The leading edges of aircraft wings have to meet a very demanding set of characteristics. New research shows that a combination of steel composite metal foam (CMF) and epoxy resin has more desirable characteristics for use as a leading-edge material than the aluminium currently in widespread use. A paper on the work is appeared in Applied Surface Science.
“We call our hybrid material ‘infused CMF’,” says Afsaneh Rabiei, a professor of mechanical and aerospace engineering at North Carolina State University. “And while infused CMF is about the same weight as aluminium, it is tougher and has other characteristics that make it more appealing from a flight performance, safety and fuel efficiency standpoint.”
CMF is a foam that consists of hollow, metallic spheres – made of materials such as stainless steel or titanium – embedded in a metallic matrix made of steel, aluminium or metallic alloys. For this study, the researchers used steel-steel CMF, with both the spheres and the matrix made of steel. Previous work has found that the metal foam is remarkably tough: it can withstand .50 calibre rounds, resist high temperatures and block blast pressure from high-explosive incendiary rounds.
Infused CMF is produced by immersing the steel-steel CMF in a hydrophobic epoxy resin and using vacuum forces to pull the resin into both the hollow spheres and the much smaller pores found in the steel matrix itself. This results in about 88% of the CMF’s pores being filled with epoxy resin.
The researchers then tested both infused CMF and aerospace-grade aluminium to see how they worked in three areas that affect the performance of an aircraft wing’s leading edge. These were: contact angle, which determines how quickly water streams off of a material; insect adhesion, or how well bug parts stick to the material; and particle wear, or how well the material stands up to erosion.
Contact angle is a measure of how well water beads up on a surface: the lower a material’s contact angle, the more the water clings to the surface. This is relevant for aircraft wings because water build-up on a wing can affect aircraft performance. The researchers found that infused CMF had a contact angle 130% higher than aluminium – a significant improvement.
Insect adhesion is measured in two ways: by the maximum height of insect residue that builds up on a material; and by the area covered by insect residue on a material’s surface. Again, infused CMF outperformed aluminium – by 60% in regard to maximum height and by 30% in regard to the surface area covered.
The researchers also conducted grit blast experiments to simulate the erosion caused by the wear and tear that occurs over time when aircraft wings are in use. The researchers found that, while grit blast did increase surface roughness for infused CMF, the material still fared better than aluminium. For example, at its worst, infused CMF still had a contact angle 50% higher than that of aluminium.
In other words, the infused CMF retained its properties through erosion and wear, which indicates that it would give leading-edge wing components a longer lifetime – and reduce the costs associated with maintenance and replacement.
“Aluminium is currently the material of choice for making the leading edge of fixed-wing and rotary-wing aircraft wings,” Rabiei says. “Our results suggest that infused CMF may be a valuable replacement, offering better performance at the same weight. By the same token, the results suggest that we could use different materials for the matrix or spheres to create a combination that performs as well as conventional aluminium at a fraction of the weight. Either way, you’re improving performance and fuel efficiency.”