Mike Richardson meets with GKN Fokker’s head of thermoplastics R&D, Arnt Offringa to hear more about the company’s revolutionary induction welding process technologies that are helping to reduce the amounts of fasteners required to join flying surfaces together.
For over two decades, GKN Aerospace has been a supplier to Gulfstream Aerospace Corporation on all of the company’s large and ultra-large-cabin programmes, including the latest G500 and G600 business jet programmes.
GKN Fokker began participating in Gulfstream’s business jet programmes in 1993. Activities began with the design and build of the empennage and thermoplastic floor panels for the Gulfstream GV model, followed by the common rudder for the G450/G550. Today, GKN Aerospace manufactures a range of products including lightweight thermoplastic solutions for every Gulfstream large and ultra-large cabin aircraft.
“GKN Fokker first began working on the Gulfstream G650 rudder in 2007 and this has been approved by the Federal Aviation Administration (FAA),” begins GKN Fokker’s head of thermoplastics R&D, Arnt Offringa. “It’s been done to reduce its weight compared to the traditional solution. Whilst it provided a slight weight reduction, more importantly, we gained a cost reduction by using a novel welding technology we call ‘induction welding’, where we heat the carbon fibre to weld the rudder together instead of bonding and riveting.
“We were already making composite rudders and elevators for Gulfstream with traditional composites and epoxies, but we had to bond - and bolt them together, which added more weight. Now, we’ve switched to a new design which is welded and is also lighter because there is less material.”
GKN Fokker acquired a basic technology licence from KVE Composites Group and ironed out all the smaller technicalities in order to industrialise the concept using production-type equipment.
“It’s been a hugely successful introduction, as we now have a common rudder product across four different aircraft - one component fits all these different aircraft. We’ve also introduced a new welding jig and the production rate has increased as a result. We’ve tripled our production rate because of the commonality - a great step towards high production rate projects.
“Component induction welding is performed using robotic automation. We can store the robot parameters, so if there is an issue, we can go back to the production records and check the equipment used at the time. Our newest welding line is fully-automatic and there are several jigs positioned in one straight line. The robot moves along the track and also around the jig. This work can also be performed at night remotely, without any operator supervision.
“We’ve been able to reduce the number of bolts on this project by 50%. This gives us a cost reduction of up to 20% over the previous generation of products.”
GKN Fokker’s induction welding process now offers an entirely different proposition altogether. Indeed, a microscopic picture reveals one homogeneous material with no split between two pieces. This makes a significant difference for the aviation authorities. It sounds so simple, but why hasn’t it been done already?
“You have to go down a long road and it takes time to perform tests and demonstrate it to the aviation authorities. The first generation of welding took about 10 years from start to finish - from first test to flying production parts. This is typical for aerospace – it just takes time.”
Offringa states that one of the reasons for GKN to want to acquire the Fokker business was for its thermoplastics technology - it’s complementary to what was already being done at GKN, a company that is very strong in infusion technology and thermosets. Fokker’s focus on thermoplastics is very complementary and provides a really good fit. The main reason for choosing thermoplastics is because the materials are very tough, lightweight and are perfectly weldable, which makes it possible to remove a lot of the manufacturing cost.
“This is an important product for us in terms of the application of thermoplastics technologies. An aircraft fuselage is typically a very thin structure that can easily be damaged. Traditionally, it’s been made from aluminium, a ductile material, whereas the Boeing 787 and Airbus A350 have carbon fibre composite fuselages.
“However, carbon fibre reinforced epoxy is a relatively brittle material – it’s lightweight, but it’s not easy to make so thin. Thermoplastics has huge potential, because it is really tough. We have reached the point where we have a concept that is ‘out of the ordinary’ and revolutionary; on the one hand, a huge weight reduction is gained in the magnitude of 30%, compared to aluminium. At the same time, cost parity with an aluminium fuselage is achieved and this really interests our customers.”
Fight against fasteners
Aircraft fuselage frames need to be mechanically attached to the skin using fasteners –this was GKN Fokker’s initial cost driver. The company’s design concept is a fastener-free solution which can be done cost-effectively with thermoplastic materials. Whilst the bonding of composites is now technically feasible, the civil aviation authorities won’t accept the concept without the presence of some mechanical bolts to secure the two parts together – these are, after all, flight safety critical items.
“The frames used in traditional aerostructures have to be bolted to the fuselage and the reason is a technical one: the frame goes over each stiffener so that when the aircraft reaches altitude, the fuselage inflates like a balloon, which pulls on the frames. Therefore, a weld is not strong enough, so it requires rivets to affix the surface to the frame.
“We have a concept that eliminates the use of bolts in an affordable manner. How is this possible? We have created a unified structure – an orthogonal grid – with stiffening elements. We take flat pieces of thermoplastic composite material and a laid up skin. Between these two we locate an additional piece of plastic. This piece will make a tough joint between the stiffeners and the skin. We then melt the skin and the stiffening elements together to create an integrated structure, onto which frames are welded. It completely eliminates any need for bolts, which are expensive and require a lot of labour to prepare.
“We still need to reach the required technology readiness level. It’s an ongoing process. The entire process has been co-funded by the Dutch government. We started this particular concept four years ago, put in a lot of money to reach the stage we’re at, and it involved a lot of testing too. We are now talking to our customers about this concept. We intend to make bigger fuselage panel components in the future, but we need customers to first express their interest in order to proceed. Watch this space!”