A key objective for aircraft designers and manufacturers is to make and maintain lightweight, durable and affordable parts. To be allowed to use parts made of new materials like composite and metal additive printed components in an industry like aviation, all of the materials must be accompanied by the right certification. NLR takes care of this entire process, from calculation and design all the way through to construction and certification. “Besides making a correct calculation we can also build the design ourselves”, says Bert Thuis, Manager of NLR’s Construction Technology department. “We examine the materials, construction concepts and manufacturing technologies. Working across the entire spectrum in this way is what makes us unique.”
“Automation is the key word”, says Thuis. “It starts at the Automated Composite Manufacturing Tech Centre (ACM Tech Centre), a facility we use to address research questions at the low to medium TLR levels or levels 3 to 6. We elaborate ideas that are not yet viable or for which the business case is not yet clear, but which have the potential to become really big. NLR does this in a research environment where we sometimes develop the hardware ourselves.”
“After the ACM Tech Centre phase, we move on to the Automated Composite Manufacturing Pilot Plant (ACM Pilot Plant) where we develop TLR 6 to 8. This is also at the point at which an aircraft manufacturer or SMEs can subsequently put an idea into production. From this point, we use equipment obtainable in the market because we don’t want to come up with something for which a customer is later unable to get the equipment. That’s why we only use equipment already on the market. The end-user must be able to do a copy-paste, so to speak, and start up production.”
The Automated Composite Manufacturing Pilot Plant
Fitting smart ‘heads’ to robots
With the increasing penetration of robots in the production process, NLR also meets the demand for automation. The ACM Tech Centre has a robot-based process for producing composite materials. The actual robots are always ‘off-the-shelf’ models to which various heads can be fitted. If necessary, NLR develops its own head, frequently in cooperation with the robot manufacturer. Thuis regards this as an important element in the cooperation with SMEs. “We possess our fundamental knowledge and understand materials, the manufacturing process and the products. NLR knows the requirements a certain robot head must meet, such as the pressure that must be exerted, how flexible it must be and so on. Our data are unique in that respect.”
Bert Thuis at the ACM Pilot Plant
The great amount of knowledge of materials ensures that customers keep coming back to NLR. Thuis thinks this is mainly due to NLR’s large knowledge base rather than possessing the largest or latest equipment. “One customer recently came back to NLR after going to a newly opened centre in England where the knowledge necessary to use the equipment properly to achieve the desired results was missing.”
Composites instead of titanium
As part of the Dutch TAPAS-2 programme (Thermoplastic Affordable Primary Aircraft Structure), NLR developed together with Airbus, TenCate and Fokker a large composite engine mounting — a pylon upper spar — 6 metres long and 28 millimetres thick. The pylon is one of three composite demonstrators being developed in this programme. The other two are a section of fuselage and a tail section, in which NLR is intensively involved.
“The pylon in an aircraft is currently made of titanium, which is expensive as a material and in its processing. The question placed before us was whether a favourably priced variant could be produced. We first conducted small-scale trials with strips of thermoplastic tape that were laid by a robotic arm and turned into a small composite sheet of the required thickness. With these small pieces, we carried out trials until there was no more entrapped air, one of the requirements for a pylon.”
“Additionally, the pylon has to withstand high temperatures because of its position near the engine. The material properties had to be tested in this respect and also at very low temperatures. Everything was tested using small components at flat plate level with materials from TenCate”, says Thuis. Following these tests, NLR knew what kind of manufacturing process was needed, how fast the machine could run, at what temperature, and the force required to press down the tape.
Testing of the pylon at its actual size — 6.5 metres tall — revealed different effects such as tape behaviours, temperature management and curvatures. “We were able to use a lot of our knowledge for this purpose so as ultimately to produce a good product that is already attracting a lot of attention”, says Thuis.
Detecting and repairing damage of composite parts
Besides the manufacturing industry, NLR is also active for MRO (Maintenance, Repair and Overhaul). There is a lot of MRO work that has to be done because, compared with metals, composites behave differently in the event of damage and maintenance. Knowledge of ageing and repair is becoming increasingly important, particularly with the growing number of composite aircraft parts.
The most important question in the aerospace industry is whether it is possible to continue flying after damage has occurred. Thuis: “Imagine a cart has driven into your composite aircraft. With an aircraft made of metal there will be a dent, but in composite material you probably won’t see anything, although there is probably damage all the same. To inspect such an aircraft, there is a need for technology and knowledge and at NLR we have the equipment and the know-how to do the job. The next step is determining whether the damage has to be repaired immediately or whether it is possible to continue flying. We can help companies because we possess that knowledge.”
If repair is necessary, the next question is how to carry it out. Thuis explains that various possibilities exist, such as milling away the damage and then using rivets to affix a new section, but this is likely to increase the damage. Another option is the bonding of repairs with a patch of stick-on composite.
The last of these options seems to be a good solution, but the repair mechanic must know for sure that the patch is firmly in place and will not simply fall off. This phenomenon is known as ‘kissing bond’ and is the subject of a lot of research internationally in which NLR is cooperating.
“The most important question in the aerospace industry is whether it is possible to continue flying after damage has occurred.”
Certification
When developing a correct joint, certification immediately has to be factored in because the authorities want to know whether such bonded joints will remain in good condition for 30 to 40 years. NLR is deeply involved in materials research so as to gain a better understanding of what happens to various materials, such as composites and printed parts. The real-life situation is often tricky and NLR ranks among the leaders when it comes to knowledge and certification. Thuis: “NLR is currently conducting research into this matter with a view to using adhesive constructions.” NLR is a member of a European research programme together with other parties including Fokker, Airborne, Kok van Engelen, and Delft University of Technology.
Bert Thuis really enjoys working in a challenging and fast-changing field of research every day where he and his team can assist a wide range of clients. “All of this knowledge benefits companies that are suppliers of large corporations like Airbus and Boeing, but we also assist defence clients and SMEs. It’s great to see that much of the knowledge of these new materials is also widely usable in other fields, such as the automotive industry, the maritime sector and the infrastructure, thus creating numerous spin-offs outside the aerospace industry”.
Technology Readiness Level, R&D and industry
In the field of construction technology, NLR develops at almost all TRL levels from 1 to sometimes 8. To be able to implement the processes properly elsewhere, the goal is to work towards generic, flexible systems. A significant portion of the research nowadays takes place virtually using simulations, which speeds up lead time and reduces costs.
TRL stands for Technology Readiness Level, a method of estimating technology maturity of Critical Technology Elements (CTE) of a program (hardware, components, peripherals, etc) to integrate this technology into an operational system or subsystem. TRL 1 is the lowest level, where scientific research begins to be translated into applied research and development (R&D). For NLR, TRL 8 is the highest level, where the actual system is completed and qualified through test and demonstration. TLR 9 is the highest TRL level.
For more information about the ACM Tech Centre and the ACM Pilot Plant:
Bert Thuis bert.thuis@nlr.nl
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