In aircraft construction, composites are in, but don’t count metal out

July 2013 - Lightweighting is an ongoing trend in the aerospace industry, challenging engineers to integrate new materials and techniques into their designs without compromising there original integrity. Composites have become popular among aerospace designers, but researchers are looking at metal’s role in laminar flow technology. 

As part of the National Aerospace Technology Strategy’s Engine Technology Roadmap, the Environmentally Friendly Engine Aerospace Technology Validation Programme aims to reduce carbon dioxide emissions by 10 percent and nitrous oxide emissions by 60 percent, with a 50 percent reduction in perceived noise levels. The Advanced Metal Forming Research Group at the University of Ulster, Newtownabbey, North Ireland, has addressed this need by developing an extended laminar flow lipskin manufacturing process.

The lipskin is the leading edge of the nacelle—the pod-like structure that houses the engine and is usually found under the wings. As air flows over the surface of the aircraft and the nacelle, turbulent flow can occur, increasing drag on the aircraft. 

“Turbulent flow can be caused by lots of different things, the actual shape but also the joints within the aircraft itself,” says Alan Leacock, Ph.D., senior lecturer in advanced metal forming at the University of Ulster and leader of
AMFoR. “Joints are inevitable because you’re dealing with a manufacturing process that will always have a tolerance. That step and gap is usually the trip point for the laminar flow, which then results in this turbulent flow and additional drag on the engine.”

Smoothing it out

A riveting process attaches the lipskin to an underlying structural component of the engine’s circumference, introducing a certain degree of mismatch in the surfaces. “The key is to not have that joint as early along the edge itself,” says Leacock. “The joint itself on most engines is about 7 to 10 inches back and we need to extend it much further than that, anywhere up to maybe 35 inches back. So at least three times what we can currently manufacture.”

Natural laminar flow can be achieved by eliminating joints on the external surface of the nacelle through the rearward extension of the lipskin. Deep drawing and spin forming processes are standard ways to manufacture these skins, but Leacock says they have their own limitations. With deep drawing “you have what’s known as the limiting ratio, which basically defines the depth you can draw a certain radius of part to, based on the radius of the engine itself,” he says. “A theoretical ratio is about 1.96, so there’s a significant limitation as to the depth where we can actually draw those parts using standard deep-drawing processes.”

Spin forming offers the ability to create an extended lipskin, but surface quality becomes an issue. “Because you have severe tool contact on the surface of the part, you need a secondary finishing operation to eliminate the surface markings, which would result in turbulent flow anyway, even with the joint being further back,” says Leacock.

“Effectively we’re forming it in a perpendicular fashion to the traditional methods and it’s because of that that we’re able to form the material with minimal thickness variation,” he says. “The key steps would be an initial form process, then there’s a draw, a redraw and a final stretch. And that’s all conducted on a single machine.”

Virtual experimentation

Leacock has worked in aerospace ship forming for 17 years and has tried various processes for extending the llipskin, such as stretch forming, flax forming, deep drawing and roll bending. His team evaluated the approach in a modeling environment using Pam Stamp software from ESI Group. 

“That allowed us to try lots of crazy ideas in a virtual environment and do it very quickly. From those ideas we were able to zero in on the one that gave the most promise,” says Leacock. “We then refined the process over another year and developed a very simple test that we conducted at a very low cost to make sure that what we were seeing in the model was what we were getting on the shop floor.”

With further funding the team was able to develop a machine to produce the parts, but Leacock says after solving the problem, the difficulty lies in commercializing the system and bringing it to market. The technology is patented in approximately 30 countries, and several companies have expressed interest in licensing.

The Department of Defense classifies the process at a Manufacturing Readiness Level of 4. “The focus now is to bring the technology to MRL 7, prove it as an industrial scale and illustrate the capabilities of the technology there,” says Leacock, who notes that his research team must be responsive to industry requirements. 

“The research work that we would do here would maybe be about 20 percent fundamental and 80 percent applied. And the 20 percent fundamental is driven by the need to solve problems in the applied research area,” he says. “We’ve always tried to focus on industry needs and providing solutions for industry.”

The composite problem

“I think the problem with the aerospace industry has been the focus on composite materials, and rightly so, composites offer an incredible number of benefits in terms of their application,” says Leacock. In fact, the new Boeing 787 Dreamliner uses 50 percent composite materials. But Leacock says the vast majority of aerospace production still is metallic components.

For many reasons, the lipskin is manufactured on virtually every aircraft as a metallic component. To prevent ice build-up, hot gas from the engine is vented to the leading edge. “That swirl gas temperature can be anywhere up to 530 degrees C, which can leave the lipskin itself sitting at 204 degrees C,” says Leacock. “So a metallic material is best suited for those kinds of temperatures.”

Metal also has a greater impact resistance and damage tolerance than composite materials. Even the 787 Dreamliner, which Leacock says is the leading technology on composite aircraft, has a metallic lipskin.

Efficient cost saving

Using lighter materials and incorporating efficient designs reduces fuel consumption, but diminishing drag on an aircraft is equally important. Specifically with this new lipskin, “you’re talking about maybe one to two percent reduction of fuel consumption, which doesn’t sound like a lot but it adds up quite rapidly,” says Leacock, noting that the low percentage amounts in large savings on Transatlantic flights. 

“The reason this starts to become important is that a lot of airports are charging additional taxes to carriers if they are using inefficient aircraft or aircraft that have a certain degree of emissions,” he says. 

On the other side of the spectrum, the lipskin manufacturing process can reduce overall cost and help suppliers meet production target rates. “The current cycle time on the process that we were on is just over five minutes to actually manufacture the parts. It’s significantly better than many of the other processes, which often involve multiple stages and multiple machines to produce a single lipskin,” he says.

Leacock admires companies that introduce new challenges for the aerospace industry but he also worries that a skilled workforce will not be there to address future industry needs. “It’s often perceived as dirty and an old area of manufacturing and therefore something that won’t last that much longer,” he says. “But we’ve been building metallic aircraft now for almost 100 years and it’s something that will continue for the foreseeable future. I think manufacturing as a whole is incredibly important to economies and it’s only by manufacturing and exporting that we’re really going to drive things forward.” MM