A large composite outer wing section designed to prove the operational feasibility of drag-reducing natural laminar flow (NLF) for future aircraft is being evaluated by Airbus for flight test on a modified A340-300 following work by GKN Aerospace.

The 9-meter-long (30-ft.) section is one of two experimental wing sections intended for testing on the A340 Blade (Breakthrough Laminar Aircraft Demonstrator) program under the European Clean Sky Smart Fixed-Wing Aircraft effort. The program will simultaneously evaluate two different integrated structural concepts for an advanced passive laminar wing, which is designed to improve cruise efficiency by delaying the onset of turbulent flow over the wing surface.

The experimental wing sections will replace the existing wing outboard of the outer engines on the A340, which has been flown to Tarbes, France, for modification. Instrumentation and structural changes will be made over the course of the next year, with “power on” of the systems set to begin in late 2016. NLF flight tests are scheduled for 2017.

GKN is providing the experimental right outboard wing section, and Saab is supplying an alternate NLF concept for the port wing. The GKN section, built at the company’s Western Approaches facility near Bristol, England, incorporates a metallic leading edge and mechanically fastened composite cover, while the Saab section includes a one-piece leading edge and upper cover. The sections will also be used to evaluate anti-contamination surface coatings and a shielding Krueger leading-edge flap.

Although both sections are expected to achieve adequate levels of NLF over the upper-wing surface, the key focus for the program is whether laminar flow can be consistently achieved in operational conditions using a high-quality design that can be manufactured at high production rates. The GKN section has a double curvature skin profile and includes co-cured stringers and co-cured spar caps. “It’s been quite a challenge,” says GKN’s head of airframe and special product technologies, John Cornforth. However, GKN believes the quality of the first article, originally intended as a production test unit, is good enough for flight tests. “We’ve made our first part, and it will be the flying part. We are going through validation right now, and we are working hard to convince Airbus that this is OK,” he adds.

As part of the program, GKN also made a 4.5 X 1-meter laminar- flow ground demonstrator to validate structural, system and manufacturing aspects. “It is a fairly shallow section, so there was a concern about the packing density and the Krueger mechanism and whether it could all fit in that space,” says GKN Engineering and Technology Senior Vice President Russ Dunn. “So we divided it into two sections. One is a conventional design and the other [is a] more innovative design, which was assembled at the Manufacturing Technology Center (MTC) in Coventry. The whole assembly went together really well.”

Developed by three GKN Aerospace technology centers in Luton, the Additive Manufacturing Center in Bristol and a group working within the U.K.’s nearby National Composites Center, the demonstrator is representative of the leading edge and part of the wing- box support structure. The conventional section comprises a monolithic composite skin and standard rib design, while the advanced section includes several features designed to meet the tight design-tolerance requirements of NLF.

The advanced section is made up of a leading edge sandwich panel incorporating GKN’s polyetheretherketone (PEEK)-based laminate electro-thermal wing ice-protection technology with an integrated erosion shield. “Even though GKN has experience in bonding on aluminum erosion shields, there was a whole technology program to bolt on a nickel-steel erosion shield and see what that might do in terms of distortion,” says Cornforth. “We worked very closely with our ice protection people in Luton to make sure we have a structure which could have ice protection. So it includes a much thinner mat that can sit much closer to the surface, and the connectors—which were previously quite big and bulky—are now more efficient. All of that was brought to this program.”

The Krueger flap mechanism is supported by additively manufactured titanium ribs, while the leading-edge panel is held by three compression-molded composite ribs. “We co-cured where we could, but at the interface of rib feet and stringers there were people who were still uncomfortable about the level of loading being carried by a bond,” Cornforth says. The section therefore includes a number of fasteners which, in turn, require an “NLF recovery” system to prevent them from impacting laminar flow.

With fewer ribs than a conventional leading edge, the design also includes a foam sandwich in the skin to maintain stiffness. “One of the difficulties is that if you produce a structure like this in a conventional way with lots of ribs everywhere, you have this kind of egg-box-like structure, as due to thermal change and loadings you get quilting. The requirements of NLF are extremely tight when it comes to waviness [the undulations of a surface under loading], so having a flow which is attached to [a skin supported by fewer] ribs but is sufficiently stiff is another approach for the future. It’s been a hugely successful piece of work,” he adds.

Airbus, which oversaw assembly of the leading-edge components at the MTC, has completed installation of the Krueger flap mechanism and is expected to begin tests soon of the unit in Bremen, Germany.