What chance a clean sky?

Jan 21, 2016

Clean Sky is Europe’s multibillion-euro programme to drastically improve the emissions profile of aerospace technology. Having begun in 2008, the programme is now transitioning to its second phase and will run to 2024, by which time an array of advanced demonstrators should have been flight tested. Alex Derber gauges the progress made thus far and considers whether the huge sums committed to Clean Sky are justified.

In the last 15 years the European Union has spearheaded global efforts to tackle aviation’s steadily rising greenhouse gas emissions. Its market-based Emissions Trading Scheme (ETS), while structurally and politically flawed, still stands as the only meaningful attempt to restrict near-term CO2 output by airlines. And while ETS is currently on hold to allow the ICAO time to devise a globally-approved mechanism, it at least forced the UN body to address the issue in earnest after more than a decade of dithering.

That said, the wheels of the ICAO turn excruciatingly slowly, and any deal on emissions won’t be announced until 2016 at the organisation’s triennial assembly. The EU has promised to reinstate ETS should the ICAO not come up with an acceptable solution, but in the meantime it is pursuing the technological prong of its campaign to ameliorate the climate change impact of aircraft.

Europe’s largest aviation research project

The EU’s Clean Sky programme kicked off in 2008 with a remit to research fixed-wing, rotorcraft, airframe, engine, systems and production technologies that could achieve big reductions in CO2, NOx and noise emissions. The programme is now transitioning to its second stage, Clean Sky 2, which aims to showcase technologies capable of contributing to future aircraft that emit 30 per cent less CO2, up to 80 per cent less NOx and 75 per cent less noise than the best commercial equipment of 2014.

As Clean Sky moves from research to the production of technology demonstrators, its budget has been ramped up from €1.6bn for Clean Sky 1 to €4bn for Clean Sky 2. Following its €800m hand-out for Clean Sky 1, the EU will contribute €1.75bn to the second phase, the follow-on funding having been allocated from Horizon 2020, Europe’s €80bn, seven-year research and innovation project.

The balance of investment comes from industry, with most of Europe’s main aerospace players involved. For example, Alenia is heading development of greener regional aircraft; Airbus and Saab are leading research into larger airframes; Safran and Rolls-Royce oversee several different engine projects; while Dassault Aviation and Germany’s Fraunhofer Institute are pursuing more ecological production methods. Other big names – such as structures specialist GKN, aircraft systems manufacturer Liebherr, engine company MTU and avionics giant Thales – are also heavily involved across their various competencies. Meanwhile, a host of academic institutions and smaller companies also participate, and many others are still (January 2015) responding to proposals from the leaders of each of the six Integrated Technology Demonstrator (IDT) programmes.

As stated above, each ITD focuses on a specific aspect of aircraft design, and several have already tested sub-assemblies and components that could eventually form part of complete technology demonstrators. The latest test, in September 2014, saw the Thales and Liebherr-led ‘System for Green Operations’ ITD experiment with a new heat transfer system on board an A320 research aircraft. The new cooling system is meant to be lighter and less power-intensive than current technology.

Several months earlier, French company Corso Magenta used Clean Sky partner ONERA’s wind tunnel to test a new type of ribbed, non-liquid aircraft paint that could boost aerodynamic performance for Airbus’ ‘Smart Fixed Wing Aircraft’ ITD. Airbus itself is leading development of a laminar flow wing and has now frozen the design of the complete wingbox panel – an integrated leading edge and upper cover – that will eventually form part of the wing of its BLADE flight demonstrator.

Revisiting history

Perhaps the most eye-catching developments, however, surround Snecma’s geared open rotor project, one of the six engine studies – open rotor, geared open rotor, large turbofan, geared turbofan, turboshaft and lean burn – that constitute the ‘Sustainable And Green Engine (SAGE)’ ITD.

Open rotors, also known as unducted fans or propfans, marry the fuel efficiency of turboprops to the power at high speeds of turbofans, and have been in development for almost 40 years. By the mid-1980s GE had developed a flying prototype – the GE36 – that was 30 per cent more efficient than jet engines of the time, but plummeting oil prices scuppered interest in the breakthrough technology.

Russian manufacturer Ivchenko Progress took up the baton in following decade, coming close to full production readiness for its D-27 open rotor. Unfortunately the engine’s host aircraft, the ill-fated An-70 transport, fell victim first to the collapse of the Soviet Union and later the crisis in Ukraine, which was meant to share development costs with Russia.

Clean Sky, however, is again devoting considerable resources to one of the most important single contributors to emissions reduction. The geared open rotor will be among the first of its six development engines to undergo ground testing – scheduled for late 2015 – and Snecma expects to produce the first blade for the engine before February. This will be followed by critical design review of the whole demonstrator by the end of March 2015 and then full-scale assembly by mid-2015.

Snecma’s main goal is to achieve 30 per cent lower fuel burn than the CFM56-7 engine which powers 737NG aircraft, and which Snecma itself manufactures with GE. While the French company has benefited from the knowledge its American partner gained on the GE36, the gas generator core of the new open rotor is based on the M88 powerplant of Dassault’s Rafale fighter.

Yet fuel burn gains are inherent to open rotors, which lacking a fan case can easily eclipse the high bypass ratios of even the most modern conventional turbofans. There is typically a trade-off, however, with noise and safety, as fan cases help to muffle jet roar and contain the high-velocity debris that threatens cabin integrity in the event of engine failure.

Snecma addressed the first of those problems last year, albeit at low speeds, when it ran its HERA rig – a 1:5 scale open rotor – in a series of air-acoustic wind tunnel tests.

“The results were really promising and we demonstrated that we can meet Chapter 14 standards of certification and achieve a technology readiness level of four – so that was a major first,” comments Olivier Jung, Snecma’s chief engineer on the open rotor project. Technology Readiness Level 6 implies equipment ready for flight testing, while Chapter 14 of the Chicago Convention sets the maximum permitted noise output of civil aircraft manufactured from 2017.

Safety, meanwhile, is improved by use of a pusher configuration that situates the rotor at the back of the aircraft. This not only reduces cabin noise compared with conventional wing mounting, but also places the engine away from most passengers. Even so, future regulations could require airframes with pusher engines to have significant shielding around the tailplane and rear fuselage, all of which would add weight, possibly compromising the fuel burn advantages of the open rotor.

Nonetheless, the benefits of Snecma’s open rotor are to be found as much in its development journey as in the end product. This also applies to other Clean Sky projects, which focus on producing technology demonstrators but also aim to develop breakthrough technologies on the way.

“The demonstrator contains key enablers for future engines, like the blade-pitch control mechanism and the contra-rotating reduction gearbox. These two technologies significantly increase the whole engine’s performance in terms of fuel burn and Snecma and its partners such as GKN, Avio, and Aircelle target a TRL of 5 for most of these technologies,” says Jung.

First flight tests of the geared open rotor, potentially aboard an A340-300, are planned for late 2020 or early 2021.

Impact assessment

While aerospace manufacturers tinker with new designs and production processes, academic and research institutions plug data into computer models to assess the real-world impact of the new technology.

Cranfield University in the UK is part of Clean Sky’s ‘Technology Evaluator’ (TE) program. TE is funded with approximately €30 million to conduct the environmental impact assessment of new fixed-wing and rotorcraft technologies. “Within the TE we are leading the environmental impact assessments for the rotorcraft configurations, so we interact very strongly with the Green RotorCraft Integrated Technology Demonstrator (GRC ITD),” says Vassilios Pachidis, Head of the Gas Turbine Engineering Group at Cranfield’s Propulsion Engineering Centre. While helicopters and tilt rotors normally fall outside the bounds of ATE&M reporting, Cranfield’s assessment methodology is similar to that employed by the TE for fixed wing aircraft, and thus bears summarising.

The GRC ITD is led by AgustaWestland and Eurocopter, and supported by Liebherr, Thales and Hispano-Suiza. To bridge the gap between their work and TE requirements, the partners created a computer simulator called PHOENIX, or ‘Platform for Hosting Operational and Environmental Investigations for Rotorcraft’.

“PHOENIX allows us to simulate virtual helicopters on virtual four-dimensional trajectories,” says Pachidis. By this he means that Cranfield can not only investigate the environmental impact of a future helicopter during a single flight, but also the effects of an entire fleet of new rotorcraft performing their predecessors’ mission profiles. In a similar fashion, other aerospace research bodies – such as ONERA in France, the NLR in the Netherlands and Germany’s DLR – will evaluate the effects of improved aircraft performance at single mission, single airport and system-wide levels.

Just as there will be distinct assessments for regional, narrowbody and widebody aircraft, rotorcraft are also categorised according to their normal mission profiles. For instance, twin-engine light helicopters are modelled against typical police and patrol flights, while simulations for single-engine light helicopters use the waypoint and location data of typical VIP flights.

For each type of rotorcraft, each simulation is then run using three separate data inputs: the standard rotorcraft design of 2000; the state-of-the-art in 2020; and the technology of 2020 incorporating the targeted improvements of the Clean Sky ITDs.

“Those conceptual configurations are analysed in great detail and we are following a bottom-up approach, designing helicopters from a subsystems level up to fully integrated configurations, so guesswork is kept to a minimum,” comments Pachidis.

Nonetheless, the most forward-looking assessments are necessarily conceptual, dealing with designs that may never make it into commercial service. Pachidis, like Jung at Snecma, isn’t troubled by this prospect. “I’m not sure if we’re going to see complete helicopter configurations emerging from Clean Sky, but clearly industry has learned a lot from the exercise so I’m confident that systems and sub-systems will enter future fleets,” he says.

Value for taxpayer money?

There is no doubt that commercial aviation needs every weapon at its disposal to arrest its climate change impact. Total kerosene fuel burn by the world’s airlines is expected to grow by about three per cent per year, reaching three per cent of all global greenhouse gas emissions by 2050, according to the IPCC. That may seem a small figure, but it assumes business as usual in all other sectors, whereas in reality many countries have committed to net carbon reductions that, if achieved, will leave aviation – which unlike other industries doesn’t have a viable alternative to burning fossil fuels – with a shamefully fat slice of the global warming pie.

Thus programmes such as Clean Sky are to be welcomed, right? Free market proponents might disagree in circumstances where Clean Sky rewards R&D that would have been undertaken anyway. The recent oil slump notwithstanding, engine and airframe manufacturers know that their customers value fuel efficiency above all but safety, so it is questionable whether they need taxpayer money to incentivise them further. Of course, aerospace development in Europe and the United States has always received truckloads of government aid – both legal and illegal – so one might argue that at least this latest manifestation has a worthy goal.

Pachidis believes that Clean Sky does fund research that wouldn’t have occurred commercially, allowing companies to be more adventurous in their thinking.

“Clean Sky is useful because it de-risks activities that industry would be less willing to undertake because of the risks and timelines involved. You think that industry is always looking ahead, trying to fully explore the boundaries of the existing envelope, but frequently their longer-term research efforts are hindered due to the tremendous pressure to deliver the next product to the customer on time and on specification,” he comments.

That Clean Sky’s participants are not under such pressure is a double-edged sword. Snecma’s open rotor, for instance, was originally due to fly in 2016, but may now only see its first flight test five years later. Airbus, meanwhile, had been scheduled to flight test its low-drag wing in 3Q 2014, but that date has been pushed back to late 2016. Such delays also occur in commercial programmes, but the need for environmental advances is particularly pressing given the diminishing number of years available before climate change becomes impossible to halt.

Even if such concerns were not a factor, however, some question Clean Sky participants’ commitment to the programme, or at least the goals it pursues.

“If they want access to the funds they should start delivering,” says Bill Hemmings, programme manager for aviation and shipping at Transport & Environment, a Brussels-based NGO focused on sustainable transport.

“The ICAO is developing a fuel efficiency standard for aircraft, but the aerospace industry is fighting to make sure it has no teeth. That’s wrong – if you have your fingers in the honeypot you shouldn’t be campaigning against the very efficiencies you’re being funded to achieve,” he adds.

It’s a depressing thought, but aviation’s future climate change impact is likely to depend as much on the dirty business of closed-door lobbying as it does on Clean Sky.