Tuesday, April 3, 2018

Boeing 787-10 technical description

Boeing has closed the 15-year-long development phase of the 787 family of aircraft with the delivery of the third and – for at least another decade – final major variant of the Dreamliner to launch customer Singapore Airlines.

The Star Alliance carrier has taken delivery of an aircraft that serves as a visual metaphor for the rise, fall and rebound of Boeing's most significant development effort in two decades. In design and execution, the 787-10 represents equally the breathless ambition of Boeing's original vision for the Dreamliner and the sober, risk-averse spirit that gripped the program after the troubled development and entry into service of the 787-8.

The largest of the three variants boasts the same advanced technologies – electrical, structural and aerodynamic – that make the 787 different from anything Boeing had attempted before or since. At the same time, the 787-10 cedes a 2,000nm (3,700km)-range advantage to its nearest competitor, Airbus's comparably sized A350-900. In exchange, Boeing offers customers – and, not least, itself – the economic advantage of an almost extreme degree of commonality, with about 95% of the 787-9 part numbers reused on the 787-10.

Although it has less range, the 787-10 is no less ambitious than its rival in the market, offering to replace an aging world fleet of A330-300s, A340s and 777-200s.

While Airbus dismisses the 787-10 as a hasty and conservative reaction to the first glimpse of its A350-900, Boeing calls it the most efficient wide-body aircraft it has ever built – one that is still capable of operating 90% of the existing routes served by airlines with an aircraft in its size class.

As a double-stretch of the original 787-8, Boeing pushed the type's computerized flight controls to compensate for the impact of the additional length of the fuselage, in the process inventing a new approach to damping flutter.

The official end of the 787-10 development phase came on 19 January, when the US Federal Aviation Administration awarded Boeing an amended type certificate. The document validated a nine-month-long test program which consumed 900 flight hours and paused only for one of the three 787-10s to make marketing appearances at the Paris and Dubai air shows. "We predicted a quiet test program and we delivered," Boeing's 787 vice-president Bob Whittington told journalists in January.

As a stretched derivative, the 787-10's most important feature is its fuselage length, which is 68.3m (224ft). That makes it almost 4.5m longer than the 777-200 and nearly 5.5m lengthier than the 787-9. Compared with the latter, the extra length is achieved by inserting a 3.04m plug forward of the wing and a 2.43m plug aft of the wing.

It was the length of the forward plug that dictated Boeing's decision to assemble the 787-10 exclusively in North Charleston, South Carolina. For 787-8s and 787-9s assembled in Everett, Washington, completed mid-fuselage sections are flown from North Charleston on the 747-400 Large Cargo Freighter (LCF). But the 3.04m extension makes the 787-10 mid-fuselage too long to fit inside the LCF's bulbous payload bay.

The impact of the double stretch, of course, extended beyond the logistics. The nearly 5.5m of extra length means the aircraft can carry four to five more rows in the economy cabin compared with the 787-9. It also adds 38.2m3 of volume in the cargo bays below the main deck.

While the impact of the additional structure on the empty weight of the 787-10 is still unknown – as Boeing has not yet released this metric – it is likely that the empty weight increased at a slightly lower rate than the stretch between the 787-8 and 787-9, when the fuselage length increased by 10.7% and the empty weight grew by 7.42%. Boeing introduced extensive design changes on the 787-9 to reduce structural weight and simplify the assembly process.

Although the empty weight is still unknown, the metric for maximum take-off weight (MTOW) has been a key selling point since the 787-10 program was launched. Despite the 8.71% increase in fuselage length, the 787-10 was certificated with the same MTOW as the 787-9. While the empty weight defines an aircraft's structural efficiency, the MTOW drives the structural design based on the maximum loads. By keeping the MTOW common between the two variants, Boeing minimized the list of structural changes required for certification.

With the exception of the fuselage plugs, the -9 and -10 are structurally nearly identical. All three variants share a 60.1m wingspan and 5.49m cabin width. The additional length of the -10 required Boeing to strengthen the wing in a few spots, Whittington says. To keep the production process as common as possible between the two variants, Boeing made the same changes to the wing of the -9.

"That really made sense to make them backwards compatible to the -9," Whittington says. "It was a small amount of strengthening. It didn’t impact the flight test or the performance."

The same philosophy was applied to the hybrid laminar flow control (HLFC) system. The concept of HLFC has been known about for decades. Airflow over any surface – such as a vertical fin or horizontal tail – at some point transitions from laminar to turbulent. Delaying the critical moment of transition as long as possible is a goal of any aerodynamicist. Beginning on the 787-9, Boeing installed an HLFC system to push the transition to turbulent flow as far back along the surface as possible.

Boeing's concept relies on the principle of passive suction. As a door opens in the middle of the vertical fin, airflow approaching the leading edge is drawn through holes in the fin instead of around it. As the air exhausts through the door, it creates a low-pressure field that delays the onset of turbulent flow. The holes in the leading edge are difficult to discern with the naked eye, but the saloon-style swinging doors on the side are visible on the 787-9 and -10.

Similar doors could be found initially in the horizontal tail of the early 787-9. As the design of the 787-10 was frozen, Boeing determined the benefit of using the system in the horizontal tail did not justify the extra cost and complexity. Adding to the commonality theme, the HLFC has been dropped from the horizontal tails of both variants, but it remains in the vertical fin.

Only one new technology is known to be unique in the 787-10. This is called the flaps up vertical mode suppression system (F0VMS, including a numerical zero). The European Aviation Safety Agency refers to the -10's F0VMS as the first "active modal suppression system for flutter compliance" in a commercial aircraft. Both EASA and the FAA developed a special condition to allow Boeing to certificate the airworthiness of the F0VMS in the absence of existing regulations to follow.

The existence of the F0VMS on the 787-10 is another example of how far Boeing was willing to push technology to maximize commonality with the 787-9 on the production line. With a longer fuselage than the -9 but an identical wing, the flutter margin for the wing on the 787-10 was inherently reduced. It is a familiar issue in aircraft design, but one with a textbook solution: either increase the torsional stiffness of the wing or add wingtip ballast weights. However, Boeing rejected both options because "they do not meet [the company's] business objectives", the FAA wrote in 2016, as part of the agency's special-condition justification for the F0VMS.

Instead of a traditional hardware solution, Boeing proposed to use software to achieve the same effect, restoring the 787-10's margin for flutter dampening to acceptable levels. In flaps-up mode only, the system activates by oscillating the elevators. As a software fix with structural implications, the F0VMS is not unlike the original 787's vertical gust load alleviation system, which adapted flight controls to offset the rollercoaster effect of vertical gusts. As a result, regulators allowed Boeing to design a lighter wing than would otherwise be required.

Software again proved useful with the sizing of the horizontal tails. As a stretch of the 787-9, textbook aircraft design would suggest the 787-10 would need larger horizontal stabilizers, offsetting the effect of the longer fuselage on pitch control. Instead, Boeing engineer Vedad Mahmulyin used software to increase the effectiveness of the existing stabilizers. Boeing gave Mahmulyin an internal engineering award for solving the problem. The software-based approach saved Boeing the cost and time of developing and testing new stabilizers for the 787-10, while also maximizing commonality with the smaller -9.

All of this effort to maximize commonality renders the 787-10 design unusually optimized for the payload and range specifications on paper today. Many Boeing models are designed with a margin to provide the option of higher-gross-weight versions, but it is not clear that the 787-10 has that ability.

"We are always looking at that. Our customers will tell us when they need a little extra," Whittington says. "This airplane is point-designed for the strength of the wing matching the landing gear, matching the engine thrust. I don't foresee significant changes. But we will continue to look to see if the market needs a couple thousand pounds more gross take-off weight, and we'll continue those studies."

Facing a four-digit shortfall on nautical-mile range against the A350-900, Boeing does not seem particularly motivated to close that gap with the 787-10. Instead, customers face a choice of buying either an aircraft with similar range and 20-40 fewer seats (the 787-9) now or one with better range and 20-40 more seats (the 777-8) in about four years.

If not on range, the 787-10 will continue to evolve in other ways. When the 777 introduced a new interior architecture 20 years ago, Boeing reapplied the same concept in the 767. Airbus adopted a similar approach with the A350 and the A330. The 787 pioneered several major advancements in cabin design, including larger windows, higher humidity and larger luggage bins. As the 777X arrives in service in 2020, Boeing has the opportunity to backfill new technologies on the 787 family.

"That's something we're always looking at: to take it to the next level, the highest level," says Tarun Hazari, regional director of marketing for Boeing. "I'm not saying it's going to happen. But it's something we’ve done historically, and it wouldn't surprise me if we did."

(Stephen Trimble - FlightGlobal News)

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