Clean Sky 2 OUTCOME project demonstrates PEEK thermoplastic wingbox cover

• OUTCOME has tested and validated the successful use of PEEK thermoplastic in a wingbox – the project represents a boost for energy efficiency in European manufacturing
• Use of PEEK thermoplastic material, instead of traditional thermoset, simplifies processing while delivering weight savings which lead to lighter aircraft and reduced fuel-burn
• The produced wingbox cover can be recycled at the end of its service life, saving valuable resources and demonstrating European commitment to the circular economy

Clean Sky 2’s OUTCOME project has devised a sustainable manufacturing process for producing a 4m x 1m thermoplastic-stiffened upper skin for the external wingbox (wingbox cover) of a regional/utility (Airbus C-295) aircraft. The project feeds into Clean Sky 2’s Airframe ITD D1-4/6 demonstrator, an advanced composite wingbox (the central structure of an aircraft that connects the wings with the main body/fuselage). 

OUTCOME’s main premise was to switch from using thermosetting plastic⁽¹⁾, which requires processing in an autoclave⁽²⁾ (a high-energy consuming type of industrial oven) to the use of thermoplastics, using an energy-efficient ‘out-of-autoclave’ ‘one-shot process’. This is a production method where sequential production steps are integrated into a unified process involving ‘lamination and consolidation’, a method of combining different parts (in this case the wingbox cover ‘skin’ and reinforcing stiffening structures called stringers⁽³⁾. This saves time and energy resources – an environmental win. The project aligns with the aeronautics sector’s shift to thermoplastics which are lighter than their metal counterparts.

Recyclability potential

Another key environmental driver of OUTCOME was that “Thermoplastic materials are reversible, and manufactured parts can be reconsolidated, thus reducing the number of non-quality parts that could not be used in a real demonstrator,” explains Mar Zuazo Ruíz, R&D composites specialist at FIDAMC, Spain’s Centre of Excellence for research, development and innovation in composite material technologies, and project coordinator of OUTCOME. This ‘reversibility’ means that at the end of the component’s operational service life, the material can be recycled for use in future products.

An additional advantage thermoplastics offer is their environmental value in terms of reducing potential contamination. Rubén Tejarina Hernanz, Airframe R&T Manager for Integration at Airbus Defense and Space (ADS), the Topic Manager for OUTCOME, explains the background: ADS has been involved in this technology since around 2010 when we saw that thermoplastics could be an alternative to the thermosets for use on primary and secondary structures. We saw their sustainability potential in addressing not just recyclability of materials but to also help us meet EU REACH⁽⁴⁾ contamination regulations.

Around 2007 FIDAMC had also initiated projects using thermoplastics. These mutual interests converged through Clean Sky activities which Airbus, as Airframe ITD demo co-project leader, proposed, launching a call for core partners to explore the potential of thermoplastic technologies. At that point, FIDAMC, along with aerostructures design and manufacturing specialists Aernnova, formed the OUTCOME consortium.

A key sustainability enabler underpinning the project was the consortium’s decision to use a thermoplastic called Polyetheretherketone (PEEK) – a material which matches the performance properties of thermosets, even surpassing them in terms of ageing and damage tolerance, while bringing valuable weight savings. PEEK made it possible to reinforce the wingbox cover using stringers without the need for fasteners (bolts or rivets) which would have added weight and complexity.

A successful outcome

Clean Aviation Project Officer Dr. Sonell Shroff notes that “The consortium succeeded in reaching its objectives in a single trial, confirming predictions regarding spring-back⁽⁵⁾, and was successful in anticipating potential issues with the co-curing process. This could have been tricky, as when you attach the stringers you can have a lot of tension and difference in stresses in the two parts.” She emphasises that, 

“The project represents a substantial achievement in terms of replacing thermoset with thermoplastic, bringing energy savings, minimum wastage and scrapping. The consortium was spot on with their analysis and coupon tests – a lot of effort has been put into this whole process.” said Dr. Sonell Shroff, Project officer at Clean Aviation

The project results feed into Clean Sky 2’s Eco Transverse Activities (TA), working towards understanding the ecological benefits across the full lifecycle of components. ADS’s Rubén adds that, “It’s uncommon to carry out R&T projects at full scale component testing, as it is planned to do within AIRFRAME ITD, but on OUTCOME the component was designed and manufactured using the same approach as if it were to become a certified prototype. We performed the same tests that apply when submitting evidence to the airworthiness authorities and the resulting technology is applicable on horizontal or vertical tailplane structures. This could be beneficial for the future of European aviation.”

An EcoStatement study, documenting the assessment of the full wingbox is being compiled by Airframe ITD and processed by ECO-TA, with results expected at the end of 2023. In the meantime, OUTCOME technology is already being applied in Clean Aviation’s HERWINGT and FASTER-H2 projects.

OUTCOME ran between January 2016 and December 2023 with the ‘wingbox cover’ manufacturing occurring between June 2022 and January 2023. The project budget of €600 000 was provided by the EU. OUTCOME was coordinated by FIDAMC, and supported by Aernnova. Airbus Defense and Space was the Topic Manager.

References:
(1) Thermosetting plastics are strong; however, they require high temperatures and use costly energy process. Their potential for recycling is poor.
(2) An autoclave is a type of industrial oven which ‘bakes’ composite materials into a stable and permanent state. An ‘out-of-autoclave’ process stabilises the material without the need for an autoclave, thereby saving energy, time and resources – it is a more environmentally friendly process.
(3) Stringers are long and thin reinforcing structures that add rigidity to the skin.
(4) REACH is an EU regulation which improves the protection of human health and the environment from the risks that can be posed by chemicals.
(5) Spring-back is an unwanted condition whereby a composite/plastic part becomes distorted from its intended shape during the curing process after it is released from the tool.