Abstract
M.Ing. (Electrical and Electronic Engineering)
This treatise presents an investigation into the application of
multivariable frequency domain techniques in the modelling and control
of a helicopter aircraft in forward flight.
The presentation is structured in the following sectioned format:
I Hypotheses are stated which deal with the use of linear,
multivariable, frequency domain theory in the modelling and
control of helicopter aircraft.
II The stated hypotheses are investigated by the application of
relevant theories and techniques to a reference case plant - a
single rotor helicopter in forward flight.
III Conclusions drawn from the results are used to assess the
validity of the hypotheses.
The subject matter of the presentation may be summarized as follows:
The hypotheses are initially placed in perspective by a discussion
of the incentives for their formulation. In essence, the hypotheses
state that helicopter dynamics, in a multivariable systems
characterization, can be modelled and an appropriate flight control
system designed by the use of linear frequency domain theory. The
plant in reference to which the hypotheses are investigated is a
single rotor utility helicopter - the Aerospatiale Alouette III. A
single flight condition - a typical cruising condition - is
considered. A comprehensive, nonlinear digital computer simulation
of the aircraft is used as a substitute for the actual plant in the
execution of the modelling and control design processes.
The plant is modelled in terms of a linear model structure, in the
form of the frequency response function, by linearization of its
highly nonlinear dynamics about an operating point (datum flight
condition). The frequency response function model parameters are
identified by power spectral density analysis procedures. This
method, based on random signal excitation of the plant, provides a
valuable quantitative measure of the accuracy of the linearization
performed in the identification. The measure, the coherence
function, is used as a criterion for the robustness required of a
control system of which the design is based on a linear model of a
nonlinear plant.