Integrated assemblies are the Holy Grail for design
engineers because they lighten structures, reduce assembly costs, and increase
strength.
In the new approach being used by Boeing with help
from NASA, dry carbon fiber structures are stitched together and then placed in
a heated tool. A vacuum is pulled and epoxy resin is infused into the
structure.
"The composites technology used in the (787)
Dreamliner is 25 to 30 years old," says Andy Harber, senior project manager,
design engineering for Boeing. "In the new approach, there is no lay-up and no
autoclaves."
787 Composites
The Dreamliner represented a dramatic increase in aircraft composites'
use, with about half of the structure made with carbon-reinforced plastic
(CFRP). As reported by Design
News in an award-winning series, the concept
required development of autoclaves larger than those ever previously used.
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Otherwise, the technology was similar to composites technology long-used in making fiberglass composites. Hand lay-up is an open mold process in which successive plies of reinforcing material, usually fiberglass or resin-impregnated reinforcements are applied to a mandril. Curing is accelerated in an autoclave.
Deliveries of the Dreamliner are running more than three years late because of a variety of problems, including issues with fasteners and attachment points in the composite structure.
No decision has been made at Boeing about potential use of the new assembly technology for commercial aircraft. But Harber says Boeing expects to use the technology in a next-generation blended wing body aircraft designed for reduced noise and pollution.
Combat Testing
The process received its first field test as
replacement landing gear doors in C-17s used as transport
aircraft in Afghanistan. "On Sept. 17, 2009, we delivered to NATO eight landing
gear doors featuring resin-infused, stitched composites," says Harber.
The original doors were made with traditional
materials and had taken a beating in the rough landing environment in the
battle zone. Tools for the C-17 doors were developed by Process Fab Inc.
The first licensee for the Boeing-developed
technology is General Dynamics Armament and Technical Products, which was
awarded a $17 million contract by Boeing for the production of composite
components and spares for the C-17 Globemaster III aircraft. Production and
program management is being done at General Dynamics' advanced materials
facility in Marion, VA.
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The latest version of the Boeing composites technology is called Pultruded Rod Stitched Efficient Unitized Structure (PRSEUS).
"This is completely reinventing how a composite structure is designed and manufactured," says Harber. One of the keys is a vacuum infusion process in which a minimal amount of resin is pulled into a tool to cover the fibers.
Another key to the use of PRSEUS is use of a pultruded rod attached to the skin as a stiffener. This could eliminate the need for thousands of fasteners on an entire aircraft. Pultrusion is a process dating back to the 1950s in which composites are pulled through a heated die, creating a very strong linear shape. Many ladder rails are made via the pultrusion process.
Phantom Works
The test bed
for this new materials technology is an experimental aircraft developed by
Boeing and NASA called the X-48. To date, the aircraft has been unmanned and built to small scale for testing purposes.
Boeing's
Phantom Works has been developing the blended wing body aircraft concept in
cooperation with the NASA Langley Research Center. Two models have been built
under contract by Cranfield Aerospace in the UK. NASA and Boeing completed
initial flight testing of the Boeing X-48B last year.
"This
project is a huge success," says Fay Collier, manager of the project in NASA's
Aeronautics Research Mission Directorate. "Bottom line: the team has proven the
ability to fly tailless aircraft to the edge of the low-speed envelope safely."
The blended wing body is a dramatic departure
from the "tube-and-wing" approach used in commercial planes. Many engineers
feel that design has reached its potential. It's not clear though how
comfortable passengers would feel in such a radical departure as the blended
wing.
PRSEUS technology is also a leap of faith for commercial aircraft, where
use of traditional composites was a radical departure.
"Boeing is
studying a number of advanced materials including PRSEUS for use on future
airplanes," says Boeing spokesman Bret R. Gardner. "However, we have not made
any decisions about whether we will use this material or not."
But it will have life in the next phase of development for the blended
wing body aircraft, which remains an experimental aircraft.
Next year
NASA will test a mid-fuselage section of a hybrid wing body aircraft made with
the PRSEUS technology to determine its potential flight worthiness.
If all goes well, Boeing will use its jointly developed blended wing body as
an entry in the Environmentally Responsible
Aviation Project sponsored by
NASA. The project's primary goals are to develop unconventional aircraft
designs with the potential to reduce noise, fuel burn and nitrogen oxides (NOx)
emissions.
Environmental Project
Boeing is
pinning its hopes on the integrated assembly made possible with advanced
composites technology as key to its work for NASA in developing an
environmentally responsible aircraft.
For this
project, the Boeing team will define a concept for an aircraft that can achieve
speeds up to 85 percent of the speed of sound, cover a range of nearly 7,000
miles, and carry between 50,000 and 100,000 lb of payload, either passengers or
cargo.