On Boeing's sprawling compound in Everett, outside of Seattle, are two identical four-story white buildings with ample blue-tinted windows. It's here that the 787's composite wings are taking shape. From a windowless office overlooking a labyrinth of cubicles inhabited by a troop of engineers, the wing design team leader, Mark Jenks, directs the work. [See photo, " Winging It."]
"Over the last 10 years or so, design tools for composites have improved significantly," Jenks says. "It's not such a black art anymore." He and his team began the design work using supercomputers in which they ran computational fluid-dynamics software to simulate the effects of wing features and geometries. The goal back then was to determine which designs had the best aerodynamics. The computer simulations were complemented by thousands of hours of tests in a wind tunnel.
Jenks says that now the group is preparing for a series of tests that will help validate the design. One involves building a large section of the wing and attaching to it hydraulic actuators and strain gauges to assess its structural behavior. Another involves measurements to make sure the wing will have adequate conductivity characteristics and resilience against lightning strikes.
Lightning strikes on aircraft are not rare events; one study in the 1980s found that, on average, a commercial airliner is hit by lightning at least once a year. The historic tradeoff with composites, from an aviation standpoint, is their inability to conduct electricity without the addition of metal, which lessens their advantage in weight. Aircraft wings--and, indeed, fuselages, tails, nacelles, and other structures--need to conduct electricity to enable the plane to withstand lightning strikes, which generally flow over a conductive aircraft without doing any damage.
Boeing recently completed what is known as the plane's "firm configuration." Reaching this stage means that structural, propulsion, and systems architectures are not changing anymore, and the final shape of the wing has been set. Starting around last September, a group of suppliers began working on a detailed design of the hundreds of parts needed to assemble the wing, from large structural spars to thin covering skins of carbon-fiber composite. To produce that detailed design, Boeing partnered with three "heavies" of Japanese industry--Mitsubishi Heavy Industries, Kawasaki Heavy Industries, and Fuji Heavy Industries.
Jenks says that for past planes, Boeing followed a "build to print" model: the partners would come in only at a later stage and would basically fabricate the parts according to Boeing's specifications. But for the 787, the partners were brought in about four and a half years earlier, which gave them enough time to participate in the early design work and provide input based on their manufacturing expertise. The scheme also let Boeing spread the risk, because the company's partners picked up a sizable portion of the development costs in exchange for a bigger share of the profits that will accrue if the plane is successful.
The Japanese wing design team worked side by side with its U.S. counterparts at Boeing's Everett facility until a few months ago. That was when some of the engineers began returning to Japan to start the work of elaborating in detail the shapes, material composition, and types of joints and attachments of the parts they were responsible for. Boeing engineers, meanwhile, shifted into their role of overseeing the integration of the plane's fuselage, wings, and tail.
The teams are creating their drawings and specifications using a system called Catia, developed by Dassault Systèmes, in Suresnes, France. Catia is widely used in the industry, and engineers in Japan, Italy, Australia, and Everett will share the files though a global network. And, of course, team members will also stay in touch by videoconference. "We've got half a dozen clocks in our conference rooms," Jenks says. "First it was kind of a gimmick, I think, but it got to the point where we really needed them."
A wing is basically a cantilever beam consisting of three main structural parts. The largest one is the main box, the framework spanning from the fuselage to the wingtip that is the core of the wing and supports most of the load. To this main box, structural appendages called the leading and trailing edges are added to the front and back to give the wing its aerodynamic shape.
For a plane maker, the wing is "really the heart of the whole airplane and the reason that you build a new one," says William Mason, an aerospace engineering professor at the Virginia Polytechnic Institute, in Blacksburg. The design of a composite wing, he adds, "is a big change, but it was going to happen."
"The properties of composites that can give them a leg up in terms of design is first of all the reduced weight," says Paul A. Lagacé, a professor of aeronautics and astronautics and codirector of the Technology Laboratory for Advanced Composites at the Massachusetts Institute of Technology, in Cambridge. Other advantages of composites, he says, include their resistance to corrosion and fatigue--the two biggest problems that plague airplanes--and their "wonderful flexibility in design."
In the 787 wing, Jenks and his team took full advantage of this flexibility. A metal wing is a compromise between aerodynamic performance and weight. With composites, the compromise is the same, but designers are free to investigate a much larger envelope of sizes and shapes. Composites, Jenks says, allowed them to tailor features that wouldn't be practical with metals, such as subtle curvature variations all over the wing. They also make it possible for the team to create larger and more complex structures, reducing the total number of parts and making assembly less time-consuming.
Moreover, Jenks's team was able to make the 787 wing thinner than it would be with metal. Under the flight conditions of the 787, thinner wings are aerodynamically better, because they can produce the same or more lift than thicker wings while reducing drag. With metal, however, there is a tradeoff: as you shrink the wing, you need to add stronger--and thus heavier--structures. But with composites, the Boeing engineers were able to add strength only where necessary, and in the end they got a thinner and lighter wing.
Boeing officials decided to go with an all-composite wing for the 787 only after years of experience with composite structures in previous planes, notably the tail of the 777, which has been operational for a decade. Also, the company had done extensive design work on the Sonic Cruiser, the much-hyped jet with large composite wings that would have flown at close to supersonic speeds but which Boeing canceled in late 2002.
Those projects helped Boeing engineers better understand the advantages of composites--and also their drawbacks. Problems vary from moisture absorption that can reduce stiffness to tiny fissures that can go undetected and result in abrupt, larger cracks. And then there's the simple fact that designers have more experience with metals. In fact, Boeing is using aluminum, steel, and titanium in some parts of the 787.
Another issue Boeing needed to address was cost. Until now, only military projects could afford composites. Prices for these materials have steadily dropped but remain much higher than those for metal, on the order of $100 or even $200 dollars per kilogram versus a few dollars per kilogram for metal. Moreover, their fabrication requires new tools and methods, while making aluminum parts is easier and cheaper. Boeing's plan is to benefit from economies of scale and also from reduced part count.