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Green sky thinking: Eight ways to a cleaner flying future
22 February 2007
From issue 2592 of New Scientist magazine
Bennett Daviss

Enlarge Aviation's growing impact

Enlarge More people will travel by air

Enlarge Strut-baced wings

Enlarge Laminar flow control

Enlarge Fast forward

Enlarge Flying wing

Enlarge Open rotor engines

We love jetting off to faraway beaches, and the occasional business junket or foreign city break is hard to resist. Sure, we fret about the greenhouse gases and other pollutants these flights generate, but we quickly put it to the back of our minds. After all, politicians round the world seem to say that it's OK to fly.

In a television interview in January, for instance, UK prime minister Tony Blair suggested we don't need to give up flying since scientists and engineers are developing new fuels, engines and airframes to mitigate its impact. "What we need to do is look at how you make air travel more energy-efficient," he said. "If you use the science and technology constructively, your economy can grow, people can have a good time but do so more responsibly."

It's a view shared by many politicians in the US, which represents a third of the world's market for commercial flights. But with so much at stake, are such reassurances enough? On the one hand here is an industry enjoying massive growth, with ever more of us zooming off on ever cheaper jaunts. On the other is the undeniable fact that flying produces huge quantities of greenhouse gases and other pollutants. So can technology really turn aviation green? More importantly, can it do so before it's too late?

The aviation industry itself seems remarkably sanguine. According to the International Air Transport Association, today's jet engines are around 40 per cent more fuel-efficient than those designed in the 1960s, and engineers have almost eliminated pollutants such as soot and sulphur from jet exhaust. Advances like these, it promises, are set to continue. The Advisory Council for Aeronautics Research in Europe (ACARE), for instance, a body made up of representatives from the aviation industry, government and academia, has launched a strategy that includes halving carbon dioxide emissions and reducing nitrogen oxide (NOx) emissions by 80 per cent.

But before you conclude you can enjoy those complimentary nuts with a clean conscience, take a closer look. Around 85,000 commercial flights take off each day, and this number is predicted to double by 2050. Despite healthy advances in fuel efficiency over the past 30 years, experts agree that further improvements will be far more modest. The problem, according to Dennis Bushnell, chief scientist at NASA's Langley Research Center in Virginia, is that the aviation industry is mature, and conservative to boot. Much of today's aerodynamics research is a "sunset endeavour", he says. "There is not much left to gain except by the glacial accretion of a per cent here and there over long time periods."

The result is a widening disparity between the air industry's growth - over 5 per cent annually - and the projected improvement in jetliner fuel efficiency, which is nearer 2 per cent each year. Even ACARE sets no firm timetable for its goals, saying only that the cuts in emissions will come sometime beyond 2020. Reaching them will require "substantially more output" from aeronautics researchers and "important breakthroughs" in technology.

So where will these breakthroughs come from? Not from conventional aircraft design, warns Bushnell. If the industry wants to continue to grow but grow green, it will have to make some radical changes - and take some radical risks. What we need is little short of a design revolution.

One thing that will be hard to shake off is our addiction to air travel, which of course is fuelled by kerosene. Jetliners burn about 130 million tonnes of the stuff each year. A single flight across the Atlantic can guzzle about 60,000 litres - more fuel than an average motorist uses in 50 years of driving - generating around 140 tonnes of carbon dioxide, along with 750 kilograms of NOx. At around 10 kilometres up, where most airliners fly, NOx is very efficient at creating ozone, which at these heights helps to warm the planet. Additionally, water vapour in the exhaust creates contrails that act as seeds for cirrus clouds, reflecting heat back to Earth.

The net result, according to the Intergovernmental Panel on Climate Change, is that pollution from high-flying jets is up to four times as damaging to the environment as the same amount released by chimneys and exhaust pipes at ground level. Yet while other polluters are busy cleaning up, the aviation industry's continued expansion is set to transform it into one of the largest single contributors to global warming.

Trees to the rescue

That's not to say the problem is being ignored. Business is booming in the carbon offsetting trade, for instance. If you plan to fly, you can pay any of a growing number of offsetting companies to plant trees or invest in green-energy projects. The idea is that by storing carbon in forests or by building wind turbines, they can offset your personal share of the flight's greenhouse emissions. Carbon-neutral fuels are also being explored. Last September, Richard Branson, owner of the Virgin airlines, revealed plans to invest $3 billion in developing ecologically friendly plant-based jet fuel. Unfortunately, though, both carbon offsetting and biofuels are less than perfect solutions (see "Frugal flying").

So what are the aircraft manufacturers doing? Until now, their main motivation for R&D has been to reduce operating costs. They have gradually improved the fuel efficiency of jet engines, while lightweight alloys and composite materials for fuselage and wings have increasingly helped aircraft fly further on a tank of kerosene. Change has been incremental, however, and each small advance has been driven more by short-term profit than the pursuit of more energy-efficient technology. Moreover, mergers between manufacturers have reduced competition between designers.

These days, Bushnell says, manufacturers have little incentive to include cutting-edge ideas in their creations. "It's terra incognita," he says. "Dragons be here." As a result, the latest jetliners from Airbus or Boeing are essentially identical to planes developed in the 1950s.

Yet there is no shortage of innovative ideas out there. In some cases they have even made it to flight tests. Take drag control, for instance - a technique that improves efficiency by reducing friction between a plane and the air. For minimal friction, the thin layer of air closest to the surface of an aircraft should flow smoothly - something engineers call laminar flow. In practice, however, this boundary layer flowing around a jet's wings can easily become disturbed and peel away from the surface. This creates turbulence that can account for up to 40 per cent of a plane's total drag.

To eliminate this, engineers have investigated an idea called laminar flow control. Put tens of thousands of tiny holes along the top of an aircraft's wings and a fan inside can suck the disturbed boundary layer back towards the wing. This removes the fuel-wasting turbulence, leaving a smooth flow in its place (see Diagram).

From the late 1970s onwards, the aircraft industry worked hard on laminar flow control, from theory to flight tests, to the point at which the technique could reliably reduce drag by up to 20 per cent on everything from fighter jets to airliners. But work halted in the 1990s when fuel prices dropped. "We stopped doing the research because the cost of installing and maintaining the suction system didn't pay for itself over the life of the plane," Bushnell says. Complications such as the need to clear dust, insect remains and ice from the holes increased the cost of the system.

Other forms of drag control might offer even greater benefits, but have received little attention from the aviation industry. One intriguing idea aims to combine laminar flow control with a specially modified fuselage to slash a plane's drag.

The idea was conceived in the 1960s by Fabio Goldschmied, an engineer working on submarine propulsion for the US navy. He calculated that by sucking water in through a slot towards the stern of a moving submarine and blowing it out the back, the turbulent water flow in this region that causes much of the drag on the hull could be eliminated.

In the 1980s, NASA asked Bushnell to look into the idea, and after talking with Goldschmied he saw intriguing possibilities for applying the idea to aircraft. "I never worked out the numbers or studied it quantitatively," Bushnell says. "It was just an informal modelling of what Goldschmied appeared to be doing." At Bushnell's prompting, though, Goldschmied developed the concept and constructed model fuselages that he tested in a wind tunnel. Using a specially shaped conical rear fuselage and a slot much like the one he suggested for the submarine, he discovered he could almost halve the drag on an aircraft fuselage. In a 1987 conference paper, he suggested this could have a remarkable effect, reducing an aircraft's power requirement by up to 60 per cent during the important cruise phase of a flight.

Goldschmied died in the early 1990s, but he now has a disciple in David Birkenstock. A commercial pilot in Virginia, Birkenstock has spent more than a decade investigating the concept.

He envisions a jetliner with a small but key alteration. In a conventional passenger jet, the rear of the fuselage tapers to a point. In Birkenstock's vision, that section becomes a curved-sided cone rather like a lampshade (see Diagram). Just ahead of the cone, girdling the fuselage, is a slot. Here a fan sucks in air streaming past from the main body and expels it from the end of the cone. This design maintains a smooth airflow across the rear of the fuselage, eliminating turbulence and sudden changes in air speed that create drag. Expelling the air at the rear also adds thrust.

Faster and further

Despite the weight of the fan, Birkenstock suggests that jets fitted with his "aerodynamic engine" could take off from shorter runways, climb higher faster or carry heavier payloads than conventional aircraft, all for the same amount of fuel. "On longer trips, the savings should be significant," he predicts.

Birkenstock hopes to test this idea on a modified pickup truck, and then on a small aircraft. Unfortunately, Birkenstock's pickup is still parked and it isn't likely to move until he's found out a bit more about the theory. "I've mentioned the concept to a number of engineers, even some who work at Boeing. They tell me to talk to experts on the idea." Trouble is, there aren't any. "It's the position of the profession that aerodynamics has discovered all the important ideas," he says. "That's why Goldschmied's work lies dusty on a shelf in an engineering library."

That also explains why other design ideas are going begging even though they promise to raise fuel efficiency significantly. A three-year study at the Virginia Polytechnic Institute at Blacksburg, for example, has looked at bracing an aircraft's wings using struts - a design that harks back to the 1920s (see Diagram). The main aim is to cut the weight of the craft. To withstand the forces of high-speed flight, jetliner wings are built to be strong, and that means heavy, so a plane burns more fuel just to haul itself through the sky. The Virginia Tech researchers calculate that adding a support strut from the belly of the fuselage to the wing means designers can cut the weight of the wing by two-thirds without compromising its strength or the lift it generates. That could improve fuel efficiency by 25 per cent.

Bushnell sees strut-braced wings as a gateway to a host of pollution-cutting benefits. Bracing would allow engineers to lengthen wings without adding appreciable weight and help reduce the impact of "lift-induced drag", which is caused when high-pressure air under the wing meets low-pressure air above, creating a backward-sucking vortex at each wing tip. Placing the engines at the ends of the wings would suck in some of this turbulent air, further reducing drag. You could even widen the struts so they behave like wings, generating extra lift. Biplanes, it seems, might get a second shot at the big time.

Though radical ideas like struts are still languishing, there are signs that the industry is waking up to the need for change. Boeing, for example, has been reviewing a variety of fuel-efficient technologies, including the propfan. Last October nine European aircraft companies announced a ¬1.7 billion initiative called Clean Sky to develop greener aeronautics systems such as better engines. The following month, a consortium of university engineers and aircraft manufacturers unveiled the results of the Silent Aircraft Initiative, a project to design an airliner that is much quieter at take-off and landing. The result also turns out to be surprisingly fuel-efficient.

Engineers call it a flying wing, or "blended wing body". Gone is the familiar cylindrical fuselage (see Diagram). Instead, the craft has a pair of thick, swept-back wings with the engines embedded inside. The tail is also gone, replaced by gyroscopes and raised "winglets" that help keep the plane stable. Rather than flaps, the wings are also equipped with a moveable leading edge that maximises lift. The result is a streamlined plane that is about 25 per cent more fuel-efficient than conventional airliners.

It's not a new concept. US aircraft maker Northrop tried a similar design in the 1940s, but without normal elevators and rudder it proved unstable. Flaps on the wing's trailing edge can help counteract this but are aerodynamically inefficient. A more elegant but complex solution - adopted in the Silent Aircraft design - is to use raised wing tips and tailored wing contours as stabilisers that act in the same way as a conventional tail.

Constructing the cabin space is tricky too. The interior of a conventional fuselage is cylindrical, a shape that is easily strengthened to withstand pressurisation at high altitudes. The cabin space of a flying wing would be a more complex shape, making it harder to construct from lightweight composite materials. Even the smallest weakness could spell disaster.

Tom Hynes, an aeronautical engineer at the University of Cambridge, foresees other hurdles that might put passengers off. "Will people accept fewer windows?" he asks. Then there's the quality of the ride to worry about. When a conventional airliner rolls from side to side in turbulent air, passengers in the narrow, cylindrical fuselage move relatively little. But with passengers spread out across a cabin up to 20 metres wide, the slightest movements could leave those furthest from the centre feeling like they're riding a roller coaster.

Most importantly, it would take $10 to $15 billion and more than a decade of testing to develop such a plane. It's not clear who would be willing to spend this kind of money on a new jetliner, or how quickly it could be developed. "What happens if it doesn't work?" Hynes asks. "And who takes that risk?"

The US air force seems interested - in a military version, at least. It is already working with Boeing to develop a flying wing called the X-48B. The design has passed wind-tunnel tests, and last November a small-scale prototype with a wingspan of 7 metres began flight tests to assess its stability.

Though the X-48B might end up as a bomber rather than an airliner, the flying wing is clearly beginning to look like part of the aviation landscape. Even then, the team behind the Silent Aircraft Initiative is taking no chances: to ensure their design is not too radical for the risk-averse world of commercial aviation, they have drawn up a second, less ambitious version that includes conventional engine pods sitting above the wing rather than inside. Fuel efficiency might not be so good, but it's a compromise that the airline industry might find easier to stomach.

Without tighter emissions controls in force, this kind of compromise could be the only way to get ideas like the flying wing to the departure gate. "These are high-risk technologies," says MIT-based engineer Zoltan Spakovszky, joint leader of the Silent Aircraft design team. "It will take time to evolve them." Sometimes even revolutionaries have to embrace a bit of old-fashioned evolution.

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Frugal flying: Do we have to wait for new types of plane to tackle aircraft emissions? CONTROL GROWTH OF AIR TRAVEL Airliners are not covered by the Kyoto protocol, the international agreement to control emissions that contribute to climate change. However, in December European Union environment commissioner Stavros Dimas announced that from 2011 airlines flying in the EU will have to pay for the carbon they emit. The revenue this raises could be used to encourage other industries to cut emissions. Even if that does not happen, the cost could mean higher ticket prices, which might restrain the industry's growth. This strategy is politically sensitive. Airlines on both sides of the Atlantic have responded angrily, threatening to take the EU to court. On the other side, environmental campaigners complain that the measures don't go far enough. Part of the problem is one of accounting: should you count emissions in Europe from a plane that took off in the US? Or do you count them at the airport it flew from? Others argue that increasing taxation won't stop us flying. Economist Jan Bruekner at the University of Indiana, Bloomington, studies the influence of the commercial aviation industry on the US economy and calculates that an emissions tax would reduce US air travel by less than 10 per cent. ELIMINATE OIL-BASED FUEL Aside from the challenge of creating the expensive infrastructure to produce and distribute alternatives to kerosene such as biofuels or hydrogen, the fuels themselves create problems. Take hydrogen: it doesn't generate carbon dioxide, but it provides only one-quarter as much energy as the same volume of kerosene, so the fuel tanks on a hydrogen-powered jetliner would have to be four times the size to carry the plane over the same distance. This would create design difficulties, according to a 2006 study by Boeing. A hydrogen-fuelled Boeing 737 would require so much insulation, as well as pressurisation equipment to keep the hydrogen flowing, that the fuel could no longer be stored in the wings. Instead, the tanks would have to sit in the fuselage, necessitating a wider cabin. The extra drag this creates would reduce the aircraft's fuel efficiency. The other big problem is that hydrogen produces about three times as much water vapour as kerosene when it burns. Above 9000 metres - where airliners spend most time - this water would create larger than normal contrails, which in turn form cirrus clouds that contribute to global warming. Although flying lower could solve that problem, the plane would then be vulnerable to bad weather as well as use more fuel. The picture is even worse for plant-based biofuels containing ethanol, even though they might be carbon neutral. They weigh 60 per cent more than kerosene, on top of which you need 64 per cent more volume to get the same energy. An ethanol-fuelled 737, for example, would need a 25 per cent larger wing and engines with 50 per cent more thrust just to get airborne, says Boeing. The stuff also freezes at low temperatures, so fuel tanks would need to be heated. A 2002 report by the UK Royal Commission on Environmental Pollution suggests that because of these problems, planes will continue to rely on kerosene for at least 40 years. OFFSET OUR EMISSIONS A growing number of airlines and travel companies have teamed up with businesses that enable air passengers to pay a little extra on top of their tickets to help fund green projects. The idea is that by planting trees or investing in renewable energy, you will cancel out your share of the carbon dioxide released on a flight, eliminating any damage to the environment. Last year an estimated 1.5 million people in the UK paid to offset a flight. However, environmental campaigners argue that it is a distraction from the real challenge of reducing emissions. Also, the industry is unregulated, so amongst other problems it's not clear how you quantify the amount of carbon locked in a tree, much less guarantee that it stays there. CHANGE THE WAY WE FLY Immense amounts of fuel are wasted as aircraft queue to take off at airports, or in the skies waiting to land. To help reduce this, Virgin is experimenting with electric tractors to tow planes from the terminal gate to the runway before the engines are switched on. The airline estimates it could save up to 2 tonnes of fuel per flight. Far more could be saved by ending the practice of stacking incoming airliners in holding patterns while air-traffic controllers find a slot for each one to land. Instead software could calculate each plane's most efficient, unbroken descent from cruising altitude to the tarmac, beginning as much as 2 hours away from landing. Program this plan into a plane's flight controls and you can save an estimated 400 litres of fuel per landing. The International Air Transport Association predicts airlines could save 12 per cent of their global CO2 emissions if air-traffic control systems were more efficient.

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