The RACER gets its Rotorless Tail in gear
Following in the footsteps of its ground-breaking predecessor — the X³ experimental hybrid high-speed compound helicopter developed by Airbus Helicopters — Clean Sky's RACER Demonstrator has been pacing successfully through its pre-design (in 2016), preliminary design review (in 2018) and critical design review (in 2019) phases. The RACER's detailed design has been finalised in 2020, clearing the path for procurement and manufacturing of the key subsystems required for assembly and flight testing.
Along that journey there have been many major explorations of different design layouts, but despite the challenging environment, RACER is poised to reach TRL6 at full aircraft level in 2022, on-boarding many new technologies brought in by core partners and partners.
Focusing on the Racer Rotor-less Tail system, ‘Airbus investigated many configurations to find the optimal design for a compound helicopter with a pusher configuration, finally moving to a very complex asymmetrical tail configuration,’ says Clean Sky project officer Antonello Marino.
‘The design was mainly driven by the need to assure flight controllability, weight saving, high speed capability and thus, maximising aerodynamic efficiency.’ The complexity of the RACER's features has meant adding extra hardware, such as the unconventional box-wing and gearboxes for the pusher rotors – which inevitably add weight. Consequently, says Marino, ‘this extra weight has to be compensated for somewhere else. And in this case, the team did a lot of work to reduce the weight in the rotorless tail of the RACER by using innovative materials and manufacturing techniques, including additive manufacturing and resin transfer moulding composites."
To deliver the best combination of performance characteristics, RACER has been designed with an impressive check-list of innovative features. For example, RACER's anti-torque rootless tail boom has an asymmetric cross-section profile (similar to the cross section of a wing), patented by Airbus Helicopters which leads to minus 10% torque in hover. The distinctive ‘H’ shaped empennage and the “control surfaces” positions are optimized to minimise aerodynamic interference towards improved flight controllability.
Many hands make light work
Mirroring the technical complexity of the RACER’s rotorless tail is the complexity of the team assembled to take on the challenge of designing, developing and building it.
The overall project is led by Airbus Helicopters España, supported by the OUTCOME project, led by AERNNOVA. The effort is supported by two complementary projects: the first is OLFITT, which has developed, designed, manufactured and delivered the prototype manufacturing tooling to fabricate the tailboom, horizontal and vertical stabilisers, control surfaces and upper tail-boom fairing of the RACER’s tail. The second, OLTITA, has developed, designed, manufactured and delivered a ‘light’, flexible and innovative assembly tool chain for the integration of the RACER tail. These projects are being implemented by two Italian small enterprises, OMPM and Metitalia.
‘The RACER tail-boom, based on an Airbus Helicopter patent, has been a challenge, both in terms of obtaining patent approval — which is always challenging — but also because of the strict weight targets we had to reach in order to optimise the flight envelope,’ explains Juan Manuel Jiménez García, R&T Projects Coordinator and CS2 Project Manager at Airbus Helicopters España (AHE).
Late into the RACER programme timeline a decision was made to redesign the tail architecture to enable trimming of the empennage relative to the tailboom. To facilitate this, a new interface had to be designed between the empennage and the tailboom which, says Jiménez García, ‘is needed to be implemented in the airframe equivalent to the size of a piece of A4 paper.’
A related challenge was in the area of the H-shaped vertical/horizontal stabiliser. The unusual shape was devised to improve energy consumption. Additionally ‘we have also implemented innovative manufacturing technologies and solutions’ says Jimenez García, ‘and one of them, that we're really proud of, is that we are going to put in flight the first primary structure, manufactured using additive manufacturing, to fly in an Airbus Helicopter aircraft.’
The part in question is the trimming tab fitting, made using TiAl6V4, an alpha-beta titanium alloy with a high strength-to-weight ratio and excellent corrosion resistance. The additive manufacturing process used in this case is one of the powder bed fusion (PBF) processes, known as selective laser melting (SLM).
Other smaller parts of the RACER tail, such as secondary structures including brackets, camera supports, equipment supports, and antennae supports, have been manufactured using the same combination of PBF/SLM processes, using aluminium-magnesium-scandium alloy (SCALMALLOY).
Another innovation is that the torsion box of the horizontal stabiliser is built in a single piece in an 'out of autoclave' resin transfer moulding one-shot process. ‘It's a piece of around three metres and we have saved a lot of energy and avoided the use of autoclave – it's the first time that a piece of these dimensions has been produced at Airbus Helicopters using this process,’ Jimenez García explains.
‘We've implemented “out of autoclave” technology in combination with sandwich material for the control surfaces for the vertical stabiliser,’ adds Jiménez García. ‘It's good news because we've managed to combine different manufacturing technologies together to attain the optimal solutions for the future in terms of design, manufacturing, and energy savings.’
Building skills and cooperation across Europe
SMEs OMPM and Metitalia – which, despite having vast aerospace expertise were almost new to the rotorcraft sector and its specific challenges – have been instrumental in realising the resin transfer moulding techniques and associated tooling processes and delivering them to Aernnova into OLFITT (OMPM LiferCraft Fabrication Innovative Tool Tail) project. ‘The tool for the “out of autoclave” resin transfer moulding technology was one of the most innovative parts of the project because RTM is usually used to produce small parts requiring high levels of precision,’ says Gabriella Caputo, Programs Development & Budgeting Manager at OMPM. ‘But in this case, we had to produce quite a big tool, about two metres long, to manufacture the vertical tail stabiliser.’
Clean Sky Project Officer Antonello Marino notes that RTM is usually ‘more suitable for serial production’ – rather than for prototypes – ‘because it's more expensive.’ But the rationale behind its use, he says, is that AERNNOVA and OMPM ‘decided to invest more at the research level in order to maximise exploitation opportunities for the RACER programme at the end.
In OLTITA, to manufacture the jigs for the tail-boom assembly and empennage assembly, OMPM used a measurement assisted assembly (MAA) technique. This is a novel approach for assembly based on very light adjustable supporting structures whereby the nominal positioning of each support is achieved using a combination of undersized determinate assembly (DA) holes and laser tracking to fine-tune the positioning.
The main advantages of MAA are that the main interfaces such as the frames and the hinge line are held in position and continually monitored during the assembly without using a rigid linkage, (which would be the case when using traditional jigs). An extra advantage of MAA is the reduction of tooling costs, and in principle MAA setups can be repurposed to manufacture other components.
Tying all the pieces together
At Aernnova, Ana Reguero, Programme and Project Manager R&T Department, notes that the intricate multi-stakeholder set up that has enabled the design, development and building of the RACER's rotorless tail has been both challenging but rewarding, as the major elements reach realisation.
‘The collaboration is something I'm very proud of in this project,’ says Reguero. ‘The relationship that has been established between Airbus, the core partners and the smaller companies has been very close and positive, unlike a traditional leader/supplier relationship – more of a cooperative relationship amongst equals. There has been deep engagement at all levels thanks to shared values among all the stakeholders, working together towards completion of the targets. And this has been reflected throughout all stages of the process, through production planning, regular meetings and progress reviews.’
For 2021, with delivery of the complete assembled tail slated for the first quarter of the year, Reguero reports that ‘we have to finalise all the manufacturing activities associated with the empennage and the tail-boom and other smaller components, and deliver them to Airbus Helicopters for final assembly, in preparation for the ground test campaign in the north of Spain. This will be followed by the ground tests and preparation of the documentation and reports for the certification authorities together with Airbus Helicopters for obtaining the permit to fly.’