## Effective optimal control of a fighter aircraft engine

thesis

posted on 22.11.2010 by Saad M. Mahmoud#### thesis

In order to distinguish essays and pre-prints from academic theses, we have a separate category. These are often much longer text based documents than a paper.

Typical modem fighter aircraft use two-spool, low by-pass ratio, turbojet engines to provide
the thrust needed to carry out the combat manoeuvres required
by present-day air warfare
tactics. The dynamic characteristics of such aircraft engines are complex and non-linear.
The need for fast, accurate control of the engine throughout the flight envelope is of
paramount importance and this research was concerned with the study of such problems and
subsequent design of an optimal linear control which would improve the engine's dynamic response and provide the required correspondence between the output from the engine and
the values commanded by a pilot.
A detailed mathematical model was derived which, in accuracy and complexity of
representation, was a large improvement upon existing analytical models, which assume
linear operation over a very small region of the state space, and which was simpler than the
large non-analytic representations, which are based on matching operational data. The
non-linear model used in this work was based upon information obtained from DYNGEN, a
computer program which is used to calculate the steady-state and transient responses of
turbojet and turbofan engines. It is a model of fifth order which, it is shown, correctly
models the qualitative behaviour of a representative jet engine. A number of operating points
were selected to define the boundaries used for the flight envelope. For each point a
performance investigation was carried out and a related linear model was established. By
posing the problem of engine control as a linear quadratic problem, in which the constraint
was the state equation of the linear model, control laws appropriate for each operating point
were obtained. A single control was effective with the linear model at every point. The same
control laws were then applied to the non-linear mathematical model adjusted for each
operating point, and the resulting responses were carefully studied to determine if one single
control law could be used with all operating points. Such a law was established. This led,
naturally, to the determination of an optimal linear tracking control law, and a further
investigation to determine whether there existed an optimal non-linear control law for the
non-linear model. In the work presented in this dissertation these points are fully discussed
and the reasons for choosing to find an optimal linear control law for the non-linear model by
solving the related two-point, boundary value problem using the method of quasilinearisation
are presented. A comparison of the effectiveness of the respective optimal control laws, based
upon digital simulation, is made before suggestions and recommendations for further work
are presented.

### History

#### School

- Aeronautical, Automotive, Chemical and Materials Engineering

#### Department

- Aeronautical and Automotive Engineering