As the 20th century closed, the airliner industry appeared to be dominated by derivatives, with few all-new designs on the drawing board or on the horizon. Researchers and engineers are not idle, but their endeavours are being shaped more by the demands of the market, and less by the possibilities of technology.
If a reminder was needed, last year's decision by Boeing to halt work on supersonic transports and the resulting termination of NASA's high-speed research demonstrated the power of the market. In its wake, and guided by Boeing, the US agency has reshaped its aeronautics research around safety, efficiency and the environment.
Airbus needs
In Europe, in the absence of a "European NASA", research is increasingly shaped by the needs of Airbus Industrie. Not surprisingly, technology for the planned A3XX large airliner tops its priorities. But the consortium is looking across its entire product line. "We're examining all the possibilities for different members of the family," says Alain Garcia, senior vice-president, engineering.
Garcia has created two new positions to address the issue of where Airbus goes next: Juan Hererra is chief engineer, future projects and Marc Vincendon is chief engineer, new technology. Each is responsible for co-ordinating technology input from Airbus' partner companies and their national research organisations.
Hererra's brief is "to think about advanced configurations and look at their merits". In other words, Airbus is opening its future line-up to the possibility of a fundamental shift from the popular combination of low-set wing, underwing engines and single- or twin-aisle fuselage. This conventional configuration may emerge from the analysis as the most cost effective, but at this early stage, it does not look that way. "We need lighter, more efficient designs that are much easier and cheaper to produce," says Garcia.
Boeing is also looking across its product range and considering configurations other than conventional. In the large-aircraft arena, the US company continues to look at designs ranging from a high-wing airliner resembling a scaled-up C-17 transport to the promising blended wing-body configuration. Like Airbus, Boeing does not want to be channelled down one technological avenue without doing its homework.
Lockheed Martin, meanwhile, is trying to interest Airbus in a "box wing" configuration that could form the basis of a jointly developed military tanker/transport and commercial airliner/freighter.
In any new aircraft, the European consortium will use its fly-by-wire (FBW) flight control technology to enable further improvements in efficiency. The next step is to reduce the aircraft's natural stability. In the 550-seat A3XX, likely to enter service around 2005, this will allow the horizontal tail area to be reduced by around 10%, saving 700kg and lowering drag by 0.7%. The reduction in stability will be compensated for by the FBW system, which has proved so reliable in the A320 and A330/A340 that, in the A3XX, the mechanical back-up system will be eliminated.
While the A3XX will still exhibit overall positive stability, some of the designs to be considered as part of Airbus' long-term development plan will not. "If we need to stabilise the aircraft artificially, we will do it," says Vincendon. "That will be a natural evolution from what we have done already. It is not revolutionary."
One way to destabilise an aircraft is to add a foreplane. Airbus is looking at the use of a canard for long-range airliners. Studies are concentrating on whether the aerodynamic improvements are worth the extra weight and complexity. "We must win at least a few per cent in fuel consumption," says Aerospatiale Matra head of future projects Dominique Gentili. Tests on an A340 model in the company's low-speed windtunnel at Toulouse have been promising enough to support a high-speed windtunnel programme, he says.
"We think it would only be useful to incorporate a canard in long-range aircraft because they spend most of their time in cruising flight, where the aerodynamic gains are highest," Gentili adds. The first application could come "after the A3XX - maybe around 2008-10. But we have to be careful. We have learned a lot from the experience of winglets, for example. They proved to be useful only in some applications."
Boeing's winglet experience is similar, the company opting for a unique drag-reducing raked wingtip on its latest 767-400ER. But the manufacturer plans to offer improved blended winglets across its product range, beginning with the 737NG-based Boeing Business Jet and possibly extending to the 747, under an agreement with winglet developer Aviation Partners. The US Federal Aviation Administration, meanwhile, is examining whether advanced winglets can reduce wake vortex turbulence.
Another aerodynamic means of improving efficiency is by achieving laminar airflow over flying surfaces, which can reduce drag by up to 20%. Extensive work has been carried out by Airbus, which has been flying an A320 with a hybrid laminar-flow fin since 1998. This uses boundary layer suction through a minutely perforated skin to delay the transition to turbulent flow. Airbus has also conducted an extensive in-service evaluation of riblets, a finely ribbed plastic covering for flying surfaces that can reduce drag by up to 3%. While both approaches offer aerodynamic advantages, these are offset by maintenance issues.
The advent of micro-electromechanical systems (MEMS), under development in Europe and the USA, promises to allow the use of innovative flow-control devices, such as vortex generators that can fold flat or align with the airflow when not required. They may enable aerodynamicists to control turbulence, the combined sensor, computer and actuator detecting and inhibiting flow separation, probably using tiny jets of air to quell turbulent bursts in the boundary layer. As with suction systems, the main issue is the technology's maintenance impact.
Other efficiency improvements will come from the use of new materials and manufacturing processes, saving weight and reducing cost. While aircraft are super-efficient structurally - an A340 scaled down to a 0.3m (1ft) wingspan would weigh just 1.6g (0.056oz) - there is plenty of potential for improvement.
The debate over the relative advantages of composites and metallics is complicated by the development of metal composite materials such as Glare (glassfibre-reinforced aluminium). This is being considered for the upper fuselage skin of the A3XX, for a weight saving of around 10% over conventional aluminium. The lower fuselage will be an all-welded structure, eliminating rivets and reducing weight and cost, as well as enabling a design with improved structural efficiency.
Boeing and NASA are also working to demonstrate the feasibility of manufacturing large, integrally stiffened, metallic structures, with fewer parts, joints and fasteners reducing weight and simplifying assembly. Techniques being developed include high-speed machining, friction-stir welding and the rolling of completely jointless fuselage barrels.
Lighter alloy
Aluminium lithium weighs around 10% less than conventional alloy, and is 10% stiffer, but is expensive. Doubts about its fatigue performance have prevented its use in large-scale applications. The advanced alloy will probably find its way into stringers and frames on new aircraft.
Composites feature heavily in Airbus designs, with the empennage of the A320 family and A330/A340 manufactured from carbonfibre. In the latest A340-500/600, use is extending to the fuselage keel beam and rear pressure dome. This will be the first pressurised carbonfibre component in an airliner.
Improved methods of manufacturing composite structures are being developed. Boeing is working with NASA on technology to stitch composite plies together, improving damage tolerance and reducing weight. Using a NASA-developed stitching machine, Boeing's Phantom Works has produced a 12.8m-long wing box, undergoing structural verification at NASA Langley, where it will be tested to 100% of its design limit load. It will be intentionally damaged, repaired by American Airlines' engineers and tested to 150% design limit load - and then to destruction.
The test section is the first full-scale all-composite wing box for a transport aircraft and is expected to demonstrate production cost savings of more than 20% relative to a conventional aluminium wing, while weighing 25-30% less. NASA and Boeing believe this could translate into a 5% reduction in environmental emissions and fuel consumption for a current 210-seat airliner, rising to 8.5% for a 747-sized aircraft. On a more efficient design, such as Boeing's Blended Wing Body concept, savings could rise to 10.5%.
"The next step will be going to total composite structures," says Phantom Works composite wing programme manager Michael Karral. This will be accompanied, he says, by a gradual transition to advanced resin transfer moulding processes which let the composite material be handled and stitched dry, "just like carpet", before the time-sensitive, expensive resin is injected.
Composites can enable new aerodynamic concepts that promise to improve efficiency. NASA's Active Aeroelastic Wing (AAW) programme plans to make the entire wing a control surface by taking advantage of its inherent flexibility. Active leading- and trailing-edge control surfaces will shape the wing to provide roll control, and the wing structure will no longer be burdened with stiffness requirements. The result, says NASA, will be lower drag and a potential 30% reduction in take-off gross weight.
Given successful demonstration of the AAW technology, NASA believes aircraft designers will be free to consider thinner and higher aspect-ratio wings providing greater speed and range. Active aeroelastic control will also allow management of wing structural loads and drag throughout the flight, improving efficiency and extending life. The technology is to be tested in 2001 on a Boeing F/A-18 fitted on one side with an AAW-configured wing produced by the Phantom Works.
Practical application of technologies like the active aeroelastic wing requires advances in aircraft systems. Some are under way, with the aim of improving reliability and safety while reducing maintenance costs and power consumption.
The move to an "all-electric" aircraft is expected to be gradual, because of safety issues. "Today, we cannot go to all-electric in one step, but we can go more electric relatively straightforwardly," says Klaus Fuchs, technical director of TRW Aeronautical Systems (Lucas Aerospace). "More electric" could include back-up electrical actuation of the flight control surfaces, reducing the hydraulic system redundancy required. This has been considered for the A3XX, which will use a new high pressure hydraulic system that reduces weight by 1,000kg through the use of smaller lines and reservoirs and less fluid.
Fuchs believes it is possible to mix hydraulic and electric actuation at the control surface. The more-electric aircraft will require higher-power generators, but the integration of microprocessors into subsystems will allow for "smart" control and enhancements like health monitoring. The all-electric large aircraft will require the development of substantially more powerful generators and actuators. "The technology for 100-200kW generators is mature, and we can design them, but 1MW is not on the horizon within the weight and envelope [required for aircraft use]," Fuchs says.
According to a study by British Aerospace, efficiency improvements that are coming in for the new decade could bring a 20% reduction in fuel consumption for an average 250-seat aircraft. This is made up of 7% from the engine, 4.5% from materials and structures and 8.5% from aerodynamics. On a long-range aircraft, which spends most of its time in the cruise, the most gain will come from aerodynamic improvements, while in short range aircraft the main benefits will arise from material and structural advances.
Improved efficiency also comes from the simple expedient of increasing aircraft size. BAe says doubling the size of an A340-type aircraft reduces the fuel consumption per seat by 9%. So for an A3XX-sized aircraft, the total gain over a hypothetical current generation 550-seater could be as much as 30%. That assumes all of the new technologies will be introduced. "Not all of them will satisfy the criterion of being cost effective," says BAe.
Research into a second generation supersonic transport was given an unexpected boost at the 1999 Paris air show when French prime minister Lionel Jospin announced a new government-funded research and development initiative.
French initiative
The work will be led by French research organisation Onera, but will be funded by the ministry of research instead of the transport ministry, which has hitherto supported studies into supersonic aircraft. This will "change the dynamics" of the efforts, says Christiane Michaut, responsible for civil aviation and European affairs at Onera's strategy and marketing directorate.
"The objective is to reinforce the poles of excellence in supersonics," she says. "We realise there is an enormous amount of work to be done and that there are many potential barriers to a programme to build a successor to Concorde. But we believe we must assemble a range of 'technology buckets' so that we are ready if, and when, other issues such as environmental impact are settled."
An action committee has been formed of 19 specialists from industry and research organisations that will produce a list of projects by the end of the year. This will be sent to the French scientific community, says Michaut, who is one of the committee members. She adds that the work "will be open to international co-operation. This has to be a global programme". The subjects to be studied will bear on such areas as development of computer codes, variable geometry inlet studies and materials. "We want to look at a much higher use of composites for this aircraft. We must bring the weight down." She adds that one of the major differences with Concorde will be the expected lifetime of the aircraft, which will be 60,000h - more than four times that of Concorde. "This means we will have to define procedures for airframe testing that do not yet exist." Another important area will be to find ways of optimising the aircraft around transonic as well as supersonic efficiency, since it will spend a significant amount of its life flying in the high subsonic regime to avoid the shock wave associated with passing through the sound barrier.
The European Commission is also backing supersonic research through its recently approved Epistle programme, which will centre on low-speed aerodynamics. "One of the main problems with a supersonic aircraft is its noise around airports", says Michaut. "If we can find ways of improving low-speed aerodynamics for such an aircraft it will be able to approach and depart from an airport more slowly and with less power".
Rolls-Royce and Snecma have been pursuing two distinct engine concepts - the mid-tandem fan and the ejector/mixer engine. Michaut says that both exhibit problems and need further development. "Perhaps we need a third way," she adds.
A decision on which projects will be selected under the new French initiative will be taken in April. This will be submitted to the research ministry for final go-ahead by summer 2000.
Source: Flight International
