Final Report Summary - DRESS (Distributed and Redundant Electro-mechanical nose wheel Steering System) The main aim of the DRESS project was to develop a steering system that increases significantly the levels of reliability and availability. This will provide the aircraft with true, all-weather (zero visibility) operational capabilities. Additionally, it will make it compatible with an automated ground guidance system, offering significant aircraft operational improvements and enabling more efficient air transport.The first stage of the project was to define the technical specification of the new, electrically actuated system. In order to identify the needs without focusing directly on the current hydraulic system, a functional analysis was performed. The criteria were identified based on the performance, the safety, the reliability, the operability, the weight, the environmental conditions, etc. A technical specification for an airworthy system was issued. Later on, some technical requirements not applicable to the system lab demonstrator were released in order to fit with the budget and objectives of a research project. Some key specification parameters and features are summarised here. The maximum mechanical power required is around 1 kW. The maximum angular speed is 20 degrees/s. It must be possible to tow the aircraft with the nose wheel free to rotate. The loss of steering function probability objective was set to 10^-9 per flight hour, well above the current system values. The system robustness against shimmy is a key requirement. The specification was regularly updated and will be improved at the end of the project after experiments have been held and all lessons have been learnt.Trade-off studies were performed to define the mechanical path topology and size the reducers and electric motors. A top-down generation of architectures combined with a bottom-up filtering with respect to technological constraints led from a very high number of solutions at functional level to a limited number of embodied architectures. The motor and power control electronics study was run in parallel. The study validated the choice of the high power density, split-phase permanent magnet, synchronous, Stritorque motor. Fault tolerance at motor level was not needed and a state-of-the-art, single channel architecture was used for each motor.Detailed design of the actuator mechanical transmission followed after the architecture selection. Components were selected for procurement and parts were machined. Two prototypes were assembled. RDC and CPM were designed for laboratory conditions only. Control laws were developed to comply with the distributed architecture before implementation in the hardware. Each node is composed of an input / output card designed especially for DRESS, plugged to a TTP module. Compliance to develop nodes by different partners after agreement on a communication data base specification was demonstrated. This allowed for trouble-free communication between the RDC and the EMCU.One DRESS prototype was tested, supplying the electric motors without antagonist torque load on the turning tube interface. For this, one actuator path was used to load the other path. Thermal tests were performed to investigate if the system duty cycle defined in the specification was achievable. Environmental tests with controlled temperature in the range of -55 to 85 degrees Celsius were also conducted. The complete system has been tested using a bench that was designed and manufactured especially for the DRESS project. The bench includes a dummy landing gear with some adaptable parameters. The landing gear angle is controlled by the DRESS actuator, while two different antagonist torque control systems can be used. A 'classic' low frequency antagonist torque module applies the effort with a hydraulic system.Modelling and simulation was extensively used in the design, in order to define and evaluate the DRESS performance and behaviour. A simulation plan was defined. Matlab / Simulink was used as common platform for simulation at component, system, and aircraft level. Other specific tools were used for particular studies, such as structural and thermal analyses. Ground manoeuvrability studies were also performed at aircraft level to assess the steering performance. Consistency between physical tests and simulations was ensured in order to get the best possible correlation with testing and to identify the model parameters.Criteria were identified to assess the stability of the nose landing gear, damping the oscillations and defining the maximum wheel deflection due to given perturbations. The study has shown that the leg structure remains the main contributor to the shimmy phenomenon. A coupled steering system / leg structure is needed to optimise the nose gear stability. The DRESS electromechanical steering induces a lower risk of coupling between torsional and bending modes compared to the current hydraulic system. This conclusion has been confirmed by dynamic testing though some unexpected torsional resonances appeared. After investigation the resonant phenomenon has been understood and important lessons have been learnt on the integration of an electrical actuator on a landing gear.The DRESS project has proven the feasibility of an electrically driven steering, demonstrated the advantages of a distributed architecture and validated the principle of driving a redundant actuator to increase significantly the safety. It has enabled to show potential benefits compared to the classical hydraulic steering in terms of power consumption, operability, maintenance, flexibility. Nevertheless, the system weight is significantly higher than a current one and further work is needed on shimmy phenomenon. Therefore, it cannot be implemented on an aircraft immediately. To conclude, the DRESS prototype is not sufficiently optimised to be competitive with a hydraulic system yet but gives a really good basis for future development and will enable to build an optimised design system and make the right choices. Related documents Final Report - DRESS (Distributed and Redundant Electro-mechanical nose wheel Steering System)
