NCF-RTM-OOA Manufactures Wing Rear Spar

 

Airbus’s “Wing of Tomorrow” R&D project is mainly aimed at developing new technologies for the “new A320” wing. GKN Aerospace is responsible for the research and development of the aircraft’s trailing edge spar. The “new A320” wing box is 17 meters long. GKN uses the “non-crimp fabric (NCF) – resin transfer molding (RTM) – out of autoclave (OOA)” process to manufacture the 17-meter-long wing rear spar.

 

 

GKN has developed a semi-automated manufacturing process for the first batch of Wing of Tomorrow spars, with the ultimate goal being almost fully automated manufacturing. Here the spars are fully preformed and ready to be transferred to the mold for resin transfer molding (RTM) Arguably, no structure in the wing is as complex and difficult to manufacture as the spar.

 

The spar is a long C- or I-shaped main structure that runs the length of the wing, from root to tip. Depending on the size and type of aircraft, a wing box may have one or more spars. In commercial aircraft, there are usually two spars, one at the leading edge and the other at the trailing edge. They are attached to the wing skin, ribs, landing gear structure and flaps, and are designed to withstand the bending loads generated by the aerodynamic forces acting on the wing.

 

Spars are almost always tapered – getting thicker at the root of the wing and narrower at the tip. In addition, the spars must follow the contour of the wing box and are usually angled to accommodate the sweep of the wing. Airbus (Toulouse, France) decided to switch to composites for the wing skins, stringers and 34-meter spars when developing the composites-intensive twin-aisle A350 aircraft. The gull-wing shape of the A350 wing dictated the integration of two angle changes in the rear spar, so Airbus chose a design that split the rear spar into three sections, each manufactured using automated fiber placement (AFP) of carbon fiber prepreg on a C-shaped mandrel. Each spar section is just over 11 meters long and is connected using custom CFRP panels after autoclave curing.

 

This manufacturing and assembly work is performed by GKN Aerospace at its Filton, UK facility. However, the next generation of commercial aviation wing structures—especially those for high-build-rate aircraft—must and will evolve to a more integrated design that enables factories to produce co-cured, out-of-autoclave (OOA) structures that meet demanding performance, cost and production requirements.

 

Airbus is addressing this challenge with its Wing of Tomorrow (WOT) program. WOT includes components from multiple suppliers, each evaluating different composite M&A strategies. These include, but are not limited to, the lower wing skin from Spirit AeroSystems (Wichita, Kansas, U.S.), thermoplastic composite ribs from GKN Aerospace (Hoogeveen, The Netherlands), and additional thermoplastic ribs and a vacuum bag-only rear spar from Daher (Nantes, France).

 

Also on this list is the development of a 17-meter fixed trailing edge (FTE) component, which was awarded to GKN Aerospace in the west of England. The most critical part of the FTE that GKN had to develop was the 17-meter one-piece resin transfer molded (RTM) trailing edge spar.

 

Challenges of Long Tapered RTM Structures

 

Gavin Lunney, chief engineer at GKN Aerospace, said the company joined the WOT program in 2017 and was given design data on the interface geometry and mechanical performance characteristics of the spar. Beyond that, GKN was given a lot of design freedom, Lunney said, and GKN developed the structure the company now produces “from scratch.”

 

The requirements for the work GKN undertook were small but daunting: The finished, full-scale spar had to be 17 meters long, molded in one piece, and delivered in 2022. In addition, GKN had to demonstrate full-rate production capabilities through cost-effective acquisitions. Lunney said that when he took over the project, it presented a huge challenge, but it also seemed possible.

 

“For the geometry, I think they took a typical spar geometry and then pushed it to the extreme, challenging us to try and understand the limits of what we could actually manufacture,” Lunney said. GKN soon realized that it would have to take a design-for-manufacture approach to achieve the rates Airbus was demanding.

 

To meet this challenge, GKN decided to split the spar development project into three phases. The first will be to manufacture and test small structures, including flat panels. The second and most intensive phase will be dedicated to manufacturing a full 5-meter spar to prove the feasibility of the design and manufacturing. The third phase will be dedicated to manufacturing the entire 17-meter spar. In this way, the company will be able to gradually evaluate the material and process (M&P) performance and make adjustments as needed before the final build.

 

 

Double-layer carbon fiber noncrimp fabric (NCF) manufactured by Teijin was specified for the wing spar. Shown here is the NCF kit after automated cutting and before preforming

 

“This spar is probably the most demanding spar we’ve ever seen in terms of its kink sweep and flange angles, so it presents a lot of challenges in tool design,” said Craig Carr, technical director for integrated composite structures at GKN Aerospace. “A lot of the issues were resolved by the time we completed the mid-scale portion of the project, and then we could move to full-scale tooling.”

 

“We then broke the spar down into a number of discrete features, such as radii, kinks or thickness transitions, and we focused on understanding if we could develop a process that could accommodate those features,” added Lunney. The material combination specified by Airbus for the wing spar is double-layer carbon fiber NCF supplied by Teijin (Tokyo, Japan), to be infused with an epoxy resin system from Solavi Composites (Alparetta, Georgia, U.S.).

 

The final spar that GKN ultimately developed featured a C-shaped design with two angle changes – or “kinks” – to accommodate the wing’s sweep. Early in the project, as GKN began to work on panels and discrete features, the question arose of how to construct the ply architecture for the relatively heavy NCF.

 

Lunney notes that typically, placing NCFs on a long, non-straight C-shaped structure with two angle changes and variable thickness requires strategic cutting and punching-darting of the NCF to avoid wrinkles that could reduce the strength properties of the structure.

 

However, cutting and darting is inefficient for high-volume aerostructure programs. GKN wanted and needed to automate the full-rate production of the spar as much as possible. As a result, Lunney and his team decided early on that this 17-meter-long infill spar would only be feasible in a high-rate manufacturing environment with a design that did not require cutting and punching but still avoided fabric wrinkling.

 

Instead, GKN will rely on strategic and careful layup placement, which aids automation and produces a finished structure that meets Airbus’ mechanical requirements. “If you do have punch cuts, you have to introduce staggering, seam panels and a whole bunch of complexity into your manufacturing process that you really don’t want,” Lunney said. “I think having no punch cuts is good for everybody. It’s one-piece, so there are no joints. The zero-degree plies go from root to tip.

 

We don’t punch cut the material when we form around the flange, we don’t do anything. We are able to shear the NCF so that we have fiber that conforms to the contour of the part.” Craig Carr added: “If you end up putting a lot of punch cuts in a spar, the knock-down factor is not trivial. Especially with the kink locations that we have in the spar – anytime you put any punch cut in a kink, there’s going to be a reasonable knock-down factor.

 

Then you overcome that by adding weight into the spar. Then, you start to question why you’re using a composite spar in the first place.” Another challenge revolves around process. The use of RTM requires the spar to be preformed first and then transferred to a matching metal mold for resin injection and curing. The process is not new and is usually fairly manageable, but the length of the spar adds a level of complexity that GKN must address, especially in a high-speed production environment. “Scaling up the RTM to a 17-meter structure is not trivial,” Lunney noted.

 

 

The spar has a variable thickness and two angle changes or “kinks” in its length. Normally, a structure of this size and complexity would require cutting and punching the NCF to avoid wrinkles and the resulting performance degradation. Punching presented its own challenges, so GKN chose to use a novel shearing technique. Shown here is the NCF on a forming tool

 

From 5-meter to 17-meter-long spars

In 2019, GKN entered the mid-stage of product development with the fabrication of a 5-meter-long spar. This work would integrate GKN’s material and manufacturing strategies and show Lunney and his team the feasibility of large, no-cut/no-punch RTM structures.

 

Lunney says he and his team first commissioned two male dies—one for preforming and the other for the forming process. Process development began with cutting the NCF on an automated cutting table, then assembling and placing the plies by hand in designated locations on the forming tool. After each ply is placed, it is heated and pressed to stabilize it on the forming tool.

 

When all the layers and plies are applied, the mold is closed and heated to crosslink the epoxy powder binder and give the NCF stiffness. The preform is then transferred by crane from the preform mold to the production mold—just a shorter version of the full-size tool GKN eventually plans to develop.

 

The mold has integrated thermocouples, as well as sensors to measure pressure, resin arrival and cure level within the mold to help maximize control of the process. “We built a lot of automation into this tool,” Lunney said. “We built a lot of sensors and relatively expensive technology into a 5-meter spar, which allowed us to prove that the process can meet high production rates without spending so much money on a full-size spar.”

 

GKN ultimately built eight 5-meter demonstrators to evaluate the system’s material and process capabilities. Three 5-meter beams were cut for mechanical testing; another two were used for corner bend testing. The company also performed extensive nondestructive testing (NDT).

 

Lunney said all the test results were promising, including <2% porosity, and showed that the acquisition strategy developed by GKN is viable. This gave the company the confidence it needed to move to the final stage – the fabrication of the 17m moulds and 17m spars. Lunney said the project’s biggest technical hurdles were eliminated when building the 5m spars, so moving to the 17m spar seemed more manageable.

 

 

NCF being assembled on the Alpex production tool. NCF preform partially visible, draped over the side of the toolThe full-scale forming tool and production tool were delivered in mid-2020, and GKN began manufacturing the first full-scale spar shortly after. This involved automatically cutting the plies from the rolls of the Tenjin NCF, which were then placed by hand on the forming tool. After the preform is heated and pressurized, the spar is transferred from the preform tool to the production RTM tool by vacuum lifting.

 

The most complex component in spar production is probably the 17-meter Invar tool made by Alpex Technologie (s Mills, Austria). Lunney calls it “a jigsaw puzzle of an RTM tool. The middle section of the tool has to come apart and move so we can remove the spar after forming.” Other features of the full-scale tool include the same integrated thermocouples and other sensors used in the 5-meter tool, and a fluid heating system with closed-loop temperature control.

 

After it is placed on the production mold, the preform is trimmed by hand to remove excess material before resin injection. With the preform in place and trimmed, the mold is closed and sealed, and the RTM process begins with the injection of Solvay epoxy resin. The RTM equipment, supplied by Composite Integration (Saltash, UK), includes a 200-liter resin delivery system and a pump capable of producing up to 10 bar (145 psi).

 

Before removing the spar, GKN drills seven holes directly into the spar through datum holes integrated into the mold. Designed to receive fixtures used to attach the spar to the wing box, the holes include five 6-mm diameter holes; the remaining two holes, one each at the root and tip, are 8-mm in diameter. GKN would not disclose the total cycle time for the spar preforming/molding process, except to say that it is shorter than it would take to make the same part via autoclave curing.

 

Lunney says GKN hopes to eventually automate ply layup and hole drilling to increase production efficiency and reduce cycle times. “The number of molds we need to meet will depend on how quickly we can achieve the steps required for the RTM process,” Lunney said. “So it could be a multiple of a shift.” Still, assuming one set (two beams) per mold per shift, two shifts per day, five days a week, GKN is looking at 44 sets of beams per month.

 

 

A completed 17-meter-long spar is lifted from the Alpex production mold onto the assembly fixture. GKN is manufacturing four spars as part of the demonstrator phase of Airbus’ Wing of Tomorrow program. “Right now, the material is cut on a ply cutter and then manually removed and stored. It is then manually placed on a mobile table, which is then brought into the station to position the ply,” said Lunney.

 

“So it’s a semi-automated process and we’re trying to focus on areas of development that we thought were immature at the beginning of the program and that we need to mature to get there. Obviously, cutting the ply and transplanting it onto the table is a relatively high TRL technology that we will automate in the next development phase. The stations we develop are capable of full automation and we try to keep that in mind.”

 

GKN delivered the first fixed trailing edge (FTE) assembly with a 17-meter spar to Airbus in September 2021 for the first WOT demonstrator. For its part, Airbus debuted the first WOT demonstrator at the Farnborough International Air Show in Farnborough, UK, in July 2022.

 

GKN has since delivered a second spar to Airbus, with a third to follow soon. The fourth spar, which GKN has just completed, will remain at GKN for testing. Carr says GKN’s ability to develop and deliver the spar so quickly was noteworthy: “I think that was very important for the first section to go straight into a structural test program, and for us to get such a large section properly tested for the first time in such a large program, which was well received by Airbus.”

 

Looking Back and Looking Forward

When asked what was most daunting about the development of the WOT spar, Lunney admits it was the decision to forgo cutting and punching for NCF. “It was a big departure from the norm,” he says. “I was eager to get started, but as we progressed, it became easier. Ultimately, I was calmly confident that we could do it.”

 

This shows the rear side of the finished spar on the assembly fixture at GKN Aerospace. Airbus has already assembled the first WOT demonstrator. Some of the M&P from that demonstrator could potentially find its way into wing structures for Airbus’ new aircraft programmes.

 

He also said the newness of the project brought its own challenges: “Forming the NCF material was a real challenge. We didn’t have a process. We couldn’t find anyone else in the world that had a process that would allow us to form this material into such a complex and challenging geometry without any wrinkles, without any punching.

 

I think there was a lot of skepticism at the beginning that we could do it without punching, but we’ve done it”. I think the second challenge was taking the RTM technology, which is obviously quite old and mature, and scaling it up to 17 metres – we had to find suppliers that could help us do that and then put together a tool and make it work. ”

 

Lunney is not alone, of course. He credits his entire team for helping to achieve the WOT trailing edge scramble. His teammates are: Will Broom, Stephen Williams, Clement Ooi, Phillip Brown, Mark Griffiths, Tim Smith and Tom Bertenshaw. At Farnborough, the entire group was awarded the GKN CEO Award for Technology and Innovation for its development of the s-spar.

 

What’s next? The three fixed trailing edge (FTE) assemblies delivered by GKN will form part of three wing box assemblies, one of which will undergo full-scale structural testing. GKN is optimistic that its success with the WOT spar positions the company well for production contracts on next-generation aircraft programs. In the meantime, GKN is also working closely with several advanced air mobility (AAM) manufacturers who use one-piece wing designs and will benefit greatly from a one-piece composite spar that can be manufactured quickly and efficiently. “We think this is the largest one-piece spar in the world,” Lunney said. “It’s also the largest RTM structure in the world, as far as we know.” “What we’ve done here is demonstrate the quality of the molding process,” Carl said. “We’ve demonstrated the quality of the infusion process. We know what the right requirements are for future projects.”