标题: A380 hydraulic pressure for landing gear systems [打印本页] 作者: 航空 时间: 2010-8-10 14:09:18 标题: A380 hydraulic pressure for landing gear systems
作者: 航空 时间: 2010-8-10 14:09:35
David Waters, Operations Director with GE Aviation, in Cheltenham, UK, discusses how the switch to 5,000psi hydraulic pressure for landing gear systems presents certain component manufacturing challenges 16 AEROSPACEMANUFACTURING PRESSURE TAKING THE As most readers will be aware, the two commercial aircraft which have been jostling most for the media headlines during recent years are of course the Airbus A380 and the Boeing 787. While there are significant differences between the two aircraft one thing they have in common is their fuel-efficiency, which is largely down to reduced weight. Both aircraft have realised reductions through increasing their hydraulic systems pressure from the (commercial) industry norm of 3,000 to 5,000psi. The higher pressure allows hydraulic pipes to have reduced diameters. For example, the diameter of a hydraulic inlet hose on a 3,000psi system might be three inches. This diameter could be reduced to two inches on a 5000-psi system. In short, higher pressure means reduced system volume (hydraulic fluid), and a reduced system volume means smaller fittings/brackets. Both the Airbus A380 and Boeing 787 use 5,000psi pressure for their landing gear systems but, unlike most systems on an aircraft, landing gear systems are not in constant use. Indeed, they may go 12 or more hours between cycles. Further, one must consider the masses involved. For instance, the Airbus A380’s maximum operating weight is more than 600 tonnes, and the brunt of this mass is felt on landing by its four main body landing gears (which themselves weigh about 20 tonnes). Accordingly, the extension and retraction of the landing gear (within a few seconds and in sync’ with the opening or closing of bay doors) are not without challenges. In addition to narrow gauge pipes, the switch to 5,000psi has also allowed for a reduction in the operating area of the system’s actuators – again helping reduce the overall size and therefore weight of the hydraulic system. However, the high pressure has introduced a number of component challenges, including the need to address seal wear characteristics and devise new component sealing solutions. During the development of the A380 landing gear extension and retraction system (LGERS), GE Aviation (formerly Smiths Aerospace) did much work with one of its seal suppliers and Imperial College, London. Of particular interest were the effects of ‘high frequency dither’ on seals working at 5,000psi. As mentioned, with increased pressure the operating areas of the actuators can generally be reduced. Fluid flow can also (normally) be reduced rate but a landing gear system is possibly the exception to the rule. 600 tonne operating weight | The mass taken by the A380’s four main body landing gears Photo: Airbus S.A.S. Nearing completion | Building a valve assembly LANDING GEAR I HYDRAULIC SYSTEMS For example, the retraction time from cockpit ‘landing gear up’ signal to having, in some cases, more than 20 tonnes of landing gear raised and stowed, plus bay doors closed needs to be as short as possible to achieve a clean airframe: done to reduce drag and noise signature on the ground. Large valves are therefore needed to accommodate the flow rates. Traditionally, valves are made of steel or aluminium, with the former heavy (for a large valve) and the latter susceptible to fatigue at high pressures. GE Aviation has experience of designing in aluminium up to 4,000psi and has tested up to 5,000psi. However, 5,000psi aluminium valves would be bulky and production would be critically reliant on machining practices. The Airbus A380’s and Boeing 787’s hydraulic valves are therefore made from titanium. However, titanium is much harder to machine than aluminium. Cutting speed is often constrained, heat builds up quickly and vibration can also be a problem: not good news for any manufacturing process. Once again, GE Aviation is partnering with other specialists, in this case the machine and tool manufacturers and AMRC (Advanced Manufacturing Research Centre – part of Sheffield University). The partnership has created specialist monitoring and control systems that allow running speeds to be set higher than normal. In addition this is further enabled through the use of specific machine upgrades and choice of coolants. The result: cycle times are significantly improved upon previously understood limits. Also aiding manufacture of the valves is GE Aviation’s use of Single Minute Exchange of Die (SMED) – which minimises changeover time as it allows operators to change tooling whilst machines are running. SMED also allows batch sizes and inventory to be reduced, in keeping with lean practices. A number of strategies were used which, in combination, provide set-up and change-over times dramatically lower than those previously realised. Cycle time Billets are prepared and loaded outside of machine cycle time. Finished parts are also dealt with outside of cycle, and all elements of the change process are facilitated with rapid action systems and mechanisms to further reduce time and manual interactions. An intelligent tooling system and features within the machines and tool suites ensure maximum tool swaps within machine time cycles. In addition this system provides error proofing (‘poke yoke’) in that the incorrect tools cannot be fitted. Titanium provides other advantages for product concepts and as a result GE Aviation has created multi-function hydraulic valve concepts bringing further challenges to the manufacturing environment. These products have such high value that the whole approach to manufacturing must ensure very capable processes and thus very high right-firsttime figures. As a result the whole extended GE project team applied latest methodologies of design for manufacture (DFM) and advanced quality planning to ensure this outcome. In support of this approach the team ensured that the inprocess and CMM-based technologies available were fully embedded into the control plans for the products to maintain capability. The latest system requirements of the aircraft manufacturers and the flow down of these into the landing gear systems in which GE specialises created significant challenges to the manufacturing programme teams. Through close-working partnerships, the leverage of the latest technologies and the application of the latest management methodologies, GE Aviation has taken these manufacturing challenges and turned them into competitive advantages. The result is highly optimised functional products providing optimum weight with the ability to supply customer requirements in a true lean manufacturing environment. ❙ www.geae.com Supporting manufacturing | Automated CMM technologies Acquisition GE Aviation acquired UK-based supplier of integrated systems for aircraft manufacturers and components for engine builders earlier this year for $4.8 billion. The acquisition broadens GE’s offerings for aviation customers by adding Smiths innovative flight management systems, electrical power management, mechanical actuation systems and airborne platform computing systems to GE’s growing commercial and military aircraft engines and services. AEROSPACEMANUFACTURING 17 作者: 555zhenjia 时间: 2010-8-11 23:52:17
