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The cross-functional enabling tech supporting Clean Sky demonstrators

Criss-crossing Clean Sky’s Large Passenger Aircraft programme is the need for optimal aircraft structures that improve efficiency as much as possible.

ADD-ON damage or delamination sensor through a matrix of sol-gel optical waveguides @ UJM Maxime Royon
ADD-ON damage or delamination sensor through a matrix of sol-gel optical waveguides @ UJM Maxime Royon

The 'Non-specific cross functions' project focuses on innovations that can further this goal – such as novel multi-functional materials, better automation of industrial processes, improved quality control and inspection systems including non-destructive testing, CO2 reduction through waste reduction, the introduction of new damage detection technologies, and the use of virtual tools and testing to reduce waste during the manufacture of aircraft.

Funded under the European Commission’s Horizon 2020 programme, Clean Sky project officer Jimmy Tchen describes this initiative as the creation of ‘a portfolio of different enabling technologies that support the more integrated demonstrators.’

The development and deployment of these technologies is intended to reduce recurring costs by 20% and cut manufacturing lead times also by a fifth, contributing to the overall Clean Sky Platform 2 Demonstrator objectives, in order to enhance European aeronautics sector competitiveness in line with the European Green Deal.

From topic manager Airbus's perspective, the value and purpose of the non-specific cross-functions project is to bridge technology gaps using innovative solutions to bring efficiencies to European aviation by leveraging the expertise and ingenuity that exists within the Clean Sky 2 ecosystem.

‘What airlines demand today is that we look to optimise the design of aircraft structures to reduce their maintenance operations as much as possible,’ explains Marie-Anne De Smet, Leader of Airframe Structural Health Management at Airbus.

‘We're looking to deliver aircraft that will have less downtime. Therefore we need to give operators – not just airlines but also maintenance, repair and overhaul shops – strategic solutions for structural health monitoring, using sensing technologies to detect or even anticipate damage.’ 

What airlines demand today is that we look to optimise the design of aircraft structures to reduce their maintenance operations as much as possible

Advanced Damage Detection through Optical Sensor Network

A good example of that is the ADD-ON (Advanced Damage Detection through Optical sensor Network) project. The overall objective of the ADD-ON project is to investigate the feasibility of developing a unique Optical Sensor Network (OSN) to control various classes of distributed and point-based optical sensors. 

Coordinated by Université Jean Monnet Saint-Etienne, the ADD-ON project has three specific objectives; the first is to monitor aircraft health in real-time (during flight as well as during ground-based operations) using a sol-gel matrix that can be deployed on large aircraft surfaces. 

The second is to optimise a loading measurement chain based on a technology called fibre Bragg grating (FBG), using an integrated photonics system all-at-chip level adapted to the aircraft avionics needs. 

‘One of the advantages of optical sensors is that by using just a limited number of interrogators, it's possible to have a large quantity of sensing points, which means using less cable, and therefore introducing less weight on the aircraft,’ says the coordinator of the project Sylvain Girard, professor at the Université Jean Monnet Saint-Etienne and leader of the Materials for Optics and Photonics in Extreme Radiation Environments team at the Laboratoire Hubert Curien.

‘We use the sensitivity of the optical material to monitor, in real time, any damage that you might have on the aircraft, such as delamination, corrosion, or cracking – it's a reliable and less invasive way to obtain the measurements you need to control the integrity and structural health of the aircraft,’ says Girard. 

This contrasts with the conventional methods used for assessing the integrity of materials, according to Thijs van Leest, Team Lead Research & Innovation Manager at PhotonFirst, a Netherlands-based developer and provider of state-of-the-art integrated photonic sensing solutions, who says that ‘traditionally, the testing of materials is carried out in a laboratory setting. What we're doing is using integrated photonics to put optical functionalities for the interrogation onto a chip, making the solution fit for the demanding environment of an aircraft.’ 

The implication is that damage detection capabilities that today can only be accessed in a laboratory environment could soon, through the use of integrated photonics technology, be embedded within the onboard aircraft material and monitored in real time.

The third objective is to investigate the feasibility of self-diagnosis for the OSN. During the ADD-ON project the potential of distributed sensing techniques will be evaluated, the best solution will be selected and then it will be tested on a small-scale prototype panel using in-lab validation.

Two years into the project, the consortium reports that the sensing scheme options and applications have been explored and some of the principles demonstrated, including assessment of samples of the sol-gel with optical waveguide structures. In the final phase of the project the technology will be applied on representative panel sections for further testing and evaluation closer to the envisioned aircraft applications.

Traditionally, the testing of materials is carried out in a laboratory setting. What we're doing is using integrated photonics to put optical functionalities for the interrogation onto a chip, making the solution fit for the demanding environment of an aircraft

Material gain

Complementary to the damage detection activities of ADD-ON, another cornerstone of the non-specific cross functions project is research into the inherent properties of new materials that have the potential to bring performance efficiencies to the next generation of aircraft.

For example, at French aerospace lab ONERA – a Clean Sky core partner – research engineer Frédéric Laurin is working on the advanced material characterisation of new composite materials which offer many performance, logistical and environmental benefits over today's more traditional thermoset-based composites.

‘Airbus would like to use composite materials with a thermoplastic matrix for the fuselage of future medium range aircraft, and also potentially for other parts,’ explains Laurin.

‘The idea is to have a deep understanding of how this material becomes damaged and fails. So we are performing tests in the laboratory on small coupons and are using several measurement techniques to observe and develop methods to automatically detect and count the number of cracks, and then compare the crack pattern with the previous generation of materials.

‘Therefore, we perform tension tests, compression tests, shear tests etc – the challenge is to propose a mathematical model which can be used for composite structures and to be integrated into a finite element method code that is applicable at a composite structure scale. We have to propose a model which is complex enough to describe what we are seeing, but simple enough to be used in a design office.’

Once that model can be defined it will enable European aviation to take advantage of the environmental benefits of the new generation of composite materials. For example, they can be more easily recycled and, importantly, can be welded. This opens enormous design possibilities and also simplifies the reparability processes, bringing operational benefits for aircraft maintenance.

Laurin reports that the collaboration with Airbus (and with Aernnova for this particular project as they are carrying out related studies), ‘is very fruitful for everyone involved.’

‘I hope it will be just the beginning of the story,’ he says.

Demonstration of the manufacturing of a sol-gel waveguide grid on a large surface (40x40cm²) @ UJM Maxime Royon
Demonstration of the manufacturing of a sol-gel waveguide grid on a large surface (40x40cm²) @ UJM Maxime Royon

Linking up with the Airframe ITD Demonstrator

The development of new composites at ONERA, and the advancement of innovative structural health monitoring technologies through ADD-ON, are emblematic of the transverse nature of the non-specific cross functions project.

‘ADD-ON is an example of a project that was launched through ITD Airframe, but that would also be of interest for the other partners who are on the ITD Airframe, and of interest for Airbus on the LPA,’ said Marie-Anne De Smet. 

She concludes that, ‘Globally speaking, the huge benefit from the Clean Sky 2 research framework is very beneficial for Airbus, because thanks to the support from the European community we have a large range of partners who work with us, helping us to progress quicker on innovation. And innovation is one key element that helps Airbus simplify the design of new aircraft as well as helping simplify their development, internally at Airbus. We have that frame of activity in LPA platform 2 and in the non-specific cross function project.’