On the flightpath to a friendly footprint

A host of technologies are set to reduce civil aviation’s environmental impact. Stephen Harris reports

Aircraft pollution is in some ways a relatively small problem compared to that from cars. The aviation industry is responsible for just 2 per cent of global man-made carbon emissions whereas road transport contributes about 16 per cent. Yet the low-carbon revolution has already begun on the roads with the launch of fully electric cars and hydrogen-powered buses. A battery-powered jumbo jet, however, remains a fantasy and aviation emissions are rising thanks to rapid industry growth.

NASA’s blended-wing body N3-X concept plane

So what technologies could the civil aerospace sector use to help improve its environmental outlook? Greener propulsion systems are obviously essential for reducing carbon emissions, but there are also big potential changes in the design of aircraft and, crucially, how they
are manufactured, that the industry believes hold significant promise.

In particular, it’s not just the technologies themselves but how they are integrated on the aircraft that will make a difference, according to Iain Gray, director of aerospace at Cranfield University and former managing director of Airbus UK. “We continue to make aerodynamic gains through things such as new wing architectures, and lots of people forget the inefficiencies that are still associated with aircraft design at low speed with the landing gear down, for example,” he said. “But the biggest thing we can do is look at the integration of the power plant and the airframe itself.”

This idea that engines could be built into the wings of an aircraft rather than hanging below them in order to create a more aerodynamic shape (and so require less fuel) is on the long-term horizon of major companies such as Airbus and Rolls-Royce. The two firms are collaborating on a concept study known as E-Thrust to design a set of electric fans distributed in clusters along the wing span. Energy would then be generated from an advanced gas power unit that also charges the aircraft’s batteries. On descent, the fans could even be used as wind turbines to harvest more energy.

The Rolls Royce Advance concept features carbon titanium blades

“If it becomes unfeasible to put the engine under the wing you can go for a different wing design or eventually distribute more fans around the vehicle,” said Phil Curnock, chief engineer for future programmes in Rolls-Royce’s civil large engines business. “Battery technology weight has to come a long way before you can get to fully electric aircraft,” he added. “This makes it easier to go down a hybrid or electric route.”

Such a concept is still several decades away from being seen on production aircraft. In the meantime, Rolls-Royce is focusing on its Advance engine design, which will introduce a carbon titanium composite fan to reduce weight and change where most of the work is done in the engine to make it more efficient. After this will come the UltraFan, which, with its power gearbox and variable pitch blades, Rolls-Royce hopes will deliver a 25 per cent improvement in emissions compared to the first generation of its Trent XWB engine by 2025.

There’s another key technology that’s set to have a major impact on the environmental footprint of aircraft and that’s additive manufacturing or 3D printing. Although still in its early stages as a production tool, the technology can already help reduce the weight an aircraft carries by enabling components to be designed in a way that could never be manufactured with traditional techniques, for example, built from a lattice or honeycomb structure.

“We can make components significantly lighter but with the same performance,” said Billy Wu, leader of the additive manufacturing research network at Imperial College London. “By reducing weight you get fuel savings over the life of the plane. So even though the capital costs are higher you save money in the long term.”

Every gram of weight reduced contributes to fuel savings, so even being able to reduce the weight of a belt buckle from over 100g to around 70g using 3D printing is important. The Airbus A350 XWB contains more than 1,000 3D-printed parts, many contributing a small but important weight reduction.

Ultrafan - Rolls-Royce
The Rolls Royce Ultraform concept offers a 25% reduction in fuel burn

Additive manufacturing is also reducing aerospace’s environmental impact by reducing the amount of material that is wasted during manufacturing. “So much of aerospace manufacturing is about buying large lumps of aluminium or titanium and turning 95 per cent of them into swarf,” said Nick Jones, technical manager at 3D printer manufacturer Renishaw. “With additive manufacturing, we start with a powder, turn what we need into the product and the rest can go back into the machine.” The company recently joined an Airbus-led project known as WINDY to develop expertise in 3D printing wind components.

This kind of research will enable the next step in aerospace additive manufacturing: to start using 3D printing for safety-critical components. This has the opportunity not just to make these parts lighter but could also enable manufacturers to program greater functionality into components and materials to improve the way they work. For example, printing turbine blades that contain tiny ventilation channels would allow more air to be run through the components. This would increase the protective cooling effect you get from passing
a film of air across the blades and so raise the temperature at which the engine can safely operate, meaning greater efficiency and more fuel savings.

The E-Thrust is an electrical distributed propulsion system concept for lower fuel consumption, fewer emissions and less noise.
The E-Thrust is an electrical distributed propulsion system concept for lower fuel consumption, fewer emissions and less noise.

There’s a final set of technologies that could reduce aviation’s environmental impact that aren’t even located on the aircraft but involve improving the airspace and airport infrastructure. Using greater autonomous guiding and planning technology could enable air- traffic control to schedule aircraft landings and take-offs much more efficiently, reducing the time aircraft spend circling an airport wasting fuel. An aircraft’s landing position and location could even be optimised to reduce the amount of runway taxiing needed.

A further advance considered by Airbus in its recent Smarter Skies concept study is the idea of using power-assisted take-off not unlike that used by some aircraft on naval carriers to effectively catapult the aircraft into the sky, slashing the amount of fuel needed for ascent. While this is perhaps a more outlandish idea, it highlights how there are energy savings to be made in every part of the aircraft and the flying process. In lieu of a huge breakthrough in battery technology, the aerospace industry will ultimately need to look at all of them if it wants to make its business sustainable.