The application of semi-active control technology to aircraft landing gear
The purpose of the research investigation was to study the application of semi-active control technology to the design of a suspension system to be used in a landing gear of a high speed military aircraft. A semi-active system was used because it will allow a system to be driven from the hydraulic systems already existing in the aircraft without extensive modification. The research work involved establishing a theoretical mathematical model for the semi-active damping system. This model involved a large number of non-linear dynamic phenomena and elements including a two-stage gas spring, lever geometry, break out friction, square law damping and the switching function needed to achieve the semi-active control. Validation of the model was carried out by means of an extensive study of the dynamic responses obtained from digital simulation. An extended programme of laboratory experiments was also carried out to confirm the theoretical and simulated results, and to demonstrate the potential benefits in performance which can be achieved with those obtained from standard and optimized passive suspension system. The experimental rig involved a physical model which used hydraulic elements of a general industry standard, but not specially approved for aircraft use. The apparatus was arranged to permit a considerable degree of freedom for implementing the control laws which facilitated the assessment of different control schemes and allowed, at the same time, the ready simulation of various passive damping arrangements. An extensive series of trials was carried out on the final design and involved frequency response tests and subjecting the experimental suspension to inputs obtained from a simulated runway profile. The profile simulation was a discrete representation of a particular runway chosen for its roughness which was characteristic of runways from which high-speed military aircraft operate. From the research investigation and these trials it was established that semi-active control of the damping function is superior to standard techniques and achieves a substantial reduction in the energy transmitted to the airframe during ground manoeuvres.