Clean machines Part I

6 min read

In the first part of our June 30 Cover Feature, Stuart Nathan reports on how some of Europe’s biggest aerospace groups are working together on technology for greener, more efficient aircraft.

The twin presssures of environmental pressures and soaring fuel prices are challenging the civil aviation industry as never before. Despite improvements in aerodynamics and engine performance over the past decade, aircraft are still fuel-hungry, deplete resources and produce large amounts of carbon dioxide per passenger.

And while it is generally accepted that the increase in demand for air travel, rather than the performance of the aircraft, is the concern, the aerospace industry is investigating new technologies for more fuel-efficient flight.

There are many such projects around the world, but the largest is in Europe. Clean Sky, one of the first of a new type of public-funded collaborative research project, is bringing together most of Europe’s aerospace companies in an effort to transform passenger aircraft. Costing a total of €1.6bn (around £1.3bn) half from industry, half from European Union funds, Clean Sky could help to put a fleet of very different-looking airliners into the skies in 10 years.

Clean Sky is a joint technology initiative (JTI), a public-private partnership which aims to encourage industry to participate in research that is wider than their normal R&D subjects. European politicians are concerned that industry is too conservative about innovation and using new technologies, and it is hoped this approach would encourage companies to develop research into products. Clean Sky is one of four JTIs: the others are in hydrogen and fuel cells; innovative medicines and nano-electronics.

To identify subject areas for JTIs, the European Commission looked for sectors where innovations could lead to substantial returns for industry, both in meeting the needs of society or in simpler, economic terms. The trigger for Clean Sky was a set of goals set out by the Advisory Council for Aeronautics Research in Europe (ACARE), in 2004: to halve CO2 emissions from aircraft, per mile; cut NOx emissions by 80 per cent; halve perceived aircraft noise; and establish green design, manufacturing, maintenance and product disposal practices, by 2020.

To achieve this, Clean Sky has been split into six parts, known as integrated technology demonstrators (ITDs). Two are for specific types of aircraft —rotorcraft, including helicopters and tilt-rotors; and regional aircraft, designed for short-range hops. Another two are looking at eco-design, emphasising materials, manufacturing, maintenance and recycling; and green operation, studying thermal management and other aspects of running civil aircraft that can be fine-tuned to save on fuel. The other two, perhaps of most interest to engineers, look at technologies for parts of the plane responsible for fuel consumption — the engines and the wings.



With the Clean Sky consortium including such companies as Rolls-Royce, Safran, Airbus, Dassault and AgustaWestland, it could be seen as a massive case of state aid to industry. However, Nick Peacock of Rolls-Royce, co-ordinator of the engines ITD, said this is not the case. ‘This is the commission pinpointing a need, and then industry coming up with proposals to meet this need. Even though the industry agrees that there is a need for this sort of research, the sums involved are so large the companies just aren’t prepared, and in fact aren’t able, to do this research on their own.’

The Clean Sky research is not aimed at producing actual aircraft. Instead, it is pre-competitive research, looking to develop and test the essential components for environmentally-friendly civil aviation at full scale and operational conditions. ‘It’s the only way the conservative people who run industries like this will then invest in products which are themselves high risk. There’s no guarantee that these products will make our fortunes,’ said Peacock.

This means the ITD engines will not be anything like a commercial engine, he said. ‘We want to test out materials, designs for combusters, turbines, controls and so on in a realistic environment, and the only way we can do that is to build an engine; we can then simulate all the pressures, temperatures and loads those components will encounter. But it might be something that looks very unusual; it might be very heavy or uncompetitive. We can then take the results of that and design a product that meets the need of the customer, but that part — the certification and product development — will always be 100 per cent industry funded.’

The engines ITD is responsible for the largest proportion of Clean Sky’s emissions reductions aims — 15 to 20 per cent of the total reduction goal. ‘That will be verifiably delivered and ready to introduce into commercial engines,’ said Peacock. The technologies his consortium will develop are strikingly different from today’s engines.

The project aims to produce five engine demonstrators, each suited for a specific type of airliner, but Peacock said the most important target is the narrow-bodied, single-aisle aircraft that ply the medium-haul routes. These, he said, are by far the most numerous type of airliner and so the biggest polluters. ‘You might think it’s the bigger planes that are the most polluting, but there are relatively few of them,’ he said.

The engines will not be tweaked versions of the current turbofan engine, but are likely to look back to earlier days of air travel. It looks like propellors will be making a big comeback.

These new engines, of which one version will be developed by Rolls-Royce and another by Safran, are based around the turboprop concept. A turbine engine similar to a jet is used, but instead of sucking in air, burning fuel to spin the engine, and ejecting the exhaust to generate thrust, the rotating turbine is used to spin the propellor.

‘They’ve been used for donkey’s years,’ said Peacock. ‘They’re efficient, in that they don’t burn much fuel, but they’re slow — turboprop aircraft fly at Mach 0.5, whereas turbofan airliners are are about Mach 0.8. What we want to do is develop engines that will propel a 100-seat plus airliner at Mach 0.8, which will allow them to be integrated into air traffic control regimes.’

Rolls-Royce is using its experience on the TP400-D6 turboprop engine

Everyone in the engines sector agrees that, to do this, each engine will need at least two propellors on the same turbine, rotating in opposite directions. ‘But different companies have different ideas on how to achieve that; whether the propellors are driven directly or through a gearbox; whether they should be tractors — pulling the aircraft along — or pushers; and whether they should be mounted on the wing or on the tail, for example,’ said Peacock. So each company will develop its own demonstrator engine, and collate the results from each.

Propellors sound old-fashioned, but Peacock stressed that, from the engine makers’ perspective, this is unbroken ground. ‘Nobody has ever developed a contra-rotating propellor that makes money and meets noise criteria. And from Rolls-Royce’s perspective, it’s all relatively new. The last turboprop we developed on our own was in the late 1950s, although we were involved in the collaborative effort to develop the turboprops for the Airbus A400M military transport. But for whole engines, our engineering capability in that area is all gone.’

Other engine technology candidates include geared turbofans, a modification of jet engines where the fan component — the rotating blades at the front which pull air into the engine —rotate more slowly than the turbine to reduce noise. This involves running the fan via a geabox, so both components can rotate at their optimum speed.

Peacock hopes the new engines could enter service quickly. ‘Clean Sky is a seven-year project, but it needn’t be another seven before the engines enter service. The most optimistic timescale to develop a commercial engine is three years, as long as there are still concerns about fuel costs and the environment to drive development. And we think that those concerns will still be there.’

But engines are not the only contributors to fuel economy and emissions. Airframe manufacturers are also contributing, with the main focus on the wings. While these develop the lift needed to keep the aircraft flying, they also generate turbulence and drag, which slow the plane down. For Clean Sky, the research is focusing on developing ‘smart fixed-wing aircraft’, where the wings will be able to adjust the flow of air around them to minimise the amount of power needed to fly.

There is nothing new about adaptive wings — the Wright Brothers’ plane used ‘wing-warping’ for take-off lift and to steer the plane. But Jens König of Airbus, co-ordinator of the smart wings ITD, said the new wings will be able to sense the air pressure and turbulence over many parts of the wings, and manipulate the airflow. ‘We want to create the beneficial load conditions and economic conditions, that is, to create an airflow situation on the wings so that it uses less fuel,’ he said.

Originally, the ITD partners hoped to design wings with no moving parts, which would affect airflow using suction and air-blowing components in various parts of the wing. ‘But then we realised that we needed a two-stage process,’ said König. ‘For the first stage, we’ll have a passive approach, where we need mechanical moving parts to control the flow. The second phase will then look further, using the knowledge we gain from the first stage, to give active flow and load control with the suction and blow-out elements.’

The first phase models are likely to use components that look similar to current wings, but have different jobs. The wing will also look different — low drag configurations could look like a current wing facing backwards, with the leading edge perpendicular to the aircraft fuselage and the trailing edge swept forwards.

‘We want to create better lift, and adjust the flow to change the lift/drag ratio, but we also want to look much more precisely at airflow to maintain a pressure distribution, produce a beneficial flow characteristic, and give us the most efficient load on the wing in cruise flight,’ said König. ‘So the devices we’ll use for this will look similar to the flaps and ailerons on current aircraft. We can’t discard the functionality they have; they’ll both create lift and adjust pressure and load on the wing.’

The JTI approach is optimistic and ambitious, trusting companies to work together on projects where they would normally compete. But environmental goals are often said to require a large, international collaboration, and Clean Sky could be a role-model. ‘It gives us the collaborative approach and the united front we need, but it also acknowledges the need for flexibility,’ said Peacock. ‘It’s impossible for us to know what the best approach or the key technology will be five years before we test it. The world will change and so will its needs.’