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Changing Gear: Clean Sky electrifies aircraft landing gear systems for more sustainable and quicker turnarounds

Direct drive rig
Direct drive rig

In line with the trend in aircraft design which aims to retire hydraulic and pneumatic systems and shift towards more environmentally friendly and energy-efficient alternatives, Clean Sky 2’s Advanced Landing Gears project (part of a  bigger initiative called Landing Gear Systems) focuses on the electrification of landing gears for future aircraft, and builds on the achievements in landing gear developments from Clean Sky 1. Related Clean Sky landing gear activities including technological bricks for Electro-Hydrostatic Actuation Main Landing Gears systems, a direct drive motorised landing gear and an innovative new type of wheel and tyre combo, are showing measurable enefits in accelerating aircraft turnaround times and reducing noise at airports.

In most conventional airliners of today the landing gear is extended and retracted using power drawn from centralised, complex and heavy hydraulic systems which also power many of the aircraft’s flight control surfaces. By contrast, in Clean Sky’s Advanced Landing Gear Systems project, a technology called Electro-Hydrostatic Actuation (EHA) means that the landing gear can be activated and retracted using a more localised and more electrical solution which saves weight, consumes less power, and generates less heat and noise in the process. All of this neatly aligns with Horizon 2020 aspirations in respect of sustainable, energy-efficient vehicles for European mobility.

Leading the project, Safran Landing Systems (SLS) is focused around the main landing gear while Liebherr concentrates on the nose landing gear. The historical techno-bricks created at the beginning of CS2 by SLS for EHA Main Landing Gear (MLG) Systems have involved the development of the following:

  • a composite brace from Fokker for structure weight savings;
  • an intelligent brake monitoring system to optimise operational efficiency led by United Technologies Research Centre Ireland;
  • a long stroke piezoelectric actuator through the CFP08 AUDACITY project led by CEDRAT; and
  • a monitoring sensor for hydraulic systems though the CFP 08 FLUIDER project led by IK4.

’The bricks developed for EHA Main Landing Gears mainly aim at reducing the weight at aircraft level, though other benefits like power optimisation and pollution reduction are also derived,’ says Emmanuel Kuhn, R&T Program Director Deputy at Safran Landing Systems. A key initiative complementing the EHA Main Landing Gears initiative is the inclusion of electrically-driven landing gear wheels for electric taxiing (e-taxiing) which enables cleaner, quieter and more fuel-efficient movements of aircraft between the airport terminal and the runways, as Kuhn explains:

’All of our projects aim at reducing CO2 and one way to do this is through weight reduction. Another way is to increase the efficiency of the systems by using less power to actuate the extension and retraction of the main landing gear by using electrostatic actuation. A third lever to reduce CO2 is to directly switch off the engines, and this is typically what a direct drive system does. It enables the aircraft to switch off its engines when it has landed, and taxi to the gate just by using electric motors integrated within the wheels. It’s the same before the plane takes off, it uses the electric motors within the wheels to taxi from the gate to the runway.’

Novel self-cooling wheel by Safran Landing Systems (SLS)
Novel self-cooling wheel by Safran Landing Systems (SLS)

Driving force

’Within Clean Sky 1 we were working on Smart Ground Operation (SGO) R&T activity on electric taxiing of aircraft, which ended in 2016,’ says Kuhn. ’We achieved an effective design but we concluded that we have to decrease the weight and volume. Then, when Clean Sky 2 happened this opened up an opportunity to carry on and keep on improving the e-taxiing design through another activity that we called the “Direct Drive System”. This enables the taxiing phase with the aircraft’s main engines switched off which directly reduces CO2, NOx and noise pollution.’

The final Direct Drive System demonstration involves a wheel actuator made by SLS, but also features power electronics made by a company called INDRA in the ETSIN project and an energy recovery pack made by a company called ADENEO in the SUNSET project.

In relation to the environmental benefits, during a 17 minute push-back and taxi-out to the runway on a 185 seat aircraft using Safran’s Direct Drive System, ’there would be up to 51% reduction in nitrogen oxide (NOx), up to 62% reduction in UHC (unburnt hydrocarbons), up to 61% reduction in CO2, and up to 73% reduction in carbon monoxide,’ confirms Jean-Philippe Vaslin, Director of R&T Platforms at Safran Landing Systems, who manages the Direct Drive activity and some of the company’s other Clean Sky projects.

In tangible terms, this means that over the course of one year, the savings from one aircraft using Direct Drive would be equivalent to removing the NOx from 932 cars, or, put another way, it would be like adding the CO2 absorption benefit of an additional 948 trees.

’This also drastically reduces noise on the ground at the airport,’ adds Vaslin, ’as we’re not using the engine for the push-back or for the taxiing.’

Reinventing the wheel

Also integrated into the Clean Sky Advanced Landing Gear project is a related work package whereby a unique wheel and tyre design is being developed which improves the wheel cooling time for an aircraft after it has landed. Wheels must have cooled to a designated safe temperature before an aircraft can take off again. Hence, quicker wheel cooling means the potential for shorter turnaround time (TAT). The design concept is oriented around a medium-sized (185 passengers) airliner.

’Michelin is designing the tyres and Safran Landing Systems is designing the wheel, and it’s a joint design, because as soon as you modify the design of the wheel, you have to adapt the tyre,’ says Emmanuel Kuhn. ’This is all the more true since the design that enables us to reduce the cooling time [following the heat generated in the landing gear by braking during the landing phase] is an angled wheel rim. Normally an aircraft wheel rim is flat [and perpendicular parallel to the ground], but in this case there is a slight angle and therefore when the aircraft brakes are applied on landing we improve the heat convection and dissipation through this slight angle, and therefore, without having a brake cooling fan, you reduce turnaround time from 55 minutes to 46 minutes.’

The biggest advantage of this from an operational standpoint is the potential to increase the aircraft utilisation, so it really is a positive for airlines. This work package started in early 2017 and by the end of 2020 Michelin will have completed three designs of the tyres, and SLS will have produced two designs for the wheel – an intensive activity in terms of design.

’It’s a simple idea but a very efficient one which has been test proven, and in terms of TAT we have achieved 9 minutes reduction through testing – it’s not a simulation, it is test proven with the first designs. Now, the next step is to improve the weight of the tyre and the wheel by adapting the wheel structure, and Michelin will work on composite material for the tyres. We have some ideas to move forward and do testing to see if they are efficient and fulfil the spec,’ says Kuhn.

There will be some activities needed before certification and eventual commercialisation, because having a new wheel with an angled rim will mean having a new non-standard tyre, which will prompt certification and operational issues. Also, from an airline point of view, as a new tyre would be required, there would need to be discussions around acceptance of the new format.

Progress so far

’For the direct drive and for the shor t turnaround wheel/tyre combination the aim is to reach Technology Readiness Level 6 (TRL6) – the idea is to finish the tests and to validate the maturity to prototype stage by the end of 2020. And for the EHA main landing gear bricks, the idea is to finish at TRL5 before the end of 2021,’ says Kuhn.

Overall system tyre and wheel
Overall system tyre and wheel


’What is nice with Clean Sky in general is – as you can see with the e-taxiing project – Clean Sky has enabled us to have continuity to mature this system. Without that, if we had not received the funds from Clean Sky 2, we would not have been able to mature and improve the earlier system that we had developed within Clean Sky 1, so this is a real opportunity. Besides that, within CS2 we have added other projects – the maturation of different techno-bricks for future more electrical aircraft, particularly for electro-hydrostatic actuators for main landing gear. This is new and this is something that has appeared only in Clean Sky 2. Another benefit is that through European funding we have been able to propose new R&T activities that make sense with our roadmap, which otherwise would not have been possible. So the Clean Sky benefit is that European companies will be able to invest in a specific topic which is part of their existing road map, or integrate new topics that are relevant to those roadmaps. This is really the benefit,’ says Kuhn.

Although Clean Sky’s Advanced Landing Gear project still has some time to run, looking into the longer term, Kuhn concludes that; ’Of course, if we conclude at the end of Clean Sky 2 that the techno-bricks that we are currently developing deserve more maturation or that we would like to submit new activities that are relevant to our roadmap, then we will use it for future topics of research.’