Linearization of aircraft models : a flight control system and flying qualities perspective

The paper focuses on the fundamental challenge of generating linear equivalent systems and accurate frequency vs. amplitude / phase data from the nonlinear model of complex Fly By Wire aircraft. A reasonably detailed nonlinear model of an aircraft should contain all the information needed for all the tasks to be performed during the aircraft development. However, even if this detailed model is available to the designer, the extraction of the information required for the different design and validation phases is a problem that involves both modeling and simulation. In particular, advanced Fly By Wire aircraft characterized by complex and strongly nonlinear models represent an especially challenging problem with regard to linearization. In the introduction, the paper analyzes and discusses the various requirements for the linear systems derived from the nonlinear model such as control laws design, flying qualities and stability assessments. For each of these requirements the engineer needs to extract the appropriate data from the nonlinear model. In some cases these data will be in the form of state space models or transfer functions. In others the only data required is the frequency response. This is the case for many flying qualities criteria. The different linearization techniques currently used in the aircraft industry are described in combination with state of the art simulation and modeling practices. In particular, the linearization of highly nonlinear elements such as the control laws is addressed and several lessons learned and best practices are discussed. A simple and effective method to obtain the Nonlinear Frequency Response from any nonlinear system is presented. The method is shown to give the same results as the describing function technique and some applications are presented such as Pilot Induced Oscillation analysis