After several false starts, Airbus knows there is no more margin for error or delay in the design of its mid-size A350 widebody
At Farnborough last year, Airbus wowed the industry with the revelation that it had ditched its "warmed-up A330 concept" to tackle the 787 and devised a far more impressive three-pronged attack with which to strike back at the Boeing widebody twinjet armoury in the form of the A350 Xtra Wide Body (XWB).
Eleven months on, the airframer is facing the challenge of turning that concept into a firm programme, with a solid specification and performance, a cohesive production plan incorporating international partners and a robust development timetable. Service entry of the baseline A350-900 is set for mid-2013, and with this being five years behind the 787, Airbus knows it cannot afford any further slips.
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Airbus will not freeze the A350's concept until the end of next year |
Airbus executives openly admit they were "caught napping" by the 787, and are even bold enough to acknowledge they were a little shocked by the sales success of the rival twinjet. The European airframer had squandered two years heading down dead-end paths with its original A330-derived response to the Boeing, before accepting the inevitable in July last year and starting with a €10 billion ($13.4 billion) clean sheet of paper. Airbus's chief operating officer customers John Leahy even jokes about it: "Everyone was writing that we redesigned the aircraft six or seven times. We didn't. We redesigned it three times, and that was enough."
Despite the catalogue of strategic errors Airbus has made in the mid-size widebody sector, remarkably - if it plays its cards right - the European airframer could exercise an important advantage over its rival. And Airbus knows it.
"The competitive challenge is defined. The 787's risk is the A350 XWB's advantage," says Alan Pardoe, director of product marketing for the A330/A340/A350. He refers to the fact that for the first time since the A320, Airbus finds itself trailing its rival from a product development perspective. And it intends to employ all the tactics that Boeing has used successfully with the 777 against the A330/A340 to take the advantage with the A350 to ensure that it is superior to the competition in every measure.
Twin attack
But with Airbus aiming to tackle two Boeing products in the 250- to 350-seat sector with its new twinjet - the 787 and the 777 - the exact path of the battle lines is unclear, especially as no-one - not least Airbus - is sure what strategy Boeing will adopt to tackle the middle of this market, the 300-seat category. There could be a larger 787-based derivative - the "-10" - or an upgraded 777-200 model, which Airbus dubs "777X".
"With all the different two- and three-class layouts from us, Boeing and others, it's difficult to match which aircraft competes with which aircraft," says Pardoe.
But despite that, he says that each A350 variant has been "quite intentionally" sized larger than the equivalent 787. "Our smallest A350 variant [which has around 20 more seats than the 787-8] is our vision of what this marketplace will sustain in the longer term."
Pardoe says that just like a decade ago when the 200-seat A310 was "supplanted by the 767-300 as it was a bigger aircraft", he expects a similar fate to befall the smallest 787. "Thus I suspect that the 787-8 will be the first to perish as it is the smallest aircraft," he says.
Whatever Boeing decides to do, Airbus believes the A350 family has the upper hand as it is a "single family of technically superior aircraft" competing against "two families a generation apart".
The A350 will be a "step ahead" of the 787 in every area, claims Airbus. Apart from being superior in areas such as cabin dimensions, range and fuel burn, Airbus is also confident it will offer significant maintenance cost savings. "On a per-seat basis, the 314-seat A350-900 will have 10% lower maintenance costs than the 280-seat 787-9," says Pardoe.
"We achieve this by extending the check intervals by reducing the number of tasks, while materials and systems technology and a reduction in the need for highly skilled people also play a part," he adds.
Airbus says the A350 will require a maintenance base visit only every 36 months, and a structural "visit" every 12 years. "It's a question of structuring the maintenance programme so the aeroplane can fly when the operators want it to," says Pardoe.
Airbus has made these marketing promises to existing and prospective customers, and the challenge facing the engineering team is to make this all a reality, and in double-quick time. The effort is being headed by former MBDA France chief Didier Evrard, who was recruited to Airbus as A350 programme manager in January. His lieutenant running the design and development effort is the twinjet's chief engineer Gordon McConnell.
Design freeze
The XWB received its industrial go-ahead in December last year, and the engineering team is now focused on completing the design freeze - "maturity gate (MG) 5" - in late 2008. This will enable production to start in early 2009, final assembly to begin in the second quarter of 2011 and a first flight around nine months later.
Evrard says Airbus is already engaged with suppliers and intends to make all the key selections between now and the design freeze next year. This is much earlier than is traditional with Airbus programmes, as the airframer is pursuing what is now standard industry practice and involving the suppliers in a joint definition phase rather than inviting them on to the programme once the configurations are finalised.
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The CAD drawing released by Airbus shows the revised nose shape being studied for the A350 |
From the 314-seat A350-900, the 270-seat -800 evolves by eliminating four frames aft of the wing, and six forward, while the 350-seat -1000 incorporates a seven-frame plug forward and four aft. All three share common wing geometry of 64m (210ft) span, 440m2 (4,740ft2) area and 35° sweep, although Airbus says that the structure will be adapted for each variant.
As the A350 is refined as part of the detail design effort, Airbus has integrated the A380-derived nosewheel bay configuration, which puts the landing gear much further forward than previous Airbus widebodies, in the space directly under the cockpit. "There have been a number of trade-offs in the nose area, which has enabled us to maximise the volume of the cockpit and avionics bay while optimising aerodynamics and the positioning of the nose landing gear," says Evrard.
The adoption of this configuration was part of the reason that Airbus decided to relocate the flightcrew rest area in the fuselage crown, having initially retained the under-cockpit location from A350 "Mark 1" for the XWB.
McConnell says Airbus has been working "on the nose and cockpit geometry and we believe we've got a good solution for the space allocation in that area".
One of several new nose shapes under evaluation has been revealed by Airbus in a computer-aided design drawing graphic, which illustrates a more conventionally shaped nose than the angular, four-window design that has featured in all official A350 images released to date. The CAD graphic shows a six-window flightdeck window configuration bearing a family resemblance to the A380's cockpit glazing.
Airbus is making much greater use of computational fluid dynamics in the design of the A350, says McConnell: "We've now got both the software and the computing power to run whole aircraft CFD models, which we used for performance and handling qualities evaluation."
Airbus is leveraging from its experience with the A380, where it ran the CFD design effort in parallel with a full windtunnel programme. "We found we had excellent calibration for high-speed design from the CFD to the flight-test and windtunnel results," says McConnell. "This has allowed us to take the bold step to reduce windtunnel testing on this programme."
By using CFD tools, Airbus "can iterate the design much faster" and at the same time has been able to cut the windtunnel time by 40% compared with the A380, says McConnell. "We've saved six months already just by using this tool for the aerodynamic development of the aircraft."
CFD drawback
But McConnell warns that the "one thing CFD doesn't do fantastically well yet is good low-speed analysis - we use it but we don't rely on it". So Airbus began A350 low-speed windtunnel testing on 29 January at Bremen in Germany and trials have also been undertaken at its Filton, UK site and at France's ONERA institute.
Evrard says Airbus is "very happy with the results" and that they "have enabled us to optimise the engine requirements and we will freeze them very soon". Aerodynamic tweaks to the A350's double-bubble fuselage shape have resulted in the adoption of a more rounded upper lobe, says Evrard. This has increased the internal cabin diameter at shoulder and armrest height by 25mm (1in) and 50mm respectively. The A350's maximum internal diameter is now 5.6m (18.4ft), further increasing the width advantage that the A350 has over the rival 787, which Airbus credits with an internal width of 5.5m.
Leahy says that increased cabin size has prompted some airlines to ask Airbus to look at a possible high-density 10-abreast seating configuration using seats similar in width to those in a nine-abreast configured A300 or A330.
Airbus's "intelligent airframe" concept means that "we adopt the best materials taking into account the whole life-cycle of the aircraft, so our material costs are driven by performance and direct maintenance costs", says McConnell.
This results in 52% (by weight) of the airframe being made from carbonfibre, compared with 22% (excluding Glare) on the A380 - the material being used for the A350's empennage, wing, belly faring and hybrid fuselage. When the A350 was an A330-based design, Airbus had rejected Boeing's path of adopting carbonfibre for the fuselage, but has changed its mind for the XWB. McConnell says the carbonfibre rethink was a natural step.
Carbonfibre project
"When we decided to change the fuselage cross-section for the XWB, we had a blank sheet of paper so we could exploit the research and technology project we'd been running on the application of carbonfibre to the fuselage," he says.
Airbus calls the A350's fuselage construction a "hybrid" structure, as it comprises carbonfibre skin panels, doublers, joints and stringers and keel beam, while the frames are made from aluminium.
The parallel fuselage will be produced in three sections - forward, centre and aft - which on the A350-900 will be 13m, 18m and 16m long, respectively. Each section will have four long carbonfibre fuselage panels (top, bottom and two sides) that will be attached to the aluminium frames. "Because we have four separate panels, we can optimise the ply lay-up of each one for its role in the structure enabling us to optimise the weight," says McConnell. "For example, the top and bottom panels mainly carry bending loads, whereas the side ones mainly carry sheer and will be optimised in a different way."
Aluminium lithium provides "a simple weight-saving" as its density is 5-6% less than a copper alloy, says McConnell. "We'll use it extensively in the fuselage in all the so-called dry areas in the fuselage, whereas in areas that get wet such as the galleys we'll use titanium to ensure we don't have any corrosion problem."
Another advantage of the hybrid fuselage concept is that the metallic fuselage frames, floor beams and seat rails create what Airbus calls an "electrical network" enabling a carbonfibre fuselage to emulate the electrical continuity of an all-metal fuselage, says McConnell. "This is required in a carbonfibre fuselage to provide a neutral return path for electrical equipment."
To guard against lightning strikes, Airbus has adopted the concept in use on the carbonfibre tails of its current aircraft - a metallic mesh on the outer surface.
The wing is effectively all-composite, with carbonfibre skins, spars and stringers. McConnell says that aluminium lithium has been adopted for all the wing ribs after running trade-off studies against carbonfibre. "For the very heavily loaded ribs, aluminium lithium is by far the best solution. For the lightly loaded ones it's a bit more balanced, but we've decided that all the ribs will be alloy."
Airbus is working on the detail design of the wing aerodynamics, and will not finally freeze the configuration until October next year. "We are already very well advanced," says McConnell. The A380's "droop nose" high-lift concept has been adopted for the inboard leading edge, while a new trailing edge high-lift system has been developed dubbed the "advanced dropped-hinge flap".
Novel device
Although this is a "very simple hinge design", McConnell says that the flap concept is "a novel device as it is a multifunctional trailing-edge flap system where we can deflect the spoiler as well as the flap to control the gap between the trailing edge and the flap and thus optimise the performance of the system". He adds that as well as providing high efficiency in terms of its lift/drag performance, it also has a big benefit in its simplicity and weight saving.
McConnell says that other advanced functions are being studied for the dropped-hinge flap design. "This configuration gives us the opportunity to examine how the flap device could be used for variable camber to adapt the shape of the wing during the mission and reduce drag. It could also be used for load alleviation functions through the differential setting of each of the flaps," he says.
Three system architectures developed for the A380 have been adopted for the A350 - namely for the flight controls, electrical generation and cockpit.
The A350 has the A380's 2H/2E flight-control system "which incorporates two hydraulic and two separate electrically powered control systems", says McConnell, meaning that the architecture is almost exactly the same as its big sister - each primary surface has a single hydraulically powered actuator and electrically powered back-up with the exception of the outer aileron, which uses the two hydraulic systems together. "The benefit of this system is that is it limited to one hydraulic circuit resulting in fewer pipes and weight," he says. "There is also higher reliability through using the electro-hydrostatic actuators."
Airbus has adopted fully electric actuation for the slats, while the A330/A340's hydraulic ram air turbine has been dropped in favour of an electric device, due to the more electric architecture of the flight-control system.
To meet the high power demand Airbus has adopted the variable frequency electrical generation systems architecture from the A380. "We have four 150kVA variable frequency generators - two on each engine to give redundancy and enable despatch for an ETOPS flight with one generator inoperative," says McConnell.
The variable frequency generators are simpler and lighter than the integrated-drive generators that equip the A330/A340, which also makes them more reliable, he adds.
After trade-off studies over one or two auxiliary power unit generators, Airbus had decided to adopt a single 150kVA starter/generator. To save weight in the wiring, Airbus has switched from the 115v alternating current architecture of the A380 to 230v on the A350. "We can achieve this through a very minor change to the A380 generators," says McConnell.
As part of the A350 redesign ahead of the XWB relaunch, Airbus re-evaluated the bleedless technology that Boeing is introducing on the 787 for the pressurisation system, but again rejected it. "With today's technology we do not see a benefit from deleting the bleed system for the weight reduction or for the operating costs, at the aircraft level," says McConnell.
He adds that eliminating the bleed system results in the need for the aircraft to provide greater electrical capacity and incorporate new technology to drive systems such as air conditioning and "from our studies with suppliers [going bleedless] doesn't look anything other than being weight-neutral, with a big risk on the reliability side".
GE hope
As for the engines, Airbus is hopeful that General Electric - launch powerplant supplier for the A350 Mark 1 - will join the XWB programme to offer an alternative to the Rolls-Royce Trent XWB. "The airlines prefer to have two suppliers, so we prefer it too," says Evrard. "We are discussing with GE, which is a potential second supplier. They have to make up their business case in order to join us. It was always foreseen that they would join later and this is still the situation."
Meanwhile, R-R is working to finalise the design of the Trent XWB powerplant, but will not complete this "for another couple of months", says McConnell, adding that it has some "architecture choices to make".
The 75,000-95,000lb thrust (335-425kN) R-R engine will have a common design for all three A350 variants, in terms of its architecture and sizing. Although it is being developed from the bleedless Trent 1000 that powers the 787, McConnell says that Trent XWB specific fuel consumption is around 2% better than for the 787 powerplant.
Modified Trent
The engine will be rated at 87,000lb thrust for the A350-900 for service entry in 2013, and then flat down to 75,000lb thrust for the -800. An uprated, slightly modified version of the Trent XWB developing 95,000lb thrust will enter service on the -1000 in 2015.
McConnell says that to achieve the required thrust increase for the -1000, "R-R will introduce some technologies in the form of new materials in the hot end of the engine to be able to increase the thrust, without changing the flows or architecture. That will allow them to run the engine faster, so they'll have to rematch the fan to the engine. The fan diameter will be the same as the lower-thrust engines, but the fan blades will be different."
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The A350's flightdeck is effectively a replica of that on the A380 |
McConnell says that R-R wants to ensure "as much as possible" that there is a common bill of material for the engine across the entire family so it "is looking at how it could reintroduce that new technology back on to the engine for the -800 and -900".
Airbus had already adopted the A380's flightdeck for the A350 under previous studies, and continues with this thinking for the XWB. "We'll probably have slightly larger information displays at the side, but otherwise it is very much an A380 cockpit," says McConnell. "Like the other Airbus aircraft, there will be the option of single or dual head-up displays."
The A350 will also incorporate the A380's AFDX avionics technology, which provides significant maintenance cost savings over the Arinc 429 system that equips the A330/A340. "AFDX has fewer connections and better reconfiguration flexibility," says McConnell.
The switch to a composite wing has forced Airbus to rethink the way it attaches the main landing gear to the structure, and the solution the airframer has developed borrows from the design of the Vickers VC10 of the 1960s.
"Traditionally we bolt the gear to the rear spar of the wing, but with a carbonfibre wing we have a problem with the local thicknesses of the wingskins to take the load of the landing gear," says McConnell.
Each A350 main landing gear leg is attached to the rear wing spar at the front and to a gear beam at the rear, which itself is attached to the wing and the fuselage. To help reduce the loads further into the wing, a double side-stay configuration was adopted. "Both these solutions have been used before on aircraft like the VC10," says McConnell.
With the take-off weight range for the A350 family spanning 245t (540,000lb) to 295t, Airbus has devised a three-pronged main landing-gear design philosophy encompassing both four- and six-wheel bogies to ensure it can keep the pavement loading within limits. "Our target is to keep the A350's aircraft classification number for a flexible runway below 75," says McConnell.
The A350-800 and -900 will both have four-wheel bogies, although the -800's will be slightly shorter to save weight. Both will fit in the same 4.1m-long bay. "For the -1000, which has a 30t increase in take-off weight, we'll go to a six-wheel bogey and a 4.7m landing gear bay - an increase of one bay frame," he adds.
Future growth
There is provision to incorporate the six-wheel bogie and bay in future weight growth versions of the A350-900, such as the -900R extended range and -900F freighter, to ensure they retain the required pavement loadings.
In response to a request from the airframer's marketing department, the highly tapered rear fuselage design that has been a feature all widebody Airbuses until the A380 has been eliminated. "We've kept the parallel section much further aft with the object to retain the eight- and nine-abreast seat layouts right up to the door four area, to have as few special seat part numbers as possible, and increase the galley space and capacity at the back of the aircraft," says McConnell.
Source: Flight International
