Gust Load Alleviation Control for Very Flexible Aircraft

Abstract

This paper focuses on the development of a wind gust load alleviation control system for implementation in very flexible aircraft. The gust load alleviation system is designed using Linear Quadratic Gaussian (LQG) control techniques, and it is based on a nonlinear model of the coupled rigid-body and elastic modes of a very flexible aircraft. The nonlinear model contains the dynamics of the aircraft body reference frame, elastic strains, strain rates and unsteady aerodynamic flow states. The nonlinear model is linearized at a typical operating point corresponding to steady straight level flight conditions. Using model order reduction techniques, a lower order control-oriented model for the longitudinal dynamics of the very flexible aircraft is obtained. Assuming that the gust field is stochastic, the aircraft model is augmented with the disturbance model that matches the experimentally observed von Karman and Dryden power spectral density characteristics. A LQG controller is then designed to reduce the structural deflections, as the aircraft responds to the gust. Additionally, a command tracking control system is presented for longitudinal flight, which tracks a pitch angle command in the presence of a gust disturbance. It is demonstrated that the hard limits on the structural deflections while responding to the pitch angle command can be enforced using reference governor techniques.

Model-Predictive Gust Load Alleviation Controller for a Highly Flexible Aircraft

“…25 and L g =0.5, are presented in Figs. [17][18][19][20][21][22]. Note that in the absence of GLA the aircraft experienced control surface saturation, and the controller failed to stabilize the plant when it was flying through a discrete gust with L g =0.5.…”

“…The main challenge for a feed-forward controller is the availability of a reliable gust-induced angle of attack measurement. Recently, Dillsaver et al [17] designed a GLA controller for the highly flexible flying wing X-HALE [18]. A linear quadratic Gaussian (LQG) controller was designed and applied to the linear reduced-order model representing the aircraft dynamics.…”

Model Reduction of High-Fidelity Models for Gust Load Alleviation

“…The full order model is projected onto a small basis of m representative eigenvectors using a transformation of coordinates ∆w = Φ z +Φz (16) where z ∈ C m is the state-space vector governing the dynamics of the reduced-order nonlinear system. When nonlinear terms in the Taylor series expansion of the large-order nonlinear residual are neglected, a linear reduced model can be derived.…”

“…More recently, control design has been shown for stability augmentation and gust load alleviation in flexible aircraft . 16,17 A common approach was to fully account for the nonlinear structural behaviour while simple linear aerodynamic models based on two-dimensional theory and panel methods were used for the aerodynamics. Standard linear control design methods may also be inadequate for highly flexible aircraft because the dynamic behaviour is intrinsically nonlinear.…”

Robust Aeroelastic Control of Very Flexible Wings using Intrinsic Models

“…[4][5][6][7][8][9] They have provided substantial evidence of the importance of modelling large, geometrically-nonlinear deflections, and the challenges associated to the development of control systems for gust alleviation and trajectory tracking in such systems. Cook et al 10 and Dillsaver et al 11 applied H ∞ and LQG controllers respectively to full aircraft models and investigated the response to gust and continuous disturbances. They have highlighted the difficulty of achieving robust control on very flexible aircraft with linear control schemes when it was subject to large disturbances.…”