against published aircraft benchmarks, demonstrating good agreement in natural frequencies, flutter speeds, and nonlinear static deflections. Results provide quantitative guidance on when linear analysis is acceptable and when fully coupled nonlinear tools become essential.