Article from DLRmagazine 175: New approaches in lightweight aircraft manufacturing
Farewell aluminium!
The 'Multi-Tow Laying Head' robot heats strips of material using a laser and places them to produce the aircraft skin.
In addition to DLR, the supplier Premium Aerotec, the aircraft manufacturer Airbus and the company Aernnova Aerospace were also involved in the Multifunctional Fuselage Demonstrator (MFFD).
For decades now, aluminium was seen as the final word in raw materials for aircraft construction, offering the same strength as steel at roughly half the weight – until thermoplastics came on the scene, that is. Like thermosets, they belong to the group of carbon fibre-reinforced polymers (CFRPs) and are lighter than aluminium, but can be welded. For 20 years now, CFRPs have featured in aircraft construction, including in the Airbus A380, Boeing 787 and Airbus A350, with a carbon fibre content of up to 50 percent. Researchers are working to increase this proportion. New production processes are needed to enable the entire fuselage to be manufactured from CFRP in the future. Researchers at the DLR site in Augsburg are working on this.
Welding bridge tool in operation
The transverse stiffeners of the fuselage were not riveted to the aircraft skin, but resistance welded using an electric current. DLR developed a special tool for this purpose, referred to as a welding bridge.
There is much materials testing to be done before an aircraft fuselage made entirely of CFRPs finds its way into series production. It is important to understand the practical implications of working with these materials in the factory. Researchers at the Institute of Structures and Design within the DLR Center for Lightweight-Production-Technology tested this process using a true-to-scale fuselage model with A320 geometry. Falling under the European Union-funded Clean Sky 2 project, this Multifunctional Fuselage Demonstrator (MFFD), is eight metres long and four metres in diameter – the world's largest aviation structure made entirely of fibre-reinforced thermoplastics.
Working on an open fuselage
Today's Airbus A320 is manufactured using what is known as the sectional construction method. First, the lower and upper shell segments are connected by a longitudinal seam to form a section, commonly referred to as a 'barrel'. Several of these sections are then joined together by circumferential seams to form the fuselage. In aluminium fuselages, these connections are made using drilling and riveting processes.
Drilling the holes for the rivets creates metal filings that could damage installations in the lower part, so the complete basic structure is manufactured first. Only then are the interior components fitted in the flight deck and the passenger and cargo areas. This is currently done manually due to restricted freedom of movement in the closed fuselage.
The big advantage of components made of carbon fibre-reinforced thermoplastics is the fact that they can be welded. No rivets means no metal filings. As a result, the assembly order can be completely reversed: the lower part of the fuselage remains freely accessible until both parts are welded together lengthways. This working method promises a fundamental improvement in the assembly process, by opening up the possibility of using robots to lay cables, install the floor and rows of seats, and screw on panelling. Such a step would save time and reduce costs. After a consortium of DLR, Premium Aerotec, Aernnova and Airbus built the upper shell and a consortium in the Netherlands built the lower shell, both parts were welded together at the Fraunhofer Institute for Manufacturing Engineering and Advanced Materials in Stade. The demonstrator is currently located at the ZAL Center of Applied Aeronautical Research in Hamburg, where it can then be used for further research.
Demonstrating economic viability
The switch from aluminium to CFRPs must be worthwhile both for aircraft manufacturing companies and operators. Additional costs that arise because production systems have to be adjusted or because CFRPs are more expensive than aluminium must be offset by greater efficiency in the production process. In order to demonstrate that a complete switch from aluminium to thermoplastics can be profitable, the researchers have set ambitious objectives for the MFFD fuselage demonstrator – as the project progresses, they aim to show that 60 to 100 units can be produced per month and that compared with the current Airbus A320, weight can be reduced by 10 percent and ongoing production costs by 20 percent.
How to build an aircraft fuselage from carbon fibre
Step 1: The outer skin
First, thermoplastic tape is heated by a robot-operated laser to around 380 degrees Celsius and laid down with pinpoint accuracy. The tool handled by the robot resembles an oversized tape dispenser. With minimal material waste and low energy consumption, this creates a thermoplastic laminate layer by layer, which does not need to be hardened separately. This process, refined by DLR, is called 'in-situ consolidation'. Ensuring that both the deposition rate and the material quality are high requires close coordination between all disciplines, from mechanical engineering and materials science to control engineering and simulation.
Forty-four longitudinal stringers are welded onto the aircraft skin. The process used for this is called robot-based continuous ultrasonic welding. In a world first, thermoplastic films called energy directors are placed between the two parts that are to be welded together. A robot stimulates the components at 20 kilohertz. They begin to rub against each other, generating temperatures of around 380 degrees Celsius within a fraction of a second and welding the components together. The process is significantly faster than the drilling and riveting commonly used in the aviation industry.
Step 4: Connecting longitudinal and transverse stiffening elements
Resistance welding is once again used to integrate cleats between the stringers and frames. A small, lightweight robot is used alongside a standard robot, enabling operation within the smallest of spaces. The 'cobot' automatically aligns itself with stringers and frames by scanning them, thus eliminating the customary measuring of components or small-scale manual positioning, as is the case in current industrial applications.
