Lighten up: Innovative tooling enables RACER's fuselage to shed weight
In addition to the RACER (Rapid And Cost-Efficient Rotorcraft) Technology Demonstrator being a cutting edge aeronautical flagship, it's also a showcase of European coordination and integration, galvanising over 25 consortia in 13 countries across industry and academia, underpinned by an extensive ecosystem of SMEs, in pursuit of the EU's environmental aspirations for greener aviation.
This collaborative approach is exemplified not just at full aircraft level but also throughout the main subsystems, all of which need to be developed with meticulous synchronicity in order to ensure realisation of the Technology Readiness Level (TRL6) target, slated for early 2022, in the form of RACER's flight test programme.
Take RACER's fuselage, for example, a key element in the RACER's airframe integration. Clean Sky project officer Dr. Sonell Shroff says that the success of the fuselage design and production hinges on the carefully orchestrated ‘collaboration of multiple stakeholders, working closely together but at different levels of scale to produce the consolidated fuselage.’
Two Clean Sky projects, AFPMet (which concluded last year) and RoRCraft (a core partner project) are key to the design, development and manufacture of the fuselage. AFPMet was devoted to the innovative design, development, manufacture and delivery of specialised tooling to perform automated fibre placement for the lamination and curing of the RACER side-shells; RoRCraft's remit is the design and manufacture of the front and central fuselage section for the RACER demonstrator.
The fuselage central and side shells employ advanced and hybrid composite materials and techniques to reach the stringent safety, weight and strength design objectives. These include automated fibre/tape placement and out-of-autoclave curing to facilitate RACER's required cabin volume, low drag shape, wide access door for versatile mission usage, and advanced cabin insulation for acoustic and thermal comfort. Needless to say, the use of environmentally-friendly materials and production techniques is a given.
Innovative tooling for AFPMet
One of the main challenges in the AFPMet project, explains Metitalia's process advisor for composite manufacturing, Sabato Inserra, is that ‘the sideshells, which are of “sandwich” construction, required lamination using AFP to encapsulate the honeycomb core and composite skin in an uncured state.’
The implication is that the very large and complex composite fuselage side-shells were, prior to curing, still in a very flexible and easily deformable state. Transferring them from the laminating tool to the curing tool was a delicate operation which, if not handled carefully, would have caused damage to the finished side-shells.
‘So we worked with Fraunhofer on subscale tests to develop a way to transfer the uncured part, and based on these tests we developed tools and methods to make the full size transfer of the actual final parts,’ says Inserra.
The other challenge was that the geometry of the lamination tool was very complex, so Metitalia developed a tool fabrication process based on the ‘Lost Foam Casting’ technique, which was followed by precise, numerically controlled machining. Lost foam casting is a type of evaporative-pattern casting process similar to investment casting which uses foam for the pattern instead of wax, as well as surface laser tracking and comparison with 3D-models for generating the milling parameters.
‘Lost foam casting uses less raw material to produce the complex shaped tool for the side-shells manufacture, which reduces energy consumption compared to conventional tooling approaches,’ says Inserra.
‘From a technical point of view, the main challenge was to show that such large-scale complex parts could be manufactured using the automated fibre placement layup,’ says Thomas Zenker, Research Associate at the CFRP Manufacturing Engineering at Fraunhofer.
‘A lot of interfaces need to be addressed when you're developing a large part, but it worked out well because we were able to establish direct communication with all the other stakeholders and also with Airbus who helped resolve any sticking points. From the organisational point of view the project was very efficient.’
Reinforcing the fuselage side-shells with large sandwich cores increases the stiffness. However it's difficult to drape the fibre material in these complex geometries onto the layup mould, which is why, up until now, these types of complex panels are mostly carried out using hand layup operations. ‘But now,’ says Zenker, ‘we are able to optimise the automatic fibre placement process and prove that it's also applicable to such tough geometries.’
Aligning with the 'Cost Efficient' aspect – the 'CE' of RACER – Zenker notes that the AFP process brings economic advantages when applied in a serial production context: ‘Obviously, there's dependency on the rate that you want to produce, and for very lower rates, hand layup is more economically efficient. But our calculations showed that even for as little as 12 ship-sets per year, there's a break-even point between hand layup and AFP.’
If part numbers are increased, AFP shows significant cost benefits. For example, 65 sets per year would bring 20% cost savings.
Commenting on the AFPMet outcomes, Clean Sky's Shroff confirms that ‘the results from AFPMet were very good. They provided the AFP moulds and the layup is actually done – the side shells are ready. So it's really exploited. It's a very successful project where the tools have been used to produce the parts that have gone into integration on the RACER, so that's very good news.’
RoRCRAFT takes centre stage
The ongoing RoRCRAFT project will produce RACER’s hybrid front and central fuselage structure bringing a weight reduction of 15% that contributes to 20%-30% reduction in CO2 emissions, 20%-40% reduction in NOx, and 5% reduction in perceived noise in line with Clean Sky’s objectives. This will be achieved using a combination of advanced design and new manufacturing technologies including composite materials, AFP technology and additive manufacturing.
RoRCRAFT has passed its CDR and its end of design for manufacturability (DFM) milestone for the main fuselage. Outstanding steps are the manufacture of the remaining small subassemblies, completion of the subfloor group, front/engine deck, and main landing gear section, leading to the final RACER assembly and inspection in the main jig.
Once that is accomplished the completed airframe will then be transferred to Donauwörth for the start of the final RACER assembly. Currently all the large size composite panels are manufactured and declared flightworthy.
‘Now that the critical design review is completed it means that all the data for manufacturing is fixed and the choices for the hybrid materials that they're going to use in the fuselage panels are finalised. RoRCRAFT is on time for integration, and the RACER assembly has already begun, and in 2022 they'll be testing it to TRL6,’ says Clean Sky's Shroff.
Dr. Catalin Nae, President & CEO of INCAS, Romania’s National Institute for Aerospace Research, says that the final assembly of the RACER and the countdown to its flight test programme is part of a bigger story.
‘This is a project where we really want to show the hardware to the public, and we also want to show what's behind RACER, because there are many areas of R&D which are embedded in what is considered to be the future rotorcraft,’ says Nae.
Commenting on the multi-stakeholder set up of the RoRCRAFT and the wider RACER project, he says it is ‘very important to show that we can work together, because the more you go to the industrial world, there is a kind of a community which is not that open. And it's not easy for small entities or even bigger entities to be part of this type of process, so we were very happy that as part of this Clean Sky project, we had this opportunity and possibility to show it is possible.’
Moreover he says that collaboration throughout the RACER project with multiple players has been very important, ‘because otherwise it's very difficult nowadays in this competitive market. Sticking with the old traditional way of doing things is no longer possible.’
‘It's a very, integrated process. And that is one of the major benefits of having the Clean Sky Joint Undertaking. It was a huge opportunity for all of us involved in research, having these types of demonstrators as part of our programmes.’