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The NextGenCTR unfurls its wings

Clean Sky's NextGen Civil Tiltrotor (NextGenCTR) technology demonstrator programme, led by Leonardo Helicopters, progressed significantly in 2020, paving the way towards European competitiveness and sustainability in the niche sector of short/medium-range VTOL-capability air transport.

Wing main tool (INVAR material) thermal test
Wing main tool (INVAR material) thermal test

The tiltrotor aims to trim CO2 emissions by half, lower NOx by 14%, and cut noise by 30% relative to intermediate class helicopters in line with Clean Sky’s objectives and the European Green Deal. It passed an important milestone in December 2020 – its critical design review (CDR). This means that after six years of ever-improving design reiterations the overall NextGenCTR configuration is now frozen, enabling full advancement of the manufacturing phase.

What differentiates tiltrotors from other types of aircraft is the ability of their wing-mounted rotors to tilt from a horizontal to vertical position, enabling vertical take-off and landing – an invaluable asset for intra-European regional and urban connectivity, aligning with the Flightpath 2050 aspiration of door-to-door mobility anywhere in the EU within four hours.

Bringing the NextGenCTR through the CDR threshold has meant developing an ingenious wing design which has evolved considerably from earlier design configurations. 

Clean Sky project officer Antonello Marino relates that early on in the overall NextGenCTR project, ‘Leonardo and CIRA investigated alternative solutions whereby the inner part of the wing was fixed to the fuselage, with the external part of the wing tilting together with the engine. This layout has evolved in favour of an optimal configuration where the front part of transmission tilts while the rear part is fixed with the engine and wing.’

‘The wing is very complex, because one side attaches to the engine nacelles and the other side integrates complex features and systems that enable the operation of the tilting system,’ says Marino. 

Additionally, he adds, rather than having auxiliary fuel tanks in the fuselage, ‘all the fuel tanks are in the wings. Therefore the wing is designed to accommodate the fuel tank and associated fuel distribution system with cables, sensors and control unit. These activities are implemented through collaboration between Leonardo Helicopters and the T-Wing consortium with two complementary Clean Sky grant agreements, DEFENDER and NGCTR-DIGIFuel.’

Fuel tank for installation test
Fuel tank for installation test

A collaborative process

The NextGenCTR's wing is not just a technological breakthrough but also a case study in collaboration, involving an airframer (Leonardo Helicopters), a research centre (CIRA) and academia (University of Naples, Politecnico Di Milano and Politecnico di Torino) as well as a range of large enterprises (Magnaghi Group as industrial leader) and SMEs (machining expert OMI; FEA specialist SSM; IBK Innovation for loads and aeroelasticity).

The engine/wing arrangement, as finally configured, has simplified the work of the tilting mechanism, enabling the use of a smaller tilting gearbox, thus saving weight. However, whereas usually an aircraft wing is flexible to allow for aerodynamic loads, this is not possible in a tiltrotor due to the presence of the whirl flutter phenomena. Therefore, the wing has to remain rigid. But ensuring that the wing is prevented from torsion and bending has resulted in weight gain. And these factors have made it even more necessary to minimise weight for the fuel tanks and fuel distribution system within the wing.

‘The wing assembly, including the fuel system, is complex both from a technical standpoint and also from an integration perspective due to the presence of three consortia: T-WING, DIGIfuel and DEFENDER,’ says Andrea Artioli, Programme Manager at Leonardo Helicopters. 

Through regular monthly multidisciplinary technical meetings the actors involved bonded to the point that a tighter partnership has evolved. Now, while travel is restricted, the consortia have transitioned into a virtual community that works seamlessly across all the interconnected projects as well as with the broader NextGenCTR demonstrator programme. 

Pierre Abdel Nour, Chief Designer of the NextGenCTR at Leonardo, said that the wing and its integral fuel system design was one of the most important integrations that the project had undertaken because of the multiple technical requirements. Adding further complexity to the optimisation task, the wing has been designed with a thinner airfoil compared to legacy tilt rotor aircraft in order to improve the efficiency of the wing in terms of drag divergence. 

‘We're expecting the aerodynamics of the wing to provide benefit to the platform,’ says Abdel Nour. ‘With a tiltrotor there are specific dynamic requirements for the flight envelope which demand significant attention to the aero elastic behaviour of the wings, with respect to the torsional stiffness. All these different requirements make this design very challenging, but also very interesting in terms of technology advancements and results.’

‘And last but not least,’ adds Abdel Nour, ‘the wing needs to have appropriate weight – a topic we are currently managing. But now, having passed the CDR, we are happy and have a good vision of where we're going.’

DEFENDER – a lightweight fuel-storage system

The DEFENDER project, coordinated by Politecnico di Torino, is developing (and will manufacture, test and qualify) the NextGenCTR's fuel storage subsystem, which incorporates lightweight bladder tanks with self-sealing capabilities and an innovative flange system to connect with the fuel distribution system. Design and manufacturing will be carried out in accordance with EASA Part 21 requirements in order to achieve the permit to fight approval. 

The 60-month project runs until the end of 2022, and the consortium includes CIRA, leading the technical and innovative aspects of the project. They are working with industrial partner Step Sud Mare SRL (SSM) to develop high fidelity numerical methodologies to investigate stress and dynamic responses, and will perform impact analyses of the fuel tanks and parts of the wings. The other consortium partner is Aero Sekur SPA, a leading specialist in design, manufacturing and qualification of structures with flexible materials for aerospace applications, who are designing and manufacturing the fuel bladder tanks and the associated components to connect the tanks with the other subsystems. 

‘We have just closed the DEFENDER CDR and we are starting the manufacturing stage of the fuel tanks,’ reports Matteo Filippi, DEFENDER coordinator at Politecnico Di Torino.

Having a fuel tank bladder inside the wing is a common feature on fixed wing aircraft, but for a tiltrotor there are additional crashworthiness considerations, derived from rotorcraft airworthiness requirements.

‘We have the bladders to guarantee the crashworthiness requirements during the tiltrotor drop test. You have to guarantee that no fuel spillage will occur during a crash,’ explains Marika Belardo, project manager of the DEFENDER project at CIRA. ‘The tiltrotor can take off and land in VTOL mode and hence inherits the requirements from rotorcraft rather than from fixed wing.’

‘We are cooperating with Aero Sekur to equip the fuel bladders with additive manufactured metallic flanges to link up with the fuel distribution system’, Belardo says. ‘We are working to assure the safety of flight of these components and as far as I know, nobody has integrated additive layer manufacturing parts in a fuel storage system– it will be a first.’

Another innovation within DEFENDER is the development of high fidelity models for the fuel tank and also for the systems inside the fuel tank. 

‘We interact with the NGCTR-DIGIFuel consortium to have inputs for the components inside the fuel tank bladders,’ explains Belardo, ‘For safety reasons we need to verify whether the [fuel-measuring] probes threaten to puncture the tanks and check that the fuel lines can withstand the loads during a crash. We have detailed computer models to define these phenomena – even the fuel bladder material is input in the model as experimental data.’  

These models will eventually be validated by means of experimental drop tests involving dropping a wing bay, equipped with fuel bladders, fuel lines and systems, from a prescribed height, and results will be correlated against the numerical predictions. 

‘All this effort,’ remarks Luigi di Palma, T-Wing Project Coordinator, ‘is also exploited in terms of the scalability concept, because the aim of the Joint Undertaking funding is not just to produce a demonstrator and then close, but also to design a scalable process for future exploitation. It's not a matter of just completing the project – it's a projection for the future, and it's important for any subsystem we put on the wing; it is important for all the enterprises contributing to this project.’ 

The  fact that a very unconventional wing design has been fulfilled in such a short space of time is remarkable given that it is based on a highly integrated wing box concept developed by Magnaghi Group on an in-service CS-25 programme.

Additive manufactured fuel tank flanges reworked ready for installation
Additive manufactured fuel tank flanges reworked ready for installation

NGCTR-DIGIFuel - Go with the flow

Clean Sky's NGCTR-DIGIFuel (Distribution and Gauging Interconnected Fuel System for the Next Generation Civil Tilt Rotor) project is designing, developing, and will test and qualify a high reliability integrated fuel gauging and distribution system. This is being designed with active centre of gravity (CG) management to ensure even distribution of fuel between port and starboard sides of the wing.

The consortium is led by Secondo Mona, working closely with Politecnico di Milano, to develop equipment for pumping fuel to the engine and transferring fuel from one tank to another to maintain a balanced fuel load inside the wings. The project also focuses on measuring the quantity of fuel on board, delivering it to the engine while controlling and monitoring fuel-tank ventilation and filling. 

‘One of the innovations introduced is a “clean reserve fuel tank” located nearby the engine from which it can be fed in case of any failures of the fuel transfer or pump systems,’ explains Ermanno Fossa, chief technical officer at Secondo Mona.
 
The clean reserve tank is filled via a filtration system to ensure the quantity of the clean reserve is safe and that the fuel is free from contamination. The consortium is also developing a water detector, a feature normally found on larger aircraft, to increase the safety of the fuel supply.

Another innovation is the overall layout of the filling system whereby all the tanks can transfer fuel to each other as required.

‘With this framework assembly we wanted to develop some innovative components, not just the layout of the system itself, but also most of the components that are inside,’ says Fossa.

‘We initially weighed up the possibility, with Politecnico di Milano, of using fibre optic gauging probes to measure the fuel inside the tanks. Following studies and tests we concluded that we were not mature enough to go to fibre optics, however we decided to keep a connection between each probe and the avionics and the combustor. So, in the end, we are developing probes – and this is relatively innovative in the field of helicopters and tiltrotors – all the probes will be connected by a combustor with the avionics and the same for the main pumps and booster pumps.’ 

Fossa reports that the consortium is also developing brushless pumps ‘which allow, by connection with the system and with the main aircraft computers to work at different levels of performance so that we can control the transfer quantities by actually controlling the performance of the pump.’

DIGIfuel passed its CDR in summer 2020 and is now building the first ship-set to support ground testing and the first flight tests of the NextrGenCTR Demonstrator. 

As with the DEFENDER project, Fossa concludes that the design philosophy within DIGIfuel takes account of the potential to eventually apply the technology in an industrial context, noting that, ‘we need to be able to take what we have developed in the framework of Clean Sky 2 and later assess the possibility to scale up to the requirements for a serial production tiltrotor.’   

From the Leonardo perspective, as ever-growing synergies between the T-WING, DEFENDER and DIGIfuel consortia cement the integration of the wing – and with manufacturing processes now underway – the path towards qualification for the all-important 'permit to fly' brings the first flight of Clean Sky's NextGenCTR Demonstrator one step closer.

‘We wanted to follow the spirit of Clean Sky – you cannot do research on your own, you need to collaborate, and I would say that the result is paying off. Despite the technical challenges, and also external events in times of Covid, this project has resisted everything, and we're still here, teaming up for the qualification phase,’ says Artioli.

Wing upper panel trial A vacuum bag preparation
Wing upper panel trial A vacuum bag preparation