The paper presents an overview of modeling the dynamic behavior of a large four-engine flexible aircraft and considers some of the options for control system design. The first part describes how to build an integral model, which can be used for simulating the rigid motion as well as the flexible motion of the aircraft. The result is a system of nonlinear equations of motion. The second part analyzes the dynamic properties of a sample aircraft by considering the linearized equations of motion for flight in a vertical plane at several operating points in the flight envelope. Here, it is shown how the eigenfrequencies of the rigid body and the elastic motion change with the load and flight conditions. In the third part, three options for control system design are discussed: (i) a conventional SAS controller, which does not influence actively the elastic behavior; (ii) an output feedback controller; and (iii) a robust controller. It is concluded that, using an integral controller, certain flying quality criteria can be met and damping of all the elastic modes can be improved.

1 Introduction

The evolution of large transport aircraft is characterized by fuselages getting longer and wing spans getting wider, while efforts to reduce the structural weight reduce the structural stiffness.