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Cool customer: HEFESTO keeps the heat at bay

The weight-saving shift towards the use of composite materials instead of metals for aircraft structures – helping reduce fuel-burn and minimise environmental impact, in line with the ACARE 2050 targets and the European Green Deal – is at least as relevant for helicopters as it is for fixed-wing aircraft. It's a trend exemplified by the next generation of helicopters such as the Airbus H160, the first civil rotorcraft to have a composite fuselage.

Full scale test specimen as manufactured
Full scale test specimen as manufactured

However, there's one major part of the helicopter structure that still remains made of metal, even in state-of-the-art composite-bodied helicopters, and that's the engine deck/firewall. This is typically located above the helicopter's crew and passenger cabin and plays a dual role: it supports the engine, and, in the exceptionally unlikely event of engine overheating or a fire incident, the firewall has to maintain its structural integrity and provide heat shielding to the passengers and crew. It has to do so for long enough to enable the helicopter to be safely landed and evacuated, and that's the historical reason why metal has been the 'go to' material for the engine deck/firewall. 

But there's a lighter weight option on the horizon. Clean Sky's HEFESTO (Helicopter Engine Deck – Multifunctional layered insulation for carbon fibre reinforced plastic (CFRP) fire and thermal protection) project, funded by the European Commission’s Horizon 2020 programme and completed in June 2020, focused on the design of a helicopter engine deck/firewall using composite materials that aims to cut weight by 10% and improve fatigue resistance compared to metal structures, while assuring the required thermal isolation and fireproof capabilities.

Complex design criteria

This combination of highly demanding design specifications made the project exceptionally challenging, explains Sonell Shroff, project officer at Clean Sky:

‘It's extremely difficult, because carbon fibre by itself is not the most desirable material that you would use for an engine deck/firewall, so the HEFESTO consortium had to devise a specific coating for it to ensure that it would still be structurally stable and lightweight while fulfilling the stringent EASA CS-29 airworthiness certification specifications.’

The Airbus RACER was used as a model on which to base the engine deck/firewall design, though the HEFESTO solution, starting much later than the RACER Demonstrator programme, will not be integrated with the RACER itself.

‘A primary objective of the HEFESTO project,’ says Shroff, ‘was to come up with and demonstrate a novel all-in-one multilayer coating to not only thermally isolate and fireproof the CFRP material to cope with the operating conditions and fire hazards surrounding the helicopter engine, but the coating also had to have active fire-fighting characteristics.’  

The resulting structure also had to be tested under constant temperature conditions representative of the motor area to demonstrate that the coating could thermally isolate the CFRP at temperatures which would allow the underlying composite material to maintain its mechanical properties.

And to make the project even more challenging, Shroff notes that ‘the most appropriate configuration and application method for the multilayer coating had to be identified so as to make it practical, light, cheap and efficient.’

Lab stage for preliminary fire tests at AIN
Lab stage for preliminary fire tests at AIN

Material gain

To address this complex set of criteria, consortium partner SOGECLAIR, based in Madrid, was provided with the baseline design of the RACER, and from this they derived a composite-based design of the engine deck/firewall. The other partner in (and coordinator of) the project, Asociación de la Industria Navarra (AIN), set about devising the formulation of the coating.

‘The strategy,’ says AIN's Dr. Gonzalo García Fuentes, ‘was to choose materials that were already certified by the aerospace industry,’ rather than using ‘exotic formulations.’ The idea being that apart from saving development time and costs, the use of more readily available materials would make the developed solution more exploitable in an industrial context. 

‘Plastics are much more sensitive to temperature than metals. Even 150-200°C are enough to significantly degrade a plastic,’  says Fuentes, ‘so we studied material coatings that could work in two ways: firstly to act as a fire barrier, hindering the fire from reaching the composite substructure material and then the cabin. And secondly, to preserve some level of thermal isolation so that the heat produced by a fire does not affect the mechanical properties of the CFRP itself.’

The plan was to use novel materials in a special configuration so that in case of a fire event, the coating covering the CFRP material would be able to withstand direct exposure to fire for a duration of 15 minutes. It should also withstand a heat flux of 116 kilowatts per square metre, to protect the underlying CFRP from exceeding temperatures higher than 130°C.  

‘We used combinations of already certified materials based on silicon oxides, magnesium oxide, and alkaline earth silicates,’ Fuentes explains, ‘these materials are safe from the point of view of human health as they don't contain asbestos or anything like that.’ 

AIN investigated numerous combinations of these with other materials that have high fire resistance, and with others that have high thermal isolation properties, to achieve the optimally balanced formula.

‘The redesigned structure led to a 20% weight reduction, while the consortium was also successful in demonstrating suppression of vibrations imposed by the specified standards,’ says Shroff.  

The coating solutions managed, in demonstration, to preserve the CFRP at 120°C (comfortably below the below the 130°C target) when tested against a 1100°C and 116 kW/m2 kerosene flame as set out by the CS-29 aviation standard, for the required duration of 15 minutes.

Further, the consortium identified three concrete exploitable results – a design method to challenge metal-based engine deck and fire wall configurations with CFRP redesign; a fireproof/thermal protection concept for retrofitting metal structure in any type of aircraft; and a testing device compliant with ISO 2685 to monitor interface temperatures of coupons as a pre-screening to CS-29 tests. The overall concept developed under HEFESTO is currently under patentability assessment.

Full scale test specimen positioned in fire test stage, front view, just before test
Full scale test specimen positioned in fire test stage, front view, just before test

The importance of Clean Sky demonstrators

‘As well as the sustainability and environmental benefits, the exploitation of each development is of great importance to us, at a Clean Sky demonstrator level and for all our partners carrying out the technical work,’ says Shroff.

Besides the feasibility demonstration of the technical solution developed by the project, Shroff reports that a significant gain in knowledge about helicopter engine deck fire testing and interaction of test parameters with different kinds of thermal/fire protection solutions was achieved. 

‘This is of great benefit for future demonstrator or technology developments to put the right solution in the right place, especially with regard to current and future helicopter certification requirements.’

That's a view shared by topic leader Airbus's Manuel Kempf who says that ‘it was good to see that a CFRP engine deck would be a feasible option. We clearly identified some weight saving potential, which on one hand will increase performance, and on the other hand, reduces fuel burn and is good for the environment.’

Of course, with a one and a half year research project like HEFESTO, it's not possible to resolve all the issues that are needed for serial application, such as all the supply chain, all the manufacturing issues, all the maintenance and in-service requirements, and repair. These aspects are still open. But, says Kempf, ‘now we know it basically works, and we'll continue working on that to increase the maturity of the technology.’ 

‘From a technical point of view, what was proven in HEFESTO is that it really makes sense to go towards this composite engine deck, and although this won’t fly on the RACER, hopefully it will be on one of the next helicopter programmes or demonstrators,’ he said. 
 
Beyond its use in aeronautics, the HEFESTO project has produced heat-shielding and fireproof coatings for composites that have exploitation paths in other sectors, such as in trains, road vehicles and in the gondolas of wind turbines.

Back side of the full scale test specimen prior to ISO2685 fire tests. Thermocouples positioned
Back side of the full scale test specimen prior to ISO2685 fire tests. Thermocouples positioned