7E7 fuselage becomes material issue

The Boeing 7E7 program continues to evaluate several materials for use on the primary airplane structure.

"Historic belief has held that for commercial jet transport aircraft, carbon-fiber-reinforced plastic composites provide lower weight and higher durability, but that they're too costly," said Walt Gillette, vice president of Engineering, Manufacturing and Partner Alignment for the 7E7 program. "Aluminum has always provided acceptable weight and durability at acceptable cost levels."

But with challenging goals for cost performance, durability and weight, historic solutions are not feasible for the 7E7. That has driven Boeing to work with international suppliers to look for less expensive composite structures and better-performing aluminums.

"Improving weight performance of aluminum structures while maintaining or lowering costs is a real challenge," said Karen MacKenzie, a leader of the structures development team. "But it is a challenge to which we believe we have found a design solution that, when used with advanced aluminum alloys, can make aluminum structure competitive with composite structure."

Boeing has developed a monocoque design—in which the outer skin carries the majority of the stress—for the fuselage, that incorporates several cost- and weight-reduction features while meeting all regulatory structural requirements. The new concept is referred to as Monocoque with Integral Shear Ties or "MIST."

To test the reliability of the monocoque concept, Commercial Airplanes Product Development—with the participation of some of the 7E7 technology development supplier team members—designed and built a full-scale single-aisle fuselage section. That structure was then subjected to the equivalent of five lifetimes of wear and tear (378,000 pressure-fatigue cycles). MacKenzie said the post-test inspection found no crack initiation.

For further evaluation, Boeing designed and built a full-scale twin-aisle fuselage section. In the months ahead, Boeing will subject it to a full set of fatigue loads, such as body bending and shear, floor loading and pressure.

Gillette has called the competition between composites and advanced aluminum a "horse race." Both sides are making progress rapidly as they zero in on the performance and cost targets the program established.

A decision on materials for the primary structure is expected near the middle of this year.

"It's a distinct possibility that we will end up with a combination of both aluminum and composite materials in the primary structure," Gillette said. "Both are showing a great deal of progress. The competition to meet the targets has been excellent."

MIST also is being considered for other applications, including derivative airplanes or current airplane redesigns.

"I don't yet know to what degree MIST will make it onto the [7E7] airplane, but we have certainly learned a great deal in the development effort, and it will help us to make the best decision possible," Gillette said.