Boeing’s 777X design team is tasked with introducing significant structural and propulsive advances while keeping other systems changes to a minimum, so it is no surprise that in most respects the next-generation twin will be difficult to distinguish externally from the 777-300ER of today.

Yet as Boeing digs deeper into the design of the new 777-8X and -9X models that will become its flagship long-haul products for the 2020s and beyond, the company is invoking more visible changes on its way to firm configuration in mid-2015. While system changes will remain invisible, the first lessons learned from wind-tunnel tests, in addition to new propulsion-related noise-reduction technology, will result in some new external design features that differ from Boeing’s initial concept.

As a result of this process, two visible changes have been made to the wing, a fourth-generation composite design that builds directly on Boeing’s experience with the 787. Not only is the overall span increasing, but the outboard flap will now be split into two to stiffen the sections and guard against flutter—a potentially catastrophic coupling of the structure with aerodynamic forces. These two changes are partially connected, as the greater span drives a higher overall wing-aspect ratio, further thinning out the trailing-edge flap sections.

The 777X wing is a direct adaptation of the 787-9 design which, in turn, is an improved derivative of the optimized structure introduced with the block point change on the seventh-production 787-8. “We are [incorporating] that body of knowledge into the 777X wing, and refining that design,” says Terry Beezhold, 777X chief project engineer and vice president. However, while the overall design can be scaled up for the greater size of the 777X, certain features could not, he says. “One of things we encountered as we refined the design was that the outboard flap was really long and really thin. To make it stiff enough to avoid flutter risk, it became apparent that we needed to split the outboard flap.” The wing will now feature three flap sections: outboard, intermediate and inboard, with two additional hinge fittings and fairings added to each wing.

The span is also growing by 2 ft. to 235.5 ft., with the additional length being added outboard of the folding wingtip that is being developed to enable the 777X to maintain the same 213-ft. Code E span of the 777-300ER at taxiways and gates. The added foot means each folding tip section will be 12 ft. in length, adding 24 ft. to the overall span when fully extended. The decision to grow the span follows results from wind-tunnel tests at Boeing’s transonic wind-tunnel facility in Seattle and low-speed tests at the Qinetiq-owned 16.4 X 13.7-ft. low-speed wind tunnel in Farnborough, U.K.

“We have put a lot of focus on the design of the folding tips, the hinge and the structural architecture,” says Beezhold, who adds that the span extension will have a very minimal impact to loads on the tip, either through the structure or through the hinged actuation system. “We adjusted the overall span to optimize the entire wing and to deliver the best overall aerodynamic lift. The design is aimed at the lowest drag for the minimum weight, and the sweet spot indicated a slightly larger span,” he says. Boeing characterizes the span increase as a chance to optimize efficiency and increase lift distribution, rather than as a necessary recovery from a predicted performance shortfall. It also emphasizes that, given the folding wingtip, the aerodynamic benefit of the greater span is effectively a bonus. “We are able to affect the lift-drag ratio in a positive way and in a straightforward manner,” Beezhold adds.

“The big deal is that the model performed as predicted by the CFD (computational fluid dynamics), and if you get those sort of results, you can make detailed decisions and be confident you’ll get the gain you predict,” says 777X Vice President/General Manager Bob Feldmann.

Thanks to the close agreement between the modeling and wind-tunnel results, “we have pretty good confidence” in meeting the predicted performance guarantees established for the two big twins, says Beezhold. He adds that by closing in on the final configuration sooner, Boeing expects to avoid the -chaos that radically delayed development of the 787-8 and 747-8. “We are able to optimize the definition to bring stability into the design process. We have actually begun the preliminary load cycle, so we have firmed up the key architecture and begun that.”

One of the main structural decisions related to the wing was whether to make the entire wing, including the center wing box (Section 11) that runs through the fuselage, from composite or to stay with aluminum. Although the former offered additional weight savings, the potential integration issues tipped the balance in favor of a conventional structure.

“So last year we made the decision to make the wing center section out of aluminum, and that is all about what you are pulling on [in terms of loads],” says Beezhold.

The decision makes it easier to manage the differing thermal expansions and corrosive considerations of the interfaces between the metal and composites. “The interfaces are more defined and are fewer, and it simplifies the wing-body join,” he adds. Fuji Heavy Industries, which makes the current 777 center wing box, has announced plans to develop a 10 billion yen ($97.4 million) factory for the 777X program.

Boeing also reveals that it has developed a new means of reducing engine noise, which will enable it to dispense with the distinctive nacelle sawtooth-shaped chevrons that were part of the original 777X concept. The chevrons are standard on the trailing edge of the fan cowling on the General Electric GEnx-powered 747-8 and 787, and are expected to be featured on the GE9X in development as the exclusive engine for the new twin family.

“We are replacing the chevrons with a new nozzle design technology,” says Beezhold. “It provides equivalent levels of noise for the cabin and community, but is lighter in weight and has lower drag.” The proprietary design disrupts shock cells by mechanically mixing fan stream in the bypass duct with the ambient flow, but does not require the forced mixing provided by the chevrons which, as a result, cause some drag. Beezhold declines to discuss details and says simply: “We have found a way to do it.” The nacelle will therefore look conventional and more like today’s GE90-powered 777.

Boeing also is evaluating other potential propulsion system advances including an advanced ceramic matrix composite (CMC) exhaust nozzle similar to that poised for flight test on the company’s 787 EcoDemonstrator. Significantly lighter than conventional nozzles, the CMC unit can withstand higher temperatures, which in turn provides longer life and better compatibility with the higher operating conditions of new-generation engines. The CMC test is designed to bring the nozzle to a technology readiness level of seven, or to a point where it could be considered for the 777X. “We still have time for that decision and we certainly have not eliminated it,” says Feldmann.

The newest concepts also provide a clearer visual indication of the increased overall size of the GE9X, which saw fan diameter increase to 133.5 in. from 132 in. late last year, in response to Emirates Airlines’ requirement for a “thrust bump” to raise maximum takeoff thrust from 102,000lb. to 105,000lb. Although the thrust increase was acknowledged by GE President/CEO David Joyce at the official launch of the 777X at the Dubai air show, the increase in fan diameter has only recently been disclosed.

The images also appear to indicate a minor change in the design and positioning of the pylon and strut fairing; the overall positioning of the engine is now cantilevered slightly higher.