Magnetorheological (MR) fluids enable the rapid and continuous alteration of flow
resistance via the application of a magnetic field. This unique characteristic can be
utilised to build semi-active dampers for a wide variety of vibration control systems,
including structural, automotive, and aeronautical applications. As an example, MR
fluids could enhance the performance of aircraft landing gear, which are subject to
widely varied and unpredictable impact conditions with conflicting damping
requirements.
In this thesis, a numerical sizing methodology is developed that enables the impact
performance of MR landing gears to be optimised. Using real data provided by landing
gear manufacturers, the sizing methodology is applied to both lightweight aircraft, and
large-scale commercial jets in order to demonstrate scalability. For both aircraft types,
results indicate that the peak force and the severity of fatigue loading can be enhanced
over a wide range of impact conditions. However, it is shown that MR landing gears
can be heavier than passive systems. To validate the numerical approach, a prototype
MR landing gear shock strut is designed, fabricated, and tested. Good correlation
between the model and experiment is demonstrated, particularly for low velocity
excitations.
MR dampers exhibit highly non-linear force-velocity behaviour. For landing gear
impacts, it transpires that this behaviour can be used to an advantage, where it is shown
that an acceptable performance can be obtained using open-loop control i.e. with a
constant magnetic field. However, this non-linear behaviour is highly undesirable for
other scenarios (e.g. an aircraft taxiing), and as a consequence, the choice of an effecti\'e
control strategy remains an unresolved problem. A further aim of this thesis is therefore
to develop effective control techniques for broadband excited MR vibration systems.
Through an extensive series of numerical and experimental investigations, case studics
representative of the general single-degree-of-freedom and two-degree-of-freedom
vibration isolation problem are presented. In the experiments, the hardware-in-the-Ioopsimulation
method is adopted, which provides an excellent means to bridge the gap
between theory and practice when the behaviour of a specific component is complex.
Here, the MR damper is physically tested, whilst the remainder of the structure is
simulated in real-time. The results demonstrate that the chosen control strategy can
provide significant performance benefits when compared to more commonly used
strategies and equivalent passive systems. Furthermore, the control strategy is shown to
be insensitive to factors such as the type of input excitation.