Boeing's effort to ensure as much industrial and design commonality between the two largest variants of its Dreamliner family has evolved into a two-way transfer of technology, with major elements of the 787-9, including the wing and hybrid laminar flow control (HLFC) system, now matching those of its bigger brother, the -10.

Less than two years since reviewing the value of HLFC on the 787-9 empennage altogether, Boeing has decided the system does have merit, but in a different configuration. The 787-9 entered service in 2014 with drag-inhibiting HLFC devices embedded into the vertical and horizontal stabilisers. By contrast, the 787-10 – which performed its first flight on 31 March – has HLFC only in the vertical tail, says Ken Sanger, vice-president of 787 development. To maximise commonality between the two variants, HLFC will be removed from the horizontal stabilisers of future -9s, but will remain in the vertical tail.

The 787-10 has already influenced the design of the inside of the wing for the 787-9. Though the aircraft share a common 60.1m (197ft) wingspan, Boeing engineers originally expected the wing for the 787-10 to weigh several hundred pounds more to compensate for the higher aerodynamic loads caused by the longer fuselage, Sanger says. Instead, analytical refinements reduced the required strengthening until the 787-10 wing "wasn't very much heavier" than the 787-9's.

"So we made a decision from a programme perspective to back-tab the -10 wing design back on the -9," Sanger says. "We've already done that, and that's been part of the production system for several months. Airplanes have been delivered with the -10 wing on the -9."

Such changes reveal Boeing's new level of commitment to making the two variants look as much as possible like industrial twins, apart from their length. Aligning the designs of the pair is expected to improve the efficiency of the production system, boosting profits as the programme continues to dig out of a $27 billion-sized hole of deferred production costs linked primarily to the supply-chain breakdowns and redesigns required for the 787-8.

The strategy was built into the design of the 787-10 from the beginning, but it was expected to flow mainly in the other direction. In 2015, for example, Boeing awarded "engineer of the year" honours to Vedad Mahmulyin, who devised a way to use the 787-9's horizontal stabilisers on the 787-10 by changing only the flight control software.

Sanger lists several other ways the 787-10 matches the 787-9's configuration despite a 5.49m longer fuselage, including a 3.05m plug in the Kawasaki Heavy Industries-built section 43 and a 2.44m stretch of the Boeing-made section 47. Training, maintenance and spares requirements remain mostly the same, except for several changes to accommodate the 787-10's longer fuselage.

That stretched fuselage means Boeing has had to alter the main landing gear to avoid tail-strikes as the 787-10 rotates on take-off. Borrowing a feature from the 777-300ER, the 787-10 uses a semi-levered gear to increase tail clearance for the same pitch attitude. At rotation, the wing bears most of the weight of the aircraft, allowing the 787-10's two-bogie landing gear to pivot around the aft bogie, which extends the length of the gear.

Boeing has also increased the capacity of the environmental control system, accounting for the greater cabin pressurisation volume of the large fuselage. Its engines have been uprated too, with Rolls-Royce and GE Aviation supplying 75,000lb-thrust (333kN) versions of their respective Trent 1000 and GEnx-1B powerplants.

Otherwise, the 787-10 and 787-9 are an exact match in terms of part numbers. Where the two designs most diverge, however, is with performance. For the price of carrying 40 extra passengers with the same maximum take-off weight as the 787-9, the 787-10 is listed with a maximum range of 6,400nm (11,800km) against the respective figures of 7,600nm and 7,300nm for the -9 and -8.

Some of Boeing's customers – notably Air Lease Corp founder Steven Udvar-Hazy – advocated for the 787-10 to possess range capability up to 7,000nm, or about 8.5% more distance than Boeing has delivered. The airframer argues that such performance is unnecessary, since the 787-10 can serve 90% of existing commercial routes. Unlocking more range, moreover, would drive the 787-10 to a higher maximum take-off weight and erode the hard-won gains in commonality.

In previous models, Boeing has offered sub-variants with a higher gross weight to offer customers with a requirement for more range. But that could prove difficult on a design so optimised for preserving commonality, as Sanger acknowledges.

"I think, to your point, trying to pursue more [performance], particularly if you're chasing range, you have to open up the design more," he says.

In any event, Boeing pronounces itself satisfied with the order backlog for the 787-10. With a year to go until entry into service, nine customers have ordered a total of 149 787-10s. The launch operator, Singapore Airlines, placed a firm order for 30 in 2013, and signed a letter of intent two months ago for another 19.

"We feel very good about the customer base that we have," Sanger says.

Boeing launched the 787-10 nearly four years ago at the Paris air show. Two years later, a top Boeing executive acknowledged an ongoing review of one of the major features – the HLFC system.

Scott Fancher, then vice-president and general manager for airplane development, said the 777X would not use HLFC, and the technology remained under review for the 787-9 and 787-10.

It seemed a surprising shift in Boeing's assessment of the technology's value, coming only a year after the 787-9 became the company's first aircraft to enter service with HLFC installed.

Boeing provides few details about the workings and benefits of the system, but the principle is clear. A stream of air flowing over a moving surface can change from a smooth channel into a turbulent maelstrom. As the laminar flow switches to turbulent, aerodynamic efficiency is lost. HLFC is designed to delay the onset of turbulent airflow as long as possible.

On the 787-9, the HLFC system is visible in the form of two small doors on the bottom of each tailplane and on both sides of the fin. These doors swing inward like the doors of a saloon, creating a force that sucks airflow through tiny holes embedded in the leading edges. As this air is vented through the doors, it creates a low-pressure zone along the latter half of the fin and tailplane surfaces, forcing the air into a laminar stream for longer than normally possible.

Boeing, however, is reluctant to talk about the HLFC's measured performance in service with the 787-9: "I don't know that we've actually shared that," Sanger says.

Source: Cirium Dashboard