Dynamic Performance Analysis of a Fighter Jet with Nonlinear Dynamic Inversion

Abstract: A modern view of aircraft performance analysis, is to quantify aircraft manoeuvrability with agility metrics. There are several different agility metrics, which can be seen as indexing of the aircraft agility performance. The quantified unit is the time it takes for the aircraft to perform a specific manoeuvre relevant to a given agility metric. In this thesis, estimations are done of two agility metrics, the CCT (Combat Cycle Time) and the T90 (Time to capture 90◦ bank angle) for the F-18 HARV aircraft.Estimations of the agility metrics were obtained by simulating a six-degree-of-freedom aircraft model of the F-18 HARV aircraft performing the specific manoeuvres. To control the aircraft model during the simulation a control sys-tem was developed based on the NDI (Nonlinear Dynamic Inversion) method with time scale separation assumption. The method uses the feedback from the controlsystem for linearizing the aircraft system, which results in that simple linear controllers can be applied to the nonlinear aircraft model.In this case simple proportional controllers were implemented and in the case of estimating the T90 agility metric additional gain scheduling as functions of altitude and Mach number was required to extract maximum performance. Although the control system was developed for these two specific agility metrics, results indicates that the NDI method provides an effective way to implement controllers for complex systems, especially when considering a high nonlinear flight regime.