The Seattle Times (MCT)
Boeing Co. announced a new twist Wednesday morning 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 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 its new engines and various small aerodynamic improvements.
Renderings provided by Boeing show the wingtip swept slightly backward, or raked, and with a tip that splits in two, one longer end pointing up and the other shorter one down, known as dual feather.
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.”
Winglets in general add extra wing surface without extending the overall wing span. By so doing, they increase the wing’s lift while at the same time their shape reduces the drag caused by air vortices at the wingtip.
Teal said an upward swooping winglet alone has to be taller to increase the span sufficiently, adding extra weight that negates the fuel saving.
He said the split ends of the new wingtip move the system’s center of gravity downward and maximize the aerodynamic gain without adding as much weight.
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, he said.
Low-speed testing was done at a wind tunnel in Farnborough, England, operated by U.K. aerospace research firm QinetiQ. Boeing conducted high-speed tests at its own trans-sonic wind tunnel in Seattle.
“The winglet concept performed as expected,” said Teal. “While testing was under way, our airframe, structures and manufacturing teams met with the aerodynamics team to make sure we have plans in place to integrate the winglet onto the wing and overall airframe design of the MAX in a producible way.”
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. Details of the design and the build plan will be pinned down next year when the MAX design is finalized.
The MAX, to be built in Renton, Wash., is competing against the Airbus A320neo. The neo adds winglets to the current A320 design — Airbus calls them “sharklets,” but they are very similar to the upward swooping winglets on today’s 737s.
Because Boeing’s current 737 already has winglets, Airbus executives have argued that the MAX cannot gain the extra 3.5 percent fuel-burn improvement the European planemaker 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, though Airbus hotly disputes that. Boeing spokeswoman Lauren Penning said the new MAX wingtip’s incremental fuel burn advantage adds up to a further 1.5 percent advantage compared to the neo.
Separately, Boeing revealed that the MAX will have to have a slight bump in the door of the nose landing gear.
That’s because the nose gear has to be extended by 8 inches, compared to 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 further dramatic design changes ahead before firm configuration next year, Teal said the design window is “closing fast.”
“This is pretty much it,” he said.