Air pressures

Peter Chivers, research head of Airbus, explains why rising fuel costs and environmental demands are forcing a re-think on aircraft design. Stuart Nathan reports.

Close your eyes and picture an airliner. You will probably see something with a cylindrical fuselage sitting on top of the wings, a vertical tailplane with horizontal stabilisers, and two or four jet engines slung under the wings. Most civil aircraft have looked like that for the past three decades, and for good reasons.

But the new circumstances facing the aviation industry, stemming from the rising fuel prices that now dominate airlines’ costs like never before and the overriding need to reduce carbon emissions, may be about to change airliner design in radical ways.

Peter Chivers, who heads research and technology strategy for Airbus, Europe’s largest aerospace manufacturer, has to look carefully at the finely-balanced priorities that dictate aircraft design, before he commits the company to this step into the unknown.

‘Aircraft are incredibly optimised, and not just in terms of their performance,’ he said. ‘They have to deal with very stringent safety laws, very long lifecycles, and many other topics that drive the design. The result of that is a finely balanced solution against a complex set of requirements, both in terms of the aeronautical engineering and the business case which they have to serve.’

But when one or more of these requirements changes significantly, that can be enough to force airlines to demand something different from their suppliers. ‘You can tip yourself away from a fundamentally known balance that you’re continually honing and improving — such as the familiar airliner architecture — to some kind of alternative solution.’

Leading aircraft manufacturers are now looking seriously at these alternatives and in Europe the search is embodied by two EU/aerospace industry co-funded research projects.

NACRE (New Aircraft Concepts Research), a four-year project that began in 2005, is looking at a wide range of options for technologies on civil airliners, from new types of engine to refined wing aerodynamics, both of which will improve fuel efficiency, and radical new architectures, such as huge long-haul blended wing designs and multiple fuselage options, which would allow more passengers to be carried for the same amount of fuel.

Many of the results of NACRE will feed into Clean Sky, a much larger project — €1.6bn (£1.3bn) compared with NACRE’s €30m — which aims to design new versions of many types of civil aircraft, and is now in its start-up phase (The Engineer, 30 June).

The changes being considered by NACRE are radical, including forward-swept wings and rear-mounted engines with open rotors, mounted above an H-shaped tailplane similar to World War II Lancaster bombers. But Chivers observed that similarly radical changes have happened before.

There is a theory that intrigues him, borrowed from biology, called evolutionary landscapes. In this, life-forms evolve until they reach the peak of specialisation in their particular niche of diet and environment. Then a change — in the climate, for example —will make life more difficult for the dominant life-form. Extinction follows, and another form of life will begin to evolve. The obvious example is dinosaurs giving way to mammals.

This has parallels in aircraft design. Many versions have been tried — from triplanes to biplanes to monoplanes, rear-mounted pusher propellers to front-mounted tractors, propellers to turbojets to turbofans, rear-mounted engines to under-wing designs. In each case, the design was optimised until it reached its peak, then another technology came along and displaced it.

‘And that’s where we’ve been for some time, optimising the evolution of that particular form,’ he said. ‘You reach a point where something causes you to trip away from that; whether it’s the environmental target you’re delivering towards, or something about the available constituents of the solution.

‘So if we look at where we are today, with massively increasing environmental pressures and the shift in emphasis from acquisition costs to fuel costs, that will change the emphasis on the aircraft.’

 


Designs being considered by NACRE include blended wings for long-haul, such as the NASA-Boeing X-48B experimental aircraft (above left) and forward swept wing designs with new engine types for medium-haul (above right)

Many people in the aircraft industry believe the standard airliner architecture is at its evolutionary peak, and the time has come to make one of those jumps. Chivers is not so sure. ‘I think in some areas, the potential for development is starting to level off and we need to look for more opportunities. Some of the concepts we’re looking at, both in NACRE and in Airbus, are evolutionary. Others are revolutionary.’

But caution is absolutely necessary, because of the huge costs involved. The more speculatory research in NACRE is relatively cheap; Clean Sky, which is geared more towards application and developing technology demonstrators, is more than 100 times more expensive.

‘You start to quantify the sort of benefits you might get out of taking those steps, and that helps you decide whether to invest all that extra effort required to build a bridge to where we might take aircraft design,’ said Chivers. ‘Building an aircraft is a phenomenally long process and phenomenally expensive, and if you make a dramatic configuration change, you just add to the cost, the time and the risk.’

Even for the huge rival aircraft manufacturers, Airbus and Boeing, the necessary investment is immense. ‘You don’t make a commitment to a new programme without huge confidence that you can deliver a solution, and obviously if you’re making a significant configuration change, you must have a lot of confidence that all the problems you’re going to face are addressable.

‘We could never make that step without a 100 per cent convinced view that we are going to reap the benefits from it. We bet the company on it.’

Chivers said the new constraints on fuel costs could change the way operators view their aircraft. At present, they favour the standard design because maintenance has represented one of their dominant costs.

‘So from an operator’s point of view, maintenance costs are staying still, but fuel costs are rising steeply. They might say, “OK, if you can do something that will add 20 per cent to the maintenance cost, but take 20 per cent out of the fuel cost, well, 20 per cent of the fuel is much more than 20 per cent of the maintenance.” So they’d be happy to have that.’

This means technologies that had been discarded by the aerospace industry because of their associated costs are now being reconsidered. ‘If we take something like a hybrid laminar flow wing, which reduces turbulence and improves fuel economy, one of the key reasons it had never progressed is that the maintenance cost, the system’s complexity and so on didn’t provide a business case for the operator versus the fuel savings it provides. Now, with the fuel costs swinging so heavily the other way, maybe — only maybe — the business case is more favourable.’

In other cases, the increasing power and decreasing cost of computing has helped resurrect old technology. ‘Open rotor engines were first developed in the early Nineties, but one of the challenges was that the aerodynamic skills and knowledge and the calculation methods were nowhere near mature enough to design an efficient and noise-acceptable fan. The technology has now moved on along a number of fronts, and it’s now believed those problems can be solved.’

But the required investment is so large that Clean Sky could not take place without co-funding from industrial partners and from the EU.

Chivers is quick to defend the use of public money in these projects. ‘It is a fundamental part of the European economy to have an aerospace industry,’ he said. ‘It’s a major, major wealth-creator for Europe, if you look at the number of jobs and the GDP generated throughout Europe. And I don’t just mean Airbus — our contribution to Clean Sky is less than a quarter of the total budget. We’re not the only ones who benefit.’