Integrated design process for the development of semi-active landing gears for transport aircraft

Abstract

Aircraft landing gears are currently optimised to have optimal performance in the rare case of a hard landing. The resulting suspension layout may lead to unsatisfactory oscillations when the aircraft is taxiing on a rough runway; runway unevenness can excite elastic structural modes, leading to passenger and crew discomfort. Although there are existing modifications of aircraft shock absorbers to reduce the problem, the basic design conflict between the requirements for landing and for rolling cannot be fully overcome by a passive suspension layout. Semi-active suspension techniques promise a solution to this problem. A semi-active suspension, i.e. a damper with a variable, controlled orifice cross-section, is capable of reducing fuselage vibrations effectively while being relatively light-weight and low system complexity. In this thesis, three control laws, a skyhook-type controller, a fuzzy-logic controller, and a state feedback controller are designed for the application to a semi-active suspension for an aircraft nose landing gear. Regarding the aircraft flexibility, the landing gear can no longer be designed independently from the aircraft. The layout of the controllers is therefore undertaken using an integrated design approach. Airframe and landing gear properties are determined taking into consideration models from different engineering disciplines involved in the aircraft development process, making the oleo design part of the concurrent engineering loop. The aircraft model is set up in a multibody simulation environment. The control laws are developed in a control design tool; special consideration is given to the requirements of semi-active actuators. The controllers are exported into the simulation environment and their parameters are optimised by means of multi-objective optimisation. In a further step, the performance of the three control strategies are compared with each other and additionally with passive and fully active approaches. The dependence of the control performance on operational parameters (aircraft weight and speed, runway roughness) is assessed, and limitations due to realistic actuators restrictions are discussed. Finally, the benefits and disadvantages of semi-active nose landing gear control are summarized and open problems are addressed.