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No Fly Zone: CHOPIN and STELLAR debug the cutting edge

Large scale tests of the coatings on a full size two-seater aircraft planned in STELLAR
Large scale tests of the coatings on a full size two-seater aircraft planned in STELLAR

Reducing drag on aircraft is one of the key ways to improve aerodynamic performance, reduce fuel burn, and cut CO2 emissions, so it’s a vital step on the road towards fulfilling the ACARE Flightpath 2050 targets, in line with the European Green Deal.

Clean Sky is developing hybrid laminar flow control (HLFC) and natural laminar flow (NLF) technologies to reduce drag on the next generation of passenger aircraft. But to ensure an efficient flow of air around the wings and fuselage, external surfaces must be scrupulously clean at all times. Flying through the air, the leading edges of aircraft accumulate a build-up of insect residue which can interfere with the intended aerodynamic flow.

HLFC and NLF are predicted to be able to produce meaningful fuel savings – in the region of 5% to 10% – so there's potentially a lot at stake. That's where Clean Sky's CHOPIN and STELLAR projects come in – working on specialised coatings to address this issue, part-funded by European Commission’s Horizon 2020 programme.

CHOPIN is focusing on the development of highly durable hydrophobic coatings that can be applied to micro-perforated surfaces used for drag reduction on HLFC aircraft (see Clean Sky's HYPERDRILL project). And follow-up project STELLAR builds on the learnings of CHOPIN, including a deeper understanding of insect residue properties, and concentrates on developing efficient and durable anti-contamination coatings and cleaning solutions applicable to NLF aircraft.

The coatings, explains Clean Sky project officer Jimmy Tchen, will be ‘an enabling technology to ensure surface quality of the aircraft leading edges, preventing any contamination that could disturb the aerodynamic flow, which would be detrimental to laminarity.’

Both projects are coordinated by coatings development specialist Materia Nova, based in Mons, Belgium, an entity which Tchen describes as being ‘at the frontier between chemistry and material development.’

In HLFC technology, micro-drilled holes in the aircraft's skin suck the air in so that it 'sticks' closely to the aircraft for a fuel-efficient flow. But this typically presents a practical problem, says Tchen: ‘Insects can adhere to the surface and can clog the holes.’

Flight testing of the coatings on drones planned in CHOPIN
Flight testing of the coatings on drones planned in CHOPIN

Orchestrating the players

To take on the challenge, the CHOPIN consortium brings together expertise from research centres across Europe, to complement Materia Nova’s work on the coatings. These include Cidetec Surface Engineering from San Sebastian, Spain; Norce of Norway; and Berthier Etudes, from France. The Von Karman Institute for Fluid Dynamics (VKI) in Belgium is providing facilities for wind-tunnel testing.

The objective is to develop durable hydrophobic/omniphobic coatings that mitigate or prevent the holes from clogging to keep the efficiency of the HLFC leading edges. Though there's another challenge too:

‘In CHOPIN you not only have the problem of the plugging of the holes, but also, the build-up of insect residue creates roughness on the surface of the aircraft skin, and this is also a problem,’ says Mireille Poelman R&D Program Manager, Materia Nova.

The coatings have to repel the insects as much as possible mainly by having effective anti-adhesive properties against the insect and through having self-cleaning potential.

‘The coatings have to repel the insects as much as possible mainly by having effective anti-adhesive properties against the insect and through having self-cleaning potential,’ 
adds Poelman, who explains that to achieve self-cleaning involves harnessing nature:

‘The self-cleaning works mainly thanks to water or ice crystals during flight operations so that you will have complete removal of the residues of the insects. However, rain and ice crystals impact very aggressively on any type of materials, so they have to also resist the harsh weather.’

Therefore, the idea is for the coating to be as durable as possible. Poelman points out that ‘it's also important to consider technologies that will be reparable, and in some cases you have to remove the slat from the wing completely and reapply the coating.’ 

Ionogel coatings developed in CHOPIN @CIDETEC
Ionogel coatings developed in CHOPIN @CIDETEC

Variations on a theme

A variety of coatings have been devised, taking into consideration such factors as the adherence to the substrates (thin titanium foil in the case of micro-drilled leading edges), hardness, flexibility, UV resistance, rain and sand erosion resistance, and resistance to aircrafts liquids (such as hydraulic and de-icing fluids).

The coatings explored in CHOPIN include ‘ion implantation’ and ‘Sol-Gel’ developed by Materia Nova, ‘anti-stick spray’ developed by Berthier and ‘ionogels’ by Cidetec.

From topic manager Airbus’s perspective, surface technology specialist Dr. Silke Grünke of the Airframe Research & Technology – Surface Technology department says that whilst the airframer has been looking at anti-contamination coatings since the inception of Clean Sky in 2008, the key issue still remains the long-term durability of such coatings.

On aircraft there are really harsh environmental conditions – UV radiation, high speed, and the impact of rain or sand.

‘On aircraft there are really harsh environmental conditions – UV radiation, high speed, and the impact of rain or sand. These anti-contamination coatings are of special interest for the leading edges which are mostly prone to rain and sand erosions. We also have to look at the special adhesion properties between the contaminants produced by the insects and the aircraft surfaces,’ says Grünke, “and that's why it's such a big challenge to develop such coatings.’

She adds that in CHOPIN, coatings have been tested for their contamination behaviour and cleanability, durability and erosion using simple lab tests, and are now in the closing stages of wind tunnel tests, carried out at VKI, which include simulation of insect impacts.

‘These wind-tunnel tests are now almost finished,’ says Grünke, ‘and what we see with these tests is that it's probably not realistic to have a coating which has absolutely no contamination. There will always be some insect residue, however we can have less residues and easier and improved cleanability so that the coating is cleaned during the flight – but we're still in the development process.”

The final work package in the CHOPIN project will be the flight testing of the coatings on drones, scheduled for later in 2021. This will enable correlation of the wind tunnel test results with realistic flight tests. 

But that window of opportunity is slim. Airbus R&T Project Leader for Laminar Flow Technologies Adrian Eberle says “there are limited time slots dictated by nature -- the plan is to carry these out in Norway, during the summer.”

The Norwegian summer skies offer high concentrations of insects – thereby presenting a challenging environment for the flight tests.

‘There are different types of insects – bigger ones with higher masses and smaller ones. And insects fly at different altitudes,’ says Eberle. ‘This makes the coating, and the specification of what is required on these coatings, quite complex – all these different environments need to be considered.’

Insect impacts on a leading edge @VKI
Insect impacts on a leading edge @VKI

STELLAR takes it to a higher pitch

While CHOPIN heads towards its concluding work package, Clean Sky’s STELLAR project is already underway, building on the achievements of CHOPIN with further development of efficient and durable anti-contamination coatings and cleaning solutions to tackle the build-up of insect residues. But this time the focus is on NLF aerodynamics.

The knowledge acquired and the coating and cleaning solutions developed will be evaluated through large scale tests. As in CHOPIN, wind-tunnel tests will allow the simulation of weather conditions and insect impacts on coatings. However, instead of using drones, short flight tests (on full-size piloted test aircraft of SONAIR) and long flight tests (on commercial aircraft) will be carried out. This will allow analysis of how the coatings perform at higher altitudes for a fuller validation of the newly developed coating solutions.

But what do the potential benefits look like?

‘Out of the 5 to 10% of fuel-burn benefit brought by laminarity overall, a maximum of 1 to 2% could be achieved by using these innovative coatings.’ says Tom Gibson, Technical Lead for Component Aerodynamics, Airbus.

However, that figure needs to be qualified. When flying an aircraft in a very contaminated environment, says Gibson, ‘you may lose a large proportion of your laminarity and that’s when your coating will bring you its maximum benefit. Conversely, on a day when there are no insects around, your coating will give you nothing.’

The proof will be in another couple of years when the STELLAR flight tests are carried out. But in the meantime, stepping back from the project and looking at the broader context, Clean Sky’s Jimmy Tchen notes that Clean Sky is already ‘looking at drawing synergies across projects on specific topics.’ 

This, he says, has been facilitated by means of recent workshops specifically on coatings that were organised by Airbus, with the objective of convening all the actors in the wider Clean Sky community that have an active interest in coatings, including the CHOPIN and STELLAR projects.