A new composite
material that contains layers of aluminum and glass fiber could save operators
of transport aircraft many billions of dollars in maintenance and operating
costs, researchers say.
The U.S.
aluminum giant Alcoa, materials-technology company GTM Advanced Structures and
scientists at Delft University of Technology in the Netherlands have patented a
fiber metal laminate (FML) called CentrAl reinforced aluminum, or CentrAl, for
use in aircraft manufacture.
CentrAl
provides some 25 percent more tensile strength than high-strength aluminum
alloys, is extremely resistant to metal fatigue and is highly damage-tolerant,
said Rinze Benedictus, chair of the Department of Aerospace Materials at Delft
University of Technology.
At the same
time, it is so light that a transport-aircraft wing made from a combination of CentrAl
and aluminum -- which is better than CentrAl at resisting the compression
strains on surfaces such as upper wingskins -- would not only be much stronger
than a wing made from carbon-fiber
composites, but also could be 20 percent lighter, said Benedictus.
"We
think you can save 600 to 800 kilograms in a large aircraft (over carbon-fiber
composite) -- we estimate that the saving could be around 15 to 20 percent of
the weight of the wing," he said. This hasn't been proved, because nobody
has yet made a wing using the new material, but it is likely that a wing made
using CentrAl would be much easier to repair and maintain than a carbon-fiber
composite wing.
Different
kinds of layers
Similar in
concept to the Glare material that Airbus is employing extensively in the upper
fuselage and leading edges of stabilizers in
the huge A380, but incorporating thicker laminate layers, CentrAl starts
with layers of glass fiber/epoxy sandwiched between layers of aluminum.
Between the
fiber and the aluminum are layers of a proprietary resin-rich material that its
developers call "BondPreg." These layers cause the aluminum to adhere
to the glass fiber and also help to spread stress loads evenly throughout the
laminate. Thick layers of advanced aluminum, attached strongly to the CentrAl
laminate using BondPreg, form the outside of the sandwich.
CentrAl is
"very crack-resistant," said Colonel Robert Fredell, military
assistant to the chief scientist of the U.S. Air Force. In test panels
simulating lower wingskins, fatigue cracks from artificial saw cuts grew 10 to
100 times slower in CentrAl than in conventional aluminum structures, even at
stress levels 25 percent higher.
So
resistant is CentrAl to metal fatigue that Air Force scientists are considering
using it to replace structures in many of the service's aging transport
aircraft -- particularly the hundreds of Lockheed
Martin C-130 tactical transport aircraft that it operates.
Air Force,
Reserve and Air National Guard units operate C-130s intensively in harsh environments
such as Iraq and Afghanistan. Some of the 1970s-design structures in older
C-130Es and C-130Hs require major structural inspections every 100 flight
hours, said Fredell.
Huge
potential savings on the C-130
He estimates
that using CentrAl to replace aluminum in fatigue-prone areas of the C-130,
such as its lower wingskin, could save the U.S. Air Force at least $20 million
in maintenance costs on every new C-130 throughout its life.
With the
C-130, "there's a history of replacing the center wingbox mid-life, after
about 20 years," said Fredell. "Unfortunately, we replace it with the
same thing we remove" -- a 1970s-design, fatigue-prone aluminum wingbox
that often needs replaced again a decade or two later.
Every time
the U.S. Air Force replaces a C-130 center wingbox, it spends up to $8 million
removing the aircraft's wing. Additionally, every six years each C-130 requires
a major overhaul that the U.S. Air Force calls a "Program Depot
Maintenance." Equivalent to a "heavy 'D' check" on an airliner,
each overhaul costs the U.S. Air Force millions of dollars.
However,
with CentrAl, "the technology exists to build a wing skin where the first
inspection that needs to be done is at least 50 years into the aircraft's
life," said Fredell. "So, as the aircraft ages, you don't need to
increase the frequency of inspections. Effectively you end up with a structure
where the pilot does a (pre-flight) walk-round and, if a fork-lift hasn't
crashed into the aircraft in the night, just powers it up and takes off."
The most
effective use of CentrAl would be in manufacturing new aircraft, he said. Operators
then would obtain maintenance and operating cost savings throughout each
aircraft's service life.
But, though
the Air Force and other arms of the U.S. military continue to receive new C-130Js,
there are no plans to use anything but traditional 7000-series T6 aluminum alloys
in their construction. Nor are there plans to re-wing older C-130Es and C-130Hs
with wingboxes made of new materials. This is understandable: The U.S. military
services have enormous experience with the 1970s-era aluminum materials that
are used to make their C-130s and many other aircraft.
"The
devil is extremely expensive"
"We
know the devil -- but the devil is extremely expensive," said Fredell. Given
the research that has already been done, particularly in Europe, it would only
take $20 million to $40 million to develop a new wingbox for the C-130 made with
CentrAl, he added.
While the
idea of using CentrAl "is still a proposal, not a plan," the Air
Force Research Laboratory is funding Boeing, Lockheed Martin and Northrop
Grumman through its Materials Affordability Initiative to conduct initial
studies into characterizing and demonstrating the material.
The C-130 probably
provides the most compelling case for the U.S. Air Force to use CentrAl, but the
Fairchild
A-10 Thunderbolt II -- affectionately known as "the Warthog"
because of its less-than-glamorous looks -- offers another useful opportunity.
Hugely effective in U.S. Air Force service, A-10s are now being given mid-life
renovations to extend their useful lives for decades.
If CentrAl was
employed in making even a tenth of the commercial jets on order, airlines would
save billions in maintenance and operating costs annually, said Benedictus,
noting that the manufacturers of two new Russian regional jets plan to use the
material. Meanwhile, the
A350 XWB isn't fully designed yet. Has Airbus shown interest? "No
comment," he said.
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