Characterization of nonlinear aeroservoelastic behaviour

The characterisation of the behaviour of nonlinear
aeroelastic systems has become a very important re-
search topic. Nevertheless, most of the work carried
out to date concerns the development of unsteady
CFD solutions in the transonic region. Important
though this work is, there is also a need for research
which aims at understanding the behaviour of non-
linear systems, particularly the occurance of Limit
Cycle Oscillations (LCOs). The purpose of this pa-
per is to study the stability of a simple aeroservoelas-
tic system with nonlinearities in the control system.
The work considers both structural and control law
nonlinearities and assesses the stability of the system
response by use of bifurcation diagrams. It is shown
that simple feedback systems designed to increase
the stability of the linearised system also stabilise the
nonlinear system, although their effects can be less
pronounced. Additionally, a nonlinear control law
designed to limit the control surface pitch response
was found to increase the flutter speed considerably
by forcing the system to undergo limit cycle oscilla-
tions instead of fluttering. Finally, friction was found
to affect the damping of the system but not its sta-
bility, as long as the amplitude of the frictional force
is low enough not to cause stoppages in the motion.