Lightning reaction: Effective joining methods for SAT
Designing aircraft that can withstand lightning strikes is just part and parcel of the work of aviation manufacturers. On average, a commercial aircraft is struck by lightning about once every year, and in many cases the passengers won't even notice if it happens. That's thanks to the way in which aircraft are designed to channel the electricity towards the extremities of the aircraft (often to the trailing edges through static discharge wicks).
In the past, with aircraft having metal skins, this was a fairly straightforward procedure, as aluminium alloys are good conductors of electricity. But with the increasing use of weight-saving composites, metallic meshes have to be embedded in the composite panels to allow the electrical charge to safely transition between adjoining external surfaces of the aircraft.
Clean Sky's Effective Joining Methods for SAT project involves devising and testing ways to connect up adjacent skin panels in smaller aircraft, supported through two interrelated Clean Sky projects funded under the EU’s Horizon 2020 programme; C-JOINTS, which concluded last year, and D-JOINTS, which is ongoing.
‘The aim of the two projects,’ explains Clean Sky project officer Sonia de la Cierva, ‘is to explore novel joining technologies with the ultimate goal of eliminating the need for metallic parts in the composites, to simplify manufacturing processes, reduce weight and maintenance costs, along with bringing improvement of operational safety related to lightning strike effects.’
Czech aircraft manufacturer Evektor is leading the overall project while TWI Limited is the coordinator of both C-JOINTS and D-JOINTS. Studies are modelled around the composite nose part (CNP) of the Evektor EV-55, a 9-seater commuter/utility aircraft, which is being used as a test-bed to assess the feasibility and benefits of the developed technologies at aircraft level.
‘The C-JOINTS project started with the selection of the most promising innovations from a number of technologies initially proposed. Initial screening work on laser riveting, thermal spraying and alternative manufacturing methodologies generated useful information to further the project goals,’ says Mihalis Kazilas, Business Group Manager at TWI.
Two innovative technologies were selected for further investigation: tufting and thermal spray coating. Tufting is a type of industrial stitching process, applied using robotics, whereby a thin copper thread is robotically sewn through a composite panel to facilitate electrical conductivity.
‘Tufting is a process which is quite well known though it hasn't been used before for lightning strike protection, but the automation part of the process – the robots and the programming – were already there,’ says Kazilas. ‘It was developed for the Formula One industry initially and is now finding uses in civil aviation and other applications.’
Tufting's main advantage lies in its capability for high current dissipation and thus better operational safety. The use of metallic tufting through composite components is a lower-weight alternative to embedded copper straps for transferring lightning currents through composite materials as part of the grounding path.
The other investigated process, thermal spraying coating, is a process developed at TWI where metal is sprayed onto composites to provide metallic connections that can carry energy across the composite panel. ‘It's quite a complicated process,’ says Kazilas, ‘however it's simpler than adding copper wires and then going through a dedicated bolting exercise which is what happens now.’
The coating process also makes it possible to vary the thickness of the conductive material for the critical path of high currents and effective protection of the composite structure. This contrasts favourably with today's embedded copper mesh systems, which, when damaged by lightning, are difficult and costly to repair, whereas coating repairing is fast and easy.
However, TWI says that using thermally-sprayed metallic coatings to replace the copper mesh and stripe diverters for lightning strike protection (LSP) within the composite component on the EV-55 could also be a costly technology, as robotics and special devices for spraying are needed. On the upside, spraying reduces manufacturing complexity and offers easier maintenance and lighter components through the reduced metal content for equivalent performance.
The investigated technologies may not be cheaper compared to the current CNP manufacturing process but they will certainly be better with respect to the quality of manufactured parts and their joints, enabling a faster sizing process and a high level of protection of unwanted electromagnetic interference effects (EMC).
Commenting on the results to date, Jan Kozak, Clean Sky 2 ITD Airframe Topic Leader at Evektor says that in C-JOINTS, investigation at coupon level showed potential improvement in the operational safety of composite nose part (CNP) regarding lightning strike protection (LSP) and high-intensity radiation field (HIRF) issues: ‘Coupons were exposed to mechanical and electrical tests to investigate ultimate load capacity of mechanical loading, electric current transmission capability, or impact of high current on the strength of joints. Comparison between reference and innovative coupons showed promising potential for lightning strike protection improvement, and it is assumed that there will be benefits regarding HIRF behaviour. That will be welcome due to placement of HIRF sensitive devices inside the aircraft nose cone.’
The benefits of the C-JOINTS project will be confirmed more accurately within the D-JOINTS project, where, says Kozak, ‘there is a plan to verify shielding effectiveness of thermal spraying (HIRF improvement is expected), and high current protection will be proven on the whole CNP demonstrator by execution of lightning tests.’
Building on the learnings and expertise accrued through the C-JOINTS project, D-JOINTS – which stands for ‘Design of innovative composite hybrid joints with electromagnetic compatibility’ – aims to explore new innovative composite-to-composite and hybrid composite-to-metal joints with enhanced lightning strike protection.
The project, which kicked off in May 2020 and is supported by the Brunel Composites Centre and Cranfield University, will culminate with the development of a new 'sizing tool,' a software programme based on the simulation of the mechanical, thermal and electrical responses of multi-material components involving joints. The sizing tool will help decrease the number of iterative time-consuming finite element metals (FEM) calculations, typically used by analysts and stress engineers.
‘Through the sizing tool the user would have the opportunity to input specific parameters which are important during the early design phase of a joint,’ explains Sofia Sampethai, TWI’s Adhesives, Composites and Sealants Senior Project Leader.
‘Based on these parameters, the tool would be able to give an output to enable designers to define the appropriate material to be chosen to create a specific joint that we would like to have for parts of the airplane, keeping in mind the size of the joint. This is a software programme that enables designers to identify the appropriate type of materials that could be used or combined together,’ reports Sampethai.
As with C-JOINTS, the approach will be developed and demonstrated on the reference and innovative composite nose assembly of the Evektor EV-55. The design of the nose parts and of the individual joints will be achieved through numerical calculations processed using the sizing tool.
From the Clean Sky perspective, project officer de la Cierva highlights the valuable contribution that both projects are providing in paving the way towards lighter composite-based aeroplanes with efficient lightning strike protections. The developed joints methods will ultimately be integrated into the Clean Sky SAT ‘Safe and Comfortable Cabin Demonstrator’, and ground tested in 2022.