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Clean Sky's Advanced Rear End Demo shapes up

Clean Sky's Advanced Rear End project, which is ongoing until 2023, brings together an extensive combination of interrelated aircraft rear end and empennage elements, leveraging innovative materials and novel manufacturing techniques to save weight and boost production rates for future-generation medium-sized passenger aircraft.

‘It's a pure recurring costs and weight-saving project,’ explains Clean Sky project officer Pierre Durel. ‘The objective is to integrate these new materials and manufacturing techniques, and apply more lean manufacturing processes involving less people – enabling time savings for the assembly of future aircraft.’

‘In parallel to all these activities,’ adds Durel, ‘academics are carrying out complementary analyses and simulations to assess if the design choices which have been taken are good enough from a structural standpoint. They also need to ensure that from an aerodynamic standpoint, the methodologies and material uses are suitable.’ 

Performance and environment goals 

The challenge, according to Airbus's Head of Airframe Engineering Empennage R&T (HTP/VTP), Enrique Guinaldo, ‘is to reach a balance,’ between the need to improve competitiveness and cut costs by up to 20%, and also to save weight up to 20% at component level, cut fuel burn by up to 1.5% and improve aerodynamics in line with Clean Sky’s environmental objectives. 

The Advanced Rear End project achieves this through two parallel demonstration axes, exemplified in the form of a physical industrial demonstrator with associated sub-assembly and scale part demonstrators, and complemented by virtual design configuration simulations.

Composite picture

The project focuses on the development of structures using lightweight composite materials – instead of the conventional aircraft construction techniques which use metal frames, stringers and skins fastened together with rivets. By switching to composite construction these elements can be more closely integrated, thereby accelerating manufacturing time while cutting weight, and reducing fuel-burn to enable the lowering of CO2 and NOx emissions.

‘The frames, stringers and skin are made from composite materials, the frames are made in a one shot 'out-of-autoclave' process and the stringers are made using a new technology called glide forming, which saves weight, energy and time,’ says Luis Aliaga, Executive Project Manager at Aernnova. 

All of this combines with Aernnova's work on designing a new 'pulse assembly line,' a type of moving assembly line whereby, as Aliaga explains, ‘substations can be duplicated or even triplicated when required in order not to waste time waiting for any parts.’

The context behind this is that Airbus is aiming to improve its manufacturing capacity to increase the amount of medium-sized passenger aircraft significantly per month. To achieve this, Aliaga says, ‘the pulse line incorporates the use of collaborative robots for such tasks as drilling holes and riveting.’ 

A tale of two tails

A visually obvious departure from ‘standard’ tail planes in the Advanced Rear End project is that the horizontal stabilisers point forwards. There are two rationales for this. One is directly performance driven: with the horizontal tail pointing forward, there is a benefit in terms of improving natural laminar flow on the tail, thereby reducing fuel burn.

The other reason is that by changing the sweep angle of the tail it becomes possible to make the rear end shorter, which translates into weight savings. With a shorter rear end, the passenger cabin can be extended to occupy more of the overall length of the aircraft, which means that additional seat rows (and additional revenue-generating passengers in a single aisle aircraft) can potentially be accommodated. This represents an improvement not only in competitiveness for airlines, but also means less fuel burned per passenger.

Power trip

The rear end also houses the Auxiliary Power Unit (APU), the aircraft's auxiliary power generator, for which Airbus is leading the design of an efficient air intake system as well as an advanced exhaust system.

‘We're investing in new materials and designs which will allow us to reduce the noise created by the APU when the aircraft is on the ground,’ says Guinaldo.

But by making the rear end more compact, it's a challenge to accommodate the APU – the only option is to move it slightly forward within the aircraft. 

However, by moving it forwards, says Guinaldo, ‘it starts to be surrounded by lots of things, specifically the horizontal and vertical tails, and if the APU overheats, has a fire event or an uncontained engine rotor failure, the debris must be contained so as not to damage these key parts of the airframe.’ 

To address this specially reinforced fireproofed receptacle, a set of different materials are being developed by DLR, Fraunhofer and ONERA to encapsulate the APU itself and ensure that in the event of the APU overheating or having a major failure, any debris is safely constrained.

We're investing in new materials and designs which will allow us to reduce the noise created by the APU when the aircraft is on the ground

Lightening the load

Another aspect of innovation in the overall project is in the area of loads. Loads, which come into the aircraft from the fin, the vertical tail, are transferred through special 'highly loaded frames'. Conventionally these are always manufactured in metal, but in the Advanced Rear End project these are made in composite – a Clean Sky first.

The juxtaposition of metal and composites causes issues, however. At altitude, the temperature can be up to minus 55 degrees, while down on earth in hot climates for example, temperatures can be plus 50 degrees or even more due to the heat coming from different systems. That more than 100-degree range causes an aircraft's metal parts to expand and contract, whereas composite parts remain more or less the same dimensions. This mismatch in expansion creates internal loads generated around the points of interface when metal and composite parts are fastened together.

‘You might get cracks and fatigue issues, and you have to inspect the interfaces where composites and metals join, so from an operational point of view, producing everything in composite requires potentially less maintenance,’ says Guinaldo. ‘Therefore there's both a benefit in terms of weight saving plus you have a benefit in terms of the operational life. The highly loaded frames in composite are probably the most relevant technology brick, apart from the configuration itself, which is radical in the Advanced Rear End project.’  

The highly loaded frames in composite are probably the most relevant technology brick in the Advanced Rear End project

Potential weight savings

When conventional metal structures in aircraft are replaced with their composite or thermoplastic counterparts, a weight saving ‘of around 5% or even more is possible depending on the component targeted,’ says Guinaldo. But to maximise the benefit of the efforts being invested into the Advanced Rear End, it's necessary to combine all the innovations together – the composites, the integrated panels, the relocated and encapsulated APU and the use of new high-loaded frames – with the new design configuration.

‘If you mix these technologies with the new rear end design configuration, then maybe the savings could be up to 20% compared to the rear end of a conventional single aisle aircraft,’ says Guinaldo.

Though the Clean Sky Advanced Rear End is mainly intended for exploitation on the future generation of medium-sized passenger jets, Guinaldo points out that ‘any of the technologies that we are developing in terms of industrial development could also fit into a conventional aircraft, because ultimately our demonstrators are targeting capabilities which we can also deliver in the short term through modification of existing aircraft.’

The savings could be up to 20% compared to the rear end of a conventional single aisle aircraft

The project outlook 

In February 2021 the project passed TRL3, thereby freezing the design, proving the feasibility of the project, and enabling the main manufacturing activities for the industrial demonstrator to commence.

‘We already did some prototyping and mini-serial production of small items for the industrial demonstrator,’ says Guinaldo. ‘We are focusing this year on elementary part production of the items that will be assembled in 2022, with the goal of reaching TRL6 by the end of the consortium in 2023. In parallel, the consortium is aiming to reach Manufacturing Readiness Level (MRL) 5/6 – not only for the development of the rear end industrial demonstrator but also for the associated manufacturing processes and tools that are being worked on. Finally, regarding the virtual demonstrator, we are currently carrying out tests to confirm whether the assumptions that have been made so far are still valid.’