Boeing gave a new twist Wednesday to its design for the forthcoming 737 MAX jet: A raked, "dual feather" winglet concept that it says will provide an extra 1.5 percent fuel efficiency improvement.

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Boeing gave a new twist Wednesday to its design for the forthcoming 737 MAX jet: a raked, “dual feather” winglet it says will provide an extra 1.5 percent gain in fuel efficiency.

That’s on top of the 10 to 12 percent fuel-burn improvement over the current 737 that Boeing already has claimed for the MAX with new engines and small aerodynamic improvements.

Renderings provided by Boeing show the wingtip swept slightly backward and with a tip that splits in two, one longer end pointing up and the other shorter one down.

In a conference call with journalists, Michael Teal, chief project engineer for the 737 MAX, called the concept “the most advanced wingtip technology in the single-aisle market.”

The design is innovative. No commercial airplanes in service today have anything similar, though the concept resembles a split-tip winglet design Seattle-based Aviation Partners is flight testing.

Winglets in general add extra wing surface without extending the overall wing span. They increase the wing’s lift while their shape reduces the drag caused by air vortices at the wingtip.

Teal said the split ends of the new wingtip maximize the aerodynamic gain without adding as much weight as a longer, single winglet would.

The overall MAX wing span will be just a few inches longer than the current 737. Teal said the upper feather of the new winglet is close to the size of today’s 737 winglets, which are nearly 8 feet tall. The lower feather will be shorter to ensure sufficient ground clearance.

Boeing aerodynamicists came up with the design using computer simulations, then tested it in two advanced wind tunnels, Teal said.

He said the new winglets can be incorporated into the overall design within the previously announced schedule, which will see the MAX enter service in 2017. Final details for the jet’s design and manufacturing will be pinned down next year.

Last fall, at a Las Vegas business-jet conference, a somewhat similar design called the Blended Split-Tip Scimitar Winglet was showcased by Aviation Partners, which pioneered winglet technology and works with Boeing in a joint venture that produces winglets for the 737.

Joe Clark, Aviation Partners founder and chief executive, said Wednesday his company was not involved in the MAX winglet.

“Some very senior people at Boeing have seen our split-tip winglet,” Clark said. “But we have not made any formal proposal to them.”

The MAX, to be built in Renton, is competing against the Airbus A320neo. The neo adds winglets — Airbus calls them “sharklets” — to the current A320 design, but they are very similar to the upward swooping winglets on today’s 737s.

Because the 737 already has winglets, Airbus executives have argued that the MAX cannot gain the extra 3.5 percent fuel-burn improvement the European plane-maker will gain by adding winglets to the neo.

Wednesday’s announcement of a radical new Boeing winglet design undercuts that argument.

Boeing had previously claimed a 5 percent fuel-burn advantage over the neo. Spokeswoman Lauren Penning said the new MAX wingtip’s incremental fuel-burn advantage adds up to a further 1.5 percent advantage compared with the neo.

Airbus hotly disputes Boeing’s figures. Before news of the “dual feather” winglet, Airbus had been claiming the neo would burn 8 to 11 percent less fuel than the MAX, depending on which sub-models are compared.

Airbus spokeswoman Mary Anne Greczyn said Wednesday the company’s engineers looked at the option of a split-tip winglet for the neo but decided on the “sharklet” design.

Separately, Boeing revealed that the MAX will require a slight bump in the door of the nose landing gear.

That’s because the nose gear must be extended by 8 inches, compared with the current 737, to lift the wing and allow sufficient ground clearance for the new, bigger CFM engines.

To fit the longer nose gear into the wheel well without major structural changes, Boeing is shaping the landing-gear door so that it bulges outward to accommodate the tires.

That will add some drag, but Teal said the impact is “negligible.” The projected fuel-burn improvement takes into account all the changes, negative and positive.

Asked if Boeing might have more dramatic design changes before firm configuration next year, Teal said the design window is “closing fast.”

“This is pretty much it,” he said.

Dominic Gates: 206-464-2963 or