Adaptive Feedforward Control for Gust Load Alleviation

Abstract

In this paper, an adaptive feedforward control framework is proposed for the suppression of aircraft structural vibrations induced by gust perturbations to increase the resilience of the flight control law in the presence of the aeroelastic/aeroservoelastic interactions. Currently, aircraft with nonadaptive control laws usually include roll-off or notch filters to avoid aeroelastic/aeroservoelastic interactions. However, if changes in the aircraft configuration are significant, the frequencies of the flexible modes of the aircraft may be shifted, and the notch filters could become totally ineffective. With the proposed approach, the flexible modes can be consistently estimated in real time via a proven system-identification algorithm. The identified flexible modes information is used in the proposed adaptive feedforward control algorithm to adjust the parametrization of the basis functions in a feedforward controller. Along with the recursive least-squares estimate, the feedforward controller is adjusted, and the structural vibration of the aircraft induced by the gust perturbation can be largely suppressed. An F/A-18 active aeroelastic wing aeroelastic model with gust perturbation based on the linear aeroelastic solver formulation is developed as a test bed to demonstrate the proposed adaptive feedforward control algorithm. Nomenclature A! = denominator matrix polynomial B! = numerator matrix polynomial B i q = orthonormal basis function C = damping matrix Fq = feedforward compensator F A = external aerodynamic forces F T = thrust forces F = aerodynamic forces from control surfaces Gq = transfer function from control surface to accelerometer sensor G! = frequency response function Hq = transfer function from gust perturbation to accelerometer sensor K = stiffness matrix kt = gain vector M = mass matrix M A = external aerodynamic moments M T = thrust moments M = aerodynamic moments from control surfaces Pq = all pass transfer function Pt = inverse correlation matrix Qs = aerodynamic force coefficient matrix q = forward shift operator T A = transformation matrix T g s = Dryden vertical velocity shaping filter T LPF s = low-pass filter ut = control surface command wt = gust perturbation x Le = aerodynamic lag terms yt = output signal k = coefficients of the orthonormal finite impulse response filter F = aeroelastic incremental forces M = aeroelastic incremental moments e = generalized coordinate = unknown parameters to be estimated = forgetting factor t = regress vector Subscripts ee = related to the elastic dynamics i = r; e; related to the airframe, elastic, control related states lat = related to the lateral dynamics plane long = related to the longitudinal dynamics plane re = related to the coupling between rigid-body dynamics and elastic dynamics rr = related to the rigid-body dynamics

Single-Input/Single-Output Adaptive Flutter Suppression of a Three-Dimensional Aeroelastic System

“…To overcome the disadvantages of estimation algorithm stated in previous studies [8,9], a reliable estimator, the bounded-gain-forgetting estimator, is introduced. With an online estimated model, an optimal controller of reduced order is synthesized to minimize a quadratic cost function.…”

“…On the other hand, the eigenstructure of the closed-loop system determined by the full-state eigenstructure assignment design is not optimal. Zeng et al [9] designed an adaptive feedforward filter to alleviate the gust load of an F/A-18 active aeroelastic wing model. They estimated the parameters by using a recursive least-squares algorithm involving a variable forgetting factor through a two-step approach at each sample time.…”

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

“…The application of adaptive feed-forward controllers for GLA has also been reported [14,15,16]. The main challenge for a feed-forward controller is the availability of a reliable gust-induced angle of attack measurement.…”

Adaptive Feedforward Control Design for Gust Loads Alleviation of Highly Flexible Aircraft

“…17 The environmental requirements in airworthiness regulation for future passenger airliners are extremely strict. To reduce the impact on the environment and improve aircraft global efficiency, a similar solution to the HALE aircraft is needed.…”