From the time commercial airplanes took to the skies, their design has stayed more or less the same: a long tube holding passengers and cargo, wings for lift, a tail to provide stability, engines burning copious amounts of fossil fuel for power.
Beneath their flying-cigar exteriors, today’s aircraft have undergone many changes, from advanced materials that make them ever-lighter and more accommodating, to increasingly fuel-stingy propulsion, electric controls for ease of flying and more aerodynamic design.
Carrying more passengers is crucial. By 2014, the International Air Transport Association expects there will be 3.3 billion journeys by air travellers, up from 2.5 billion in 2009.
Given soaring fuel prices and environmental concerns, that kind of growth requires innovation to make airplanes fly greener, leaner, quieter, safer and more efficiently.
“With these challenges, you need to work on a lot of elements at the same time,” says Ruben Del Rosario, manager of the subsonic fixed-wing project at NASA’s Glenn Research Center in Ohio.
Two years ago, NASA launched the program with research teams from Boeing, General Electric, Cessna, the Massachusetts Institute of Technology and Northrop Grumman.
Their mission? Make a passenger airplane for 2030 that will burn 70 per cent less fuel, give off 75 per cent fewer emissions and generate 71 decibels less noise than today’s jets.
Del Rosario, an aerospace propulsion engineer, calls these “stretch goals.” But he thinks they’re attainable. “We’re not violating physics.”
Now in the second phase of the project, NASA is investigating the designs and technologies that could make these future aircraft a reality.
The most radical configuration comes from MIT, where the fuselage is made from two tubes placed side-by-side. Nicknamed the “double-bubble,” it has a smaller tail and wings, but the larger belly produces its own lift, Del Rosario explains.
Other design suggestions have included blending a plane’s body and wings into a triangle shape; powering it with several small engines as opposed to a couple of larger ones, to reduce drag; adding little or no tail, with vertical control and stability provided by on-board computer systems, and making the outside with advanced materials and “nano-structures” that tailor themselves to be “aerodynamically optimal,” Del Rosario says. “The whole thing will be smarter.”
Advanced propulsion systems that save fuel tend to slow aircraft slightly, he says. However, with more precision in aircraft controls, “flight-management systems” will have them fly and land closer together.
NASA is also looking at aircraft that are “supersonic” (greater than the speed of sound), “hypersonic” (five times the speed of sound) and “rotary,” allowing for vertical takeoff and landing, which Del Rosario says is particularly promising for the 100- to 150-passenger market.
One of the best-known authorities in the rotary field is Canadian Paul Moller, who has worked since graduating from trade school in Calgary in 1958 on developing the volanter, which he calls “an aircraft that’s ‘streetable.’”
It’s been a long time coming. Moller, 75, of Davis, Calif., says aircraft design has benefited from new technologies, such as carbon-fibre materials and advanced avionics, which make his aircraft lighter and easier to control. “The sophistication is amazing.”
Indeed, current commercial aircraft are much more efficient than those of the past. In 1967, the first Boeing 737 carried 100 passengers about 2,775 kilometres. Today’s B737-800 can take almost twice as many passengers double the distance, while burning almost 25 per cent less fuel.
Canada’s entry into green aviation is Bombardier’s C-Series regional jet, due in late 2013. The narrow-bodied aircraft, with seating for 125 to 145 passengers, is a “clean-sheet design.” It’s all-new, says C-Series program director Sebastien Mullot.
The C-Series has geared turbofan engines, lighter materials in its wings and fuselage, fly-by-wire control, a roomier cabin and a high-tech flight deck. It is 5,500 kilograms lighter, is expected to burn 12 per cent less fuel and will emit 20 per cent less carbon dioxide and 50 per cent less nitrous oxide than other aircraft in its class.
Mullot says many aircraft still flying in the C-Series category are gas-guzzlers, dating as far back as the 1960s. Some are being “re-engined,” but they will lack other modifications.
The C-Series “noise footprint” is four times quieter, he says, due to a combination of the geared engines and improved aerodynamics, allowing it to fly outside of normal operating hours and into noise-sensitive airports.
Benny Pang, chair of the scientific committee of the Green Aviation Research and Development Network (GARDN), a Canadian network of centres of excellence focused on lowering noise and emissions in the aerospace industry, says the noise issue shows the interdependency of aircraft design. “Everything has to fit together.”
Pang says that Canada, which ranks fifth in the world in aviation projects, especially has innovative research in computational fluid dynamics, to design more aerodynamic aircraft shapes, and in the development of biofuels, which are critical to the industry’s future.
Many alternative sources of fuel are too heavy or not high enough in energy for aviation, says Pang. One promising biofuel GARDN is supporting research into is derived from an oil-rich plant that can be grown in Canada on land unsuitable for food crops.
NASA recently performed emissions testing on a renewable biofuel made from chicken fat in a jet engine. Del Rosario says the test showed the biofuel was cleaner-burning and released fewer pollutants into the air than conventional jet fuel.
Photos courtesy of Mark Drela and Adrian Pingstone.