Passenger jets get a radical green makeover
If passenger jets were a different shape, they would produce far less CO2. Danny Bradbury meets the high-fliers who are leading a green revolution at 35,000ft
Wednesday 14 February 2007
It's 55 years since the De Havilland Comet entered service as the world's first purpose-built jet airliner, and as the international media were treated to a test flight aboard the Airbus A380 a week ago, it was clear that travelling at 30,000 feet in a metal tube with wings can still leave us awestruck.
But a decade or so from now, our aircraft could be radically different. Sophisticated computer controls and new design concepts are producing something that looks more like a manta ray without the tail. In March, Boeing will begin test flights with a scaled-down X-48B, an aircraft that almost does away with the fuselage altogether and makes it part of the wing. The aim is to slash fuel consumption and carbon emissions.
With a conventional aircraft, the wings form a small part of its total bulk. The passenger cabin in the middle is dead weight and drags the plane down. Instead, with a blended-wing concept, the body becomes part of the wing. The result: a fuselage that carries passengers or cargo, while also helping to lift the aircraft. "If you can get lift from the whole surface, you get less drag, so that reduces fuel consumption and emissions," explains Keith Mann, chief executive of the Royal Aeronautical Society.
Aircraft designers have been working on these "flying wing" designs since at least the 1930s. The most famous example of a flying wing design is the B-2 Spirit Stealth Bomber. There would be more space inside the new blended-wing aircraft than there is in a B-2, which was designed primarily to avoid reflecting radar signals. This increased volume makes the designs ideal for transporting masses of cargo or large numbers of passengers.
The problem with flying-wing designs is that traditionally they have been difficult to keep in the air. "Without sophisticated electronic controls, the thing would fall out of the sky - you just couldn't control it," says Mann. A conventional plane has movable parts on both the tail and the wings to keep it stable. Without a tail, the whole back of the wing becomes an array of "elevons" - hinged surfaces doing an array of jobs that tweak the aerodynamics to keep the craft aloft. The X-48B has 20 of them - far too many for any pilot to handle manually. So, complex software must be used to translate the actions of the pilot into complex manipulations at the plane's rear.
The Boeing prototype was built primarily to test that software and the controls. "The big research challenge is in the flight-control area," explains Norm Princen, chief engineer on the X-48B project at Boeing Phantom Works, the research arm that is developing the craft. "How do you make this aircraft that doesn't have a tail as robust and safe as a conventional plane?"
Increasingly, pilots rely on computers to do much of the flying, but blended-wing body designs take this to another level, which might worry holidaymakers who fret about reaching their destination in one piece. They shouldn't worry yet: Boeing is initially considering this craft for the military, rather than for civil aviation. It could be carrying cargo and weapons for the US military within 15 years, says Princen.
The project started out as a collaboration between McDonnell Douglas and Stanford University to test blended-wing body designs for a commercial craft that would carry 800 passengers. In 1997, when Boeing and McDonnell Douglas merged, the objectives changed. The US Air Force is laden with ageing aircraft, says Princen, making it a perfect customer. "This is radically different from other commercial aircraft and the risks are huge. It's easier to develop it as a military aircraft, and prove it there."
Nonetheless, the civil industry needs this kind of development. The negative impact of air travel on the environment has been well publicised. The Intergovernmental Panel on Climate Change (IPCC) put the contribution of aviation to global warming at about 3.5 per cent in 1992. It presents varying scenarios for future emissions, drawing data from different organisations.
No wonder the Advisory Council for Aeronautics Research in Europe has asked the industry to halve its CO2 emissions by 2020 (meaning a 50 per cent cut in fuel consumption). It has also requested an 80 per cent cut in nitrogen-oxide emissions.
Aircraft are getting more efficient - the IPCC predicts an annual reduction in fuel emissions per aircraft of 1.4 per cent between 2000 and 2040 - but that's more than offset by the 6 per cent per year increase in passenger numbers, argues Peter Lockley, who works in policy development at pressure group the Aircraft Environment Foundation. "So this [new aircraft] sounds good in principle, but it won't be flying in time to save the planet," he warns.
But that's not going to stop the developers. Boeing was also a partner in a Cambridge University and MIT project to develop a blended-wing design. Their project, the Silent Aircraft Initiative, proposed using engines that could be adapted in flight to be more efficient both at take-off and while cruising. The plane would deliver about 149 passenger miles per UK gallon - that's five miles more than a Toyota Prius. But this is still a concept and the researchers face many challenges before it could fly commercially.
Aside from complex software-controlled flight, there are other challenges facing commercial blended-wing body designs. The lack of windows is one problem. Another is that the further people sit from the centre, the greater the G-force they would experience as the plane turned.
There may end up being fewer passengers in a commercial blended-wing plane than in the initial McDonnell Douglas project, says Fay Collier, principal investigator for the subsonic fixed-wing project at Nasa (which partnered on both Boeing's X-48B and the Silent Aircraft Initiative). "Other developers are looking at 200 to 225 passengers - that range seems sensible," he says. That figure, more in line with today's airliner capacities, could make it easier to locate passengers closer to the centre and reduce discomfort.
But with Boeing ruling out commercial blended-wing planes for 20 years, civil aviation needs to explore other ways to become greener. These include the use of carbon-composites, which are lighter than current materials. Beyond that, more sophisticated engine technology and adapting surfaces to reduce drag could produce further savings, while the use of biofuels is another option.
Virgin has been experimenting with towing aircraft to taxiing positions at airports, hoping to save 2 per cent on fuel per flight. And research being done in the US and Europe to use GPS, rather than ground-based radar, for air-traffic control could yield major savings by routing aircraft more efficiently. Such gains may not save the planet on their own, but they might help us to adjust our downward pitch a little.
The future of flight
Boeing 787 Dreamliner
Up to 50 per cent of the main structure of Boeing's next-generation airliner will be made of carbon composite, saving up to 20 per cent in fuel consumption over similar aircraft today. Due to enter service in May 2008.
Silent Aircraft Initiative
A blended wing-body design that can accommodate up to 215 people, this craft is still in its conceptual stage. The researchers responsible for its development have emphasised noise reduction and fuel efficiency in their design.
A Boeing Phantom Works project, the Pelican would have been the largest air-transport plane in history with a 500ft wingspan. It would have flown just 20 metres above the sea and taken advantage of the ground effect, a phenomenon that drastically reduces drag. The design was unveiled in 2002, but it has not been built.
Twenty-five per cent of Airbus's giant plane is made of carbon composites, giving a fuel burn of 2.9 litres per customer per 100km - equal to a small diesel-engined car. Due for delivery in October.
Helios and Pathfinder
These solar craft started as Nasa research projects in the 1980s. These flying solar panels travel almost 100,000 feet above the earth, staying aloft for weeks or months, serving as communications relays. Helios was destroyed during a test flight in 2003.
This bizarre-looking concept from 2000 had an extra-tall fuselage to accommodate spherical tanks of liquid nitrogen that would have been used to power the craft. It takes around four times the volume of liquid nitrogen to deliver the energy created by burning kerosene, hence the need for extra storage space. The EU-funded project didn't get off the ground.
Dirigibles are making a comeback - slowly. Designed for high-atmosphere communications, but also delivering large payloads over long distances, these ships are mostly sightseeing devices, but developers hope that they could one day be used to transport heavy loads across the Atlantic in 72 hours.
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