Boeing's big question

This story is sourced from Flight International
Subscribe today »
Is now the right time for the Seattle manufacturer to go ahead with a larger, heavily modified 747 family?

Guy Norris/SEATTLE

Decision time is looming again in Seattle. The future of the 747 and, with it, Boeing's dominance of the high-capacity long-range market, rests on whether the company commits to the proposed 747-400X and -400X Stretch.

Yet it does not seem long since Boeing faced similar decisions on a 747 stretch. Some airlines, and the industry at large, are therefore sceptical. In the 30 years since it first flew this is only the second time that the 747 has come seriously close to being extensively stretched and aerodynamically modified. The first time was in 1996 when Boeing went all out to launch the 747-500X and 600X. A go-ahead in 2000 could allow the new models to enter service by the end of 2004, beating the rival 550-seat Airbus A3XX by up to a year.

But three years is a long time in aerospace, and a gulf of hard reality separates the $7 billion-plus -500X/600X development programmes of 1996 and the $2 billion -400X and -400X Stretch proposal of 1999. The former effort foundered largely because costs spiralled out of control, and airlines were shocked when they saw the proposed price of the aircraft. As a result, Boeing simply could not attract enough interest to warrant the launch by airlines already tempted by the promise of the competing Airbus A3XX family. The subsequent collapse of the Asian "tiger" economies, added to Boeing's post-merger woes and production problems, killed off all hopes of a -500X/600X revival and forced the manufacturer to rethink.

But expensive though it was, the -500X/600X exercise was not in vain. Many valuable lessons were learned that are evident in the -400X plans of today. It proved to Boeing that there is a market for something larger than the current 747. Yet it reinforced the company's belief that the market is not large enough to justify the cost of an all-new aircraft - a lesson learned during the Very Large Commercial Transport (VLCT) studies with Airbus and its partners.

These concluded in 1995 and showed that the maximum market for an all-new aircraft with more than 600 seats was fewer than 1,000 by 2020. Detailed studies of the 500-seat plus market showed a demand for between 400 and 500 aircraft.

The abandoned development also underscored the demand for two main products - an aircraft with 747-400 capacity but about 1,850km (1,000nm) more range, and an aircraft much larger than the -400 but with roughly the same range. In response, Boeing again turned to the cheaper derivative approach, rather than opting for all-new designs. The product development (PD) team refocused on 747 developments that meet these two basic requirements, yet which avoid the pitfalls that swallowed the -500X/600X proposals.

Technology temptation

This has been possible for two main reasons. The team has not been seduced by the temptation to adopt 777-style systems and technologies wholesale. All such changes must "earn" their way on to the design, it insists. Secondly, the passage of time - and particularly the absorption of McDonnell Douglas' (MDC) engineering expertise - has provided new solutions to old problems.

The use of the trailing-edge wedge, for example - originally devised by MDC to overcome performance problems on the MD-11 - make a huge difference to the range/payload potential of the -400X without needing to change the original wing substantially. Potential changes are made easier and cheaper by improvements to production efficiency through digital definition and the Advanced Fuselage Assembly/ Fuselage Assembly Integration Team process. As a result, Boeing is better placed to go ahead with a heavily modified 747 family in 2000 than it was in 1997.

The strategy has also changed. Following the collapse of the -500X/600X proposals, the new derivative approach first adopted a cautious build-up. The "building block" approach began in 1997 with the -400IGW, an increased gross-weight variant that was aimed at long-haul carriers such as Qantas which have load-restricted routes. From this beginning, the aircraft's proposed maximum take-off weight (MTOW) gradually grew from 397,000kg to 413,000kg (875,000lb to 910,000lb).

This -400X version, as the -400IGW became, was given detailed attention in 1998 and would have entered service - if launched - as the -400ER. It also offered the chance to develop the structural foundations for the first stretch version, which was known as the -400Y. This was the first to feature a wing-root insert, and was to be stretched to accommodate a further 70 seats. Boeing hoped to have the -400Y available by 2003, beating the A3XX timetable, as it then stood, by about a year. A longer-range derivative of the -400Y was to be developed by reverting to the original -400 fuselage length, but including the larger wingbox.

Compromise design

As 1999 wore on, it became increasingly clear to Boeing that the modified wing proposal was sound, but that airline interest in the simpler -400X derivative was muted. Attempts were made to combine discrete elements of the proposed -400X stretch on the simpler derivative without adding the modified wing. The compromise design used 777 alloys, the trailing-edge wedge, low-speed drooped ailerons and nacelle chines and had an MTOW of 422,000kg.

Compared, however, to the radical advances offered by the A3XX and Boeing's own modified-wing -400X Stretch, it failed to attract much interest from airlines. Although the 422,000kg option officially remains alive, the -400X "family" PD studies are concentrated on two main versions, both of which use the modified wing.

Boeing's modified -400X proposals may be easier and cheaper to build, but are they good enough to attract airlines away from the all-new A3XX and its promised performance advantage over the 747-400? The answer is a categorical "yes", according to Boeing Commercial Airplanes Group PD director David Anderson. He believes the latest windtunnel data indicates that "we don't just equal it, we are better than the A3XX".

This claim is controversial because Boeing is eking extra performance out of a heavily modified 35-year-old wing design, while Airbus has the benefits of a clean-sheet design. Boeing's confidence is based on the dramatic range/payload improvements that come from the large insert in the root of the original wing. This stabilised at about 2.28m (7.5ft) for most of 1999, but by the end of the year had grown to around 2.66m as the design was fixed in time for a final series of windtunnel tests planned to be undertaken at Farnborough, UK, in the second quarter of next year.

Enormous lift

The huge new root provides space for an astonishing 45,400kg of extra fuel, and the additional wing area gives enormous amounts of lift and a bigger buffet boundary. The original root insert, together with a new inboard flap section, increased wing area by 14%, from 541m² (5,825ft²) on the -400 to 616m² on the -400X. The latest increase sees this area grow to 631.6m², with the span growing by 5.33m to almost 70m, compared to 64.5m on the current -400. The tips will have a drag reduction design, either in the form of raked tips similar to those developed for the 767-400ER, or possibly blended winglets such as those being studied for retrofit on the -400 and earlier 747 models.

The trailing-edge wedge, which showed fuel savings of up to 3.5% during flight tests earlier this year on a 747-400, takes up to 2.5% of the wing local chord in length, and 0.5% of the chord in height, and appears as a thickened trailing edge. This provides more inverse camber to the wing, increases aft loading while decreasing overall wing loading, and generates lift across a greater portion of the chord. To maximise these effects, the upper inboard wing-skin surface will be "re-lofted" to change the cross-section and improve long-range cruise performance.

The thicker, longer root of the modified wing results in a longer wingbox, which will make the baseline -400X version around 1.27m longer than the current -400 and increasing seating capacity by 30-50 seats. Average tri-class seating for the -400X would be around 446, compared with 416 on the standard -400. The big jump would be in range, which, thanks to the larger fuel volume and improved aerodynamics, would be up to 16,300km, versus 13,500km on the current model. The 500-seater -400X Stretch, on the other hand, would trade airframe weight for range, and end up with a range of about 14,300km - 800km more than that of the current -400. MTOW for both -400X versions would be about 473,500kg.

The overall length of the baseline -400X would grow to 72m, while the longer version would grow by 9.7m to 80.4m with the insertion of two fuselage plugs.

Engine power for the proposed family has remained relatively stable throughout the fluctuating design phase. The General Electric-Pratt & Whitney Engine Alliance continues to refine its GP7167 version for the -400X, while individual company engines such as the GE CF6 and P&W PW4000 are "on call" should Boeing revert to the derivative studies with an MTOW below 454,000kg. The GP7167 is aimed at a 67,000-73,000lb (300-325kN) thrust bracket, with a 2.51m-diameter hollow wide-chord fan.

It is "ready for launch", says Alliance president Bruce Hughes, who adds: "It will take us around 30 months to do an engine from launch to certification." The engine will differ from the GP7267/7275 being offered for the A3XX family by having one fewer low-pressure compressor and low-pressure turbine stage. "We've told Boeing we are ready to boogie when it is ready to boogie," says Hughes.

The Rolls-Royce Trent 600, combining the 2.47m-diameter fan of the Trent 700 with the core of the Trent 500, itself a scaled version of the 777's Trent 800, is also ready. The engine is provisionally rated at 68,000lb thrust for the role, although it is capable of growth to suit the demands of the -400X family, says R-R.

Last March, a flurry of speculation was caused in the industry when Boeing revealed to Flight International that it was working on a Large Airplane Product Development (LAPD) study. The admission (Flight International, 24-30 March) caused some confusion among airlines and suppliers alike, because it appeared to contradict the company's public stance against the development of all-new designs such as the A3XX, and its preference for a derivative approach.

It also appeared to indicate the renewal of the new large airplane (NLA) studies that were effectively suspended with the end of the VLCT activity, and the later launch of the 747-500X/600X study project.

Blended wing

Details were later leaked to the press indicating some of the directions taken by the LAPD team. It had visited the newly acquired McDonnell Douglas company in late 1996 and early 1997, and investigated two designs at Long Beach, California. One was the blended wing body (BWB), a flying-wing design that used the blended-in fuselage to contribute to the overall lift. MDC had configured the BWB with a double deck seating up to 800 in fore-and-aft structural cells within the fuselage and wing.

Because of the increased aerodynamic efficiency of the vast wing, designed in the first instance with a span of about 88.5m, engineers predicted that the BWB's fuel consumption would be roughly 30% lower than that of a conventional airliner of similar capacity. Intriguingly, the design also included a tri-jet arrangement which required thrust of only 50,000-60,000lb per engine.

Concerned over the emergency evacuation provisions of the double-decker and the potential costs of manufacturing, the Boeing team quickly rejected the BWB proposal. However, of its own volition, the newly absorbed Phantom Works set about redesigning the BWB. The result was a single-deck "theatre" seating arrangement with better emergency exits away from the trailing edge of the unconventional wing and its semi-podded engines. Importantly, the new design was also better tailored for "affordable" manufacturing, with smaller structural components and simpler internal geometry. In the meantime, further basic research into the BWB shape was performed in conjunction with Stanford University, which flew a scale model.

Although the BWB is not destined to be Boeing's immediate 747 successor, it is receiving renewed attention within the company and at NASA, which has selected the design as one of its first Revcon (revolutionary concept) projects. This will essentially help to "fast-track" the BWB design towards subscale flight testing of a 10.6m-span remotely piloted version by the third quarter of 2002. Flight tests of the unmanned air vehicle will take place at NASA Dryden Flight Research Center at Edwards AFB, California.

C-17 look-alike

The other design on which the LAPD team focused was the C-17, the high-wing military transporter developed by MDC for the US Air Force. Although this was at first sight a strange model on which to base a potential LAPD, the Boeing team found intriguing possibilities with the design, particularly its excellent emergency-evacuation layout.

The team also visited Lockheed Martin and researched the layout and structure of the C-5, and spent a day crawling over an Antonov An-124 that was visiting Seattle. The result was a "C-17" look-alike design, designated the Model 763-241, with a high wing, four underslung turbofans and an enormous T-tail, 20.1m to 21.3m high depending on configuration. The aircraft's span was 80m and length overall was 76.25m.

The vast, ovoid-section fuselage was about 8.2m wide at the floor level of the single main deck, with five exits on each side. The main undercarriage, comprising a six-wheel main truck on the outer station and a four-wheel main truck on each inner station, was housed in streamlined fairings that were far less obtrusive than those on the current C-17. Windtunnel tests on a model showed the design was capable of Mach 0.9-plus cruise speeds.

Despite the design's promising operational aspects, the vast size of the tail and the height of the engines above the ground posed significant problems for ground support. Also, structural analysis revealed "lots of issues" with wing loading, and the structure of the fuselage needed to support the high-set wing box and the complex undercarriage bays.

By 1998 it became clear that a more conventional design was still preferable, and work switched to a design resembling a 777 with a four-engined low wing layout. After many alterations, the resulting design (one of which was the Model 763-246) showed a 14,300km range aircraft with seats for 454. An extended-range version of the design, which included innovative features such as a composite wing, revealed the range potential for 16,600km missions. A stretch was also outlined which could seat up to 547, with optional overhead sleeping accommodation for 82.

The aircraft's 35° swept wing had a span of 80m, and a length of 73.5m for the standard version, and 80.2m for the stretch. MTOW ranged from 471,200kg to 532,500kg for the larger version, and maximum zero fuel weight ranged from 331,900kg to 340,500kg respectively. Extensive use of composites in the main wing box, flight control surfaces and empennage was also planned, as was the use of drag-reducing features such as riblets, laminar flow nacelles and "programmable" flaps. The aircraft was also envisioned with fly-by-wire flight control and a wing load alleviation system. The design also incorporated a five-post main gear with a total of 20 wheels, with the two aft gear trucks steerable to 26°.

While LAPD/NLA studies continue, Boeing intends to offer the 747-400X family as its prime contender for the future. The aim of the studies is to keep abreast of technology, says Anderson. "We don't want to miss a trick. We have looked at all sorts of designs, including double-deckers and dual bodies. We basically wanted to do our homework, and wanted to make sure we were not being trapped down a channel."

As Joe Sutter, the "father" of the 747, says: "The 747 is not an ageing dowager. She's just emerging from her teen years and avid suitors are scrambling for invitations to her coming-out party." The question now remains: will Boeing send out the invitations to the ball in the first quarter of 2000, and which suitors will come knocking?