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.