Carbon-Fiber Materials Make For Lighter, Faster Planes
SOUTH of Seattle, in one its many production facilities, the Boeing Company is trying to manufacture carbon-fiber airplane parts in a cost-effective way. It's a big challenge, since aluminum costs $6 a pound versus $50 to $75 a pound for composite materials, which are made of layers of carbon-fiber fabric baked at high temperatures.
Given the huge cost disparity, why the push for these high-tech materials?
Dick McLane, the plant's manager of technical integration, explains that this technology would be critical for the development of a new-generation plane known as a ``high-speed civil transport.'' As presently conceived, the plane would travel at 1,800 miles per hour, so fast that friction-induced heat would weaken an aluminum exterior. If titanium were used, a plane would weigh about 1 million pounds, considerably heavier than one made with composite materials.
Even in contemporary aircraft, Mr. McLane says, composite materials have advantages, being at least 20 to 25 percent lighter weight than their metal counterparts (which greatly cuts fuel costs), corrosion-resistant, and able to withstand more wear and tear. They also promise to simplify assembly somewhat: The tail section made here will use 25 percent fewer parts than are used in other Boeing planes.
The company has long used composite materials in various parts, such as landing-gear doors, to cut weight. But the tail and floor struts of Boeing's new 777 plane will be the first ``primary'' structures to go composite. The 777 will be 9 percent composite, three times greater than current Boeing jets.
``If this is a success, then the percentage [of composites] on succeeding products will be going up,'' McLane says. The next area to try would be wings, he adds, noting that the cylindrical, window-lined fuselage represents a more complicated engineering challenge. Wings have the added challenge of being fuel-storage areas.
COMPOSITES are commonly used in a range of smaller down-to-earth products, such as tennis rackets. But making 40-foot-long parts to provide safe air travel means using higher-strength fiber, applying special resins to it, and inspecting the final product rigorously, all of which adds to the expense.
In an effort to keep costs down, the Frederickson, Wash., plant puts a premium on efficiency. McLane expects the cycle time, or start-to-finish time of making the average part, to drop from 60 days to 13 days.
The factory opened this year to build vertical and horizontal stabilizers, or empennage. The new plane is scheduled to be delivered to customers starting in 1995, but must undergo a year of testing first, so the first planes are already taking shape.
The first step is the laying of carbon fiber onto forms by tape-laying machines. The thickness of the fiber tapers off from 80 plies at the root end to only 11 at the tip. The tape must be laid in strips of no more than six-inches in length: More than that could bunch up because of the contours of the mold. Even with a perfectly flat-shaped part, the strips of tape could be no more than a foot long, McLane says.
The pieces are cured for two hours in a giant pressure cooker called an autoclave at 350 degrees F. They are then tested for structural integrity in a pool of water using ultrasound. Later they go to assembly, where the empennage is put together using 62-percent composites.
Boeing designed the composite structures for the 777 to be significantly stronger, and hence safer, than those used in its defense planes such as the B-2 stealth bomber, which relies heavily on composites.
Airbus Industrie, the European consortium that competes with Boeing, also has a plane with a mostly composite empennage.