Aircraft design and production has always been a labour-intensive activity. But this is changing as the power of information technology is brought to bear
While the first 75 years of aviation saw successive revolutions in aerodynamics, structures and propulsion that took the industry from the propeller-driven, biplane, strut-and-wire Wright Flyer to the jet-powered, swept wing, all-metal aeroplane, progress over the past 25 years has involved less visible, but equally important, advances in how aircraft are designed and built. Those changes are continuing, as digital technology pervades the product life cycle.
Changes in the design office have been dramatic over the last two decades. Design engineers sit at computer screens, not drawing boards, and work with three-dimensional solid models, not two-dimensional line drawings. A design is now embodied not in drawers filled with mylar blueprints, but in a digital database constantly accessed and continuously updated to reflect the latest aircraft configuration.
The latest three-dimensional computer-aided design tools not only provide unimagined accuracy, but also unparalleled accessibility. A single product database enables engineers from all disciplines, and in any location, to work collaboratively on a design. A design task begun in Europe can be handed on to the USA, then on to Australia and back to Europe, allowing engineering work to "follow the sun".
The same digital database is used throughout the life of a product, from generating geometry for computational fluid-dynamics analysis of aircraft aerodynamics and finite-element analysis of structural loads, through producing machining programs and tooling designs, to simulating the production-line assembly and flight-line supportability of a design. The same engineering design data is used to generate graphical assembly instructions and laptop maintenance manuals.
Computer firstBoeing says its 777 was the first aircraft designed entirely on computer, using the Dassault-developed Catia software that is now the industry standard. Bombardier took the next step, using Catia to anchor a geographically dispersed network of risk-sharing partners. The Canadian manufacturer pioneered joint definition, in which partners come together in one location to define the configuration and the interfaces between their pieces of the aircraft, then disperse to complete design, manufacture and test. Lockheed Martin, Northrop Grumman and BAE Systems use Catia to create a "virtual design team" for the international F-35 Joint Strike Fighter (JSF) programme.
Solid modelling has played a key role in the JSF programme from the outset. During the competitive concept-demonstration stage, manufacturing and assembly simulations based on the Catia design data were used by the two teams to demonstrate that they could produce three different aircraft variants affordably. The same data was also used in simulations to show that the JSF could be armed, fuelled and maintained easily, whether operating from a carrier deck or an austere airstrip. Solid modelling enabled unheard-of fidelity in preliminary design, surfacing space and weight issues that previously would not have been seen until prototype manufacture.
NASA is now working with software developers to extend the design tools to include product knowledge, and not just data. This would attach to each solid models all the information about a part that is gathered during its design, manufacture and operation. Such knowledge now resides in different places - in the design office, on the factory floor and in the maintenance shop - and accumulates over service lives than can exceed 40 years. Adding the information to the database will ensure a complete product picture is available throughout an aircraft's life, facilitating upgrades and modifications even decades after it was designed.
Just as the tools designers use have advanced over the last two decades, so have the materials with which they work. The debate between all-metal or all-composite aircraft has largely passed into history, replaced by the practice of using the best material for each application. While the overall trend towards greater composite content in aircraft structures has continued, metals have stood their ground. Cheaper titanium, better aluminium alloys and tougher metal-composite laminates have helped prevent a rout by composites, encroachment of which has been slowed by the need for expensive investment in automated fabrication to offset high raw material costs, and by customer concerns about maintainability and repairability.
Metals have also benefited from development of fabrication techniques that have allowed a move away from labour-intensive built-up structures to single-piece components. High-speed machining allows items such as bulkheads and doors to be milled in one thin-walled piece from a solid billet of aluminium, replacing rivetted or bonded components with many hand-assembled parts. Superplastic forming and diffusion bonding can produce single-piece complex-curved doors and panels by shaping and fusing sheets of titanium together under high pressure and temperature. Near net-shape precision casting can produce structural parts that require minimal machine finishing before use. Laser direct sintering can create complex components from scratch by fusing powered metal under computer control. Lightweight aluminium-lithium alloy is making inroads into aircraft structures while GLARE, a fatigue-resistant aluminium-glassfibre laminate, is being used by Airbus for A380 fuselage panels.
Despite advances in metallic materials, composites account for an increasing share of aircraft structural weight. Airbus introduced composites to airliner primary structure with the A310's fin; with the A380 it has extended the use of carbonfibre to the wingbox. Boeing plans to take the next step, having selected toughened carbonfibre composites over advanced aluminium alloys for both the wing and fuselage of the proposed 7E7 airliner. Fatigue resistance and weight reduction remain the primary reasons for using carbonfibre, but advances in the automated fabrication of composites have removed many of the obstacles to their cost-effective use in aircraft structures.
Fibre firstRaytheon is the first to use automated fibre placement to produce pressurised aircraft fuselages from composites. The company's carbonfibre-fuselage Beechcraft Premier 1 light business jet was certificated in 2001 and the production process has been scaled up for the fuselage of super mid-size Hawker Horizon, scheduled for certification next year. Whereas Raytheon's 1980s-vintage Starship had an all-composite airframe, both the Premier and Horizon have metal wings, illustrating the trend for designers to select the material most appropriate to each application.
Perhaps the biggest attraction of composites for designers is the ability to reduce parts count and simplify assembly by creating large "unitised" parts. The Premier 1 fuselage shell is produced in just two parts and that of the Horizon, while substantially larger, is only three. Boeing says final assembly of the 7E7 will take only three days, instead of 13-17 days for a 737 or 777, and will involve the mating of large fully equipped subassemblies provided "just in time" by risk-sharing partners.
Today, the most visible changes in how aircraft are built are taking place on the assembly line as the aviation industry applies lean-manufacturing techniques adapted from the automotive industry. Examples include the moving assembly lines introduced by Boeing for its AH-64 combat helicopters, F/A-18 fighters and several of its airliners. Such lines, although they may move almost imperceptibly at the low production rates all too common in the aircraft industry, serve to impose a discipline on the entire production process: parts have to be where they are needed when the are needed. Shortages and snags that stop the entire line must be resolved quickly. Through innovations like moving lines, aircraft manufacturing is leaving behind its lingering reputation as a labour-intensive cottage industry and moving into the new century.
Source: Flight International
