An Integrated Process For Design And Validation Of Flight Control Laws Of Flexible Aircraft Structure

This paper recalls some problems which need to be carefully studied in relation with flexibility of large transport aircraft and control laws design. The evolution of flexible aircraft models is described, and it is shown that the evolution of the FCS design process is coming along with more interdisciplinary models. The FCS validation process is supported by models, and by flight tests. The need to perform an in flight identification of structural modes is explained, as well as the methodology which could be used for future very large transport aircraft.

Modern fighter aircraft are using high sophisticated power control and automatic flight control systems, which basically are designed to maneuver the airplane and to provide sufficient damping for the rigid body modes. Since the sensors are attached to the flexible structure, motions of the elastic aircraft will be measured and may influence the control system. In order to avoid instabilities it is necessary to predict the response of the aircraft with the control system and to correlate with flight test data. An analytical approach for the complete system including flight mechanics and unsteady aerodynamic forces is presented. The elastic structure is described by a set of normal modes which have been updated by results of ground resonance survey tests. Flutter calculations in open and closed loop on different flight conditions as well as incidence variations are demonstrated as common flutter plots. For the flutter analysis a set of notch filter is required, which should be determined in an integrated design step.

Trends to lower structural fraction of aircraft increase flexibility effects. Higher bandwidth control systems combined with these more flexible structures cause more aeroservoelastic interactions. Active, closed-loop control systems allow greater flexibility. To take advantage of this design possibility, an integrated ASE model is needed for conceptual and preliminary design stages of aircraft. This report seeks to define the equations of motion of a flexible aircraft from first principles to aid future discussions between experts in the specialties which make up ASE: aerodynamics, controls, and structures. This theoretical report documents the development of the equations, and states under what conditions the assumptions and approximations are accurate. It consists of 5 sections on different technical areas and a summary section: 1) Linearization of flexible aircraft hybrid-coordinate dynamic equations and inclusion of aerodynamic and gravitational loads; 2) Derivation of equations of motion and stability derivatives for a flexible aircraft vehicle; 3) Aerodynamics for aeroservoelasticity; 4) Model-order reduction for linear systems; and 5) Hydraulic actuator equations for aeroservoelastic modeling. Flight control systems; Servomechanisms. (edc).

Results from structural coupling investigations are presented which include the design and verification of structural filters for a flight control system. The advantages of an integrated interdisciplinary flight control system (FCS) design on the basis of the coupled dynamic model of the structural dynamic model and the flight dynamic model of the aircraft are described. The design strategy of the Flight Control System development is improved through the integrated design optimisation procedure which includes the modelling of the coupled system of the flight dynamics, the structural dynamics, the actuators and sensors as well as the effects of the digital system. Different examples are demonstrated which document the advantages of the integrated, interdisciplinary design. Methods to avoid structural mode-flight interaction are described. Especially the design of filters to minimise interaction is outlined, which is based upon a model of the aircraft describing the coupled flight dynamic, flight control dynamics and structural dynamic behaviour and on ground and in flight structural coupling tests. The paper explains design procedures, design and clearance requirements, correlation between model predictions and structural coupling tests and model update for on ground and in flight.