Technology will make planes lighter, cheaper and more sustainable
By Neil Savage
If you could step off a current jetway onto an airplane in 2035, you might not notice a big difference from today’s aircrafts. The plane might be wider, the ride a bit smoother and the cabin quieter. You might get to your destination faster, and you’d know the flight had put less carbon into the atmosphere. But will the airplanes of tomorrow seem all that different?
“It depends how close you look,” predicts Mark Drela, an aeronautics engineer at MIT. “Superficially, I don’t think they’ll be much different, but in the details, yes, they will.”
For one thing, tomorrow’s planes will be made of new materials. For example, lighter and stronger composites that can even repair themselves when damaged will double as both the hull and a medium for sending and receiving data. The aircraft’s shape might also change, having a bent nose or widened fuselage, smaller wings and an oddly shaped tail. Its engines may even sit on top of the plane rather than beneath the wings. “I think it will be an evolution, not a revolution, but, yes, I think you’re going to see some new configurations out there,” says Edward Burnett, an engineer at the Skunk Works, Lockheed Martin’s advanced development program.
This computer–generated graphic shows what the DRELFGE laminar-flow wing glove looks like and how it will be placed on the G-III research aircraft. ©NASA/Ethan Baumann
All the major aerospace companies, as well as NASA, are working to develop future airplanes that are lighter and burn less fuel, and they’re relying on a variety of technologies—from computer modeling to 3-D printing to carbon nanotubes—to get them there. “Our underlying push is always higher, further, faster, cheaper,” Burnett says.
The D-8 Design
Drela is among several researchers who have proposed a new aircraft design to achieve these goals. For the past few years, he and his colleagues have been working on what they call “a double bubble.” In contrast to the existing body structure consisting of a single long tube, Drela’s D-8 is composed of two tubes opened up and pressed together, side by side. From the passenger’s point of view, that makes for a roomier cabin. The D-8 holds 180 passengers and is meant to replace the 737 for domestic flights, but unlike the 737, it has two aisles, which Drela says should let passengers get on and off more quickly.
The big advantage of the double bubble involves a plane’s aerodynamics, shifting more of the lifting power from the wings to the body. “The fuselage can carry a bigger fraction of the weight than it normally does,” Drela says. “That shrinks the wings.” The bottom of the plane also slopes up more sharply in the front, and the cockpit sits higher than the passenger cabin. That also increases the plane’s lift, allowing for further shrinking of the wings and making the tail smaller—with just two upward fins on each side of the fuselage, and a crossbar at the top.
The shorter tail and smaller wings weigh less, and as Drela says, “Weight is everything.” Plane weight has a nearly linear relationship to fuel usage. “You make the airplane 10% lighter overall, it will burn 10% less fuel,” he says. He estimates that the D-8 burns about 40% less fuel than the 737.
Other aspects of the D-8’s design also reduce weight. For instance, while the cabin is wider, it’s also shorter, which translates into less window space and a lighter structure overall. But perhaps the most striking part of the design is that the engines sit on top of the airplane, tucked in between the upward struts of the tail.
This configuration takes advantage of the same effect that Tour de France cyclists or NASCAR drivers benefit from when they tailgate the person in front of them, a technique known as “drafting.” Similarly, placing the engines in the stream of air coming over the body in front of them cuts down on drag and requires less energy to move forward. The body of the plane also acts as an acoustic shield for the engines, making the plane sound quieter on the ground. Drela expects the plane’s noise levels to be 71 decibels below the FAA limits. Quieter airplanes are sure to be more welcome by neighbors of municipal airports, giving airlines more flexibility in scheduling and cutting down on the number of connections to be made at busy hubs. And there’s another bonus to this design: it’s harder for a bird to hit the engine.
The Shape of Wings to Come
Another method for fine-tuning a vehicle’s aerodynamics is to create shape-changing wings. Today’s airplanes have mechanical flaps, hydraulically controlled, that extend out from the wing and take on different angles during takeoff and landing to adjust the lift or drag. All those moving parts add weight, and the air moving around the flaps’ sharp edges contribute to the noise.
NASA is flight-testing one morphing wing. Engineers at the Armstrong Flight Research Center in California have modified a Gulfstream-III business jet with wings designed by Flexsys, a company founded by University of Michigan engineering professor Sridhar Kota. The wings are made from a composite material—the company won’t be more specific than that—and contain internal actuators, which bend and twist the wings, and therefore adjust their aerodynamic properties, as needed, foregoing the flaps. The bend can be as much as 9 degrees in one direction and 40 in the other, and the trailing edge can twist at a rate of 30 degrees per second. These wings change shape for takeoff, landing and during flight to get the optimum lift for flying conditions at any given time, something today’s planes don’t do. “It’s very rarely that the aircraft flies at the optimum conditions you designed it for,” Kota says. “If you can only change the shape of the flaps, you can actually hit that sweet spot throughout the mission profile.” Flexsys estimates their wings could cut down fuel use by 12% and reduce landing noise by 40%.
Altering the aerodynamics of airplanes may also help them to fly faster, although not in the expected way. The world’s only supersonic jet, the Concorde, stopped flying in 2003, partly because the Federal Aviation Administration banned faster-than-sound flight over the continental United States due to the sonic booms. “Our ability to fly faster is actually limited more by the law than by the law of physics,” says Lockheed’s Burnett.
The boom comes from a shock wave formed when the fast-moving airplane forces air streams apart, then lets them slap back together after it’s passed. So Lockheed is working on designs to reshape the sound-wave caused by an airplane flying above Mach 1 by directing some of the energy away from the ground, and making the pressure differential between the airstreams less sharp. If they can make the boom less “boomy,” they might be able to overcome objections to the flights.
SACM: What do you see as the biggist technical challenges facing today’s airline industry?
Axel Flaig: Every 15 years, the air traffic is doubling, and we need bigger aircraft to transport so many people. At the same time, this transportation must be sustainable, and that requires reducing emissions and noise.
To deal with the growth in traffic and to refresh the current fleet, we estimate there is a need for 30,000 new aircraft in the next 20 years. Although we can now produce nearly 60 aircraft a month, we’ll need to significantly ramp up production. This is quite a demanding challenge for us and our suppliers, like the engine manufacturers.
Airbus cares about the passengers’ experience, not only on the aircraft, but also on the door-to-door journey. We have concerns about the time it takes them to get from home to the airport, and the time that they stay in airport and on flight. In the interest of the passengers, we want to make the whole system operate seamlessly.
SACM: What benefits can arise from overcoming these challenges?
Axel Flaig: We will be able to meet all of the different needs of passengers. Aircraft will be available for different options in economy and comfort. The flexibility is what we want to offer as more people will be able to fly, the way they want to fly. Airbus wants to ensure more sustainability, less delays and waste. Therefore, we will be reducing the fuel that is burned and the associated emissions and noise. With reducing fuel and emissions, we are aiming at two targets: airlines want to burn the least fuel possible, and we want to ensure that the environment is impacted as little as possible.
SACM: Please describe your most recent projects that address these challenges.
Axel Flaig: Our A380 is very efficient with fuel, burning only 3 liters of fuel per 100 kilometers per passenger. Also, it provides a very smooth ride and the noise level on board is so low that it’s very enjoyable.
We’ve also improved the efficiency of our A320 with the latest evolution of the engines and improved aerodynamics, which produce a 15% fuel savings. This taught us that even on existing aircraft, new technology can help an airline operate more efficiently. We call this incremental innovation.
There are other technologies we are working on for the future. For example, a bigger ratio between the diameters of the outside and inside flow of a jet engine makes it more efficient, and the next step will appear in engines in the next 8 to 10 years. We also have projects on hybrid propulsion—mixing a turbo-fan engine and electrical motors and some batteries. That’s in an early phase.
Other technologies we are developing relate to new wings and the way the air flows over them. Plus, we aim at making everything lighter. As future airframes become continuously lighter and we improve the aerodynamics and the engines, we’ll see efficiency increasing. For example, our A350 just entered service, and it’s 25% more efficient than the older aircraft in its segment. It’s hard to always improve by 25%, but we need to be close to that.
SACM: As you mentioned previously, Airbus cares about the passenger’s door-to-door journey. How would you see the future journey to the airport?
Axel Flaig: In 100 years, you will leave your house and enter an air taxi that is completely autonomous and holds around six people, and it will fly you over congested roads to the airport and provide an easy transition to your aircraft. If you live close to the airport, you could even take an air metro, an electric aircraft that holds maybe 100 passengers and takes you to a major airport, where you might take a transcontinental flight that carries up to 1,000 passengers. It doesn’t make sense to improve only the airport and not the shuttle to the airport.
Fighting Climate Change
Making planes lighter and more efficient saves fuel and reduces the amount of carbon dioxide they pump into the atmosphere. The International Air Transport Association has set goals to make airlines carbon-neutral by 2020, and by 2050 to cut emission levels to half those of 2005. The European Advanced Biofuels Flightpath, a joint project by manufacturer Airbus, various European airlines and jet-fuel producers, and the European Commission, is working on making fuel out of plant matter—such as wood chips, camelina and even used cooking oil—that can be produced sustainably.
Such feedstocks, says Frederic Eychenne, Airbus’s energy program manager, can provide hydrocarbon molecules that are then blended into Jet A1 fuel derived from petroleum. One method, for instance, heats the organic matter to upwards of 700 degrees Celsius in a controlled atmosphere until it turns into gas, which is in turn liquefied and made into kerosene. “The specification of this fuel is exactly the same as fossil fuel,” says Airbus spokesperson Sarah La Brocq. “The big difference is the source of the fuel.” The project aims to avoid taking away land that could be used to grow food, and plans to use whatever sources are available in a particular region as the feedstock. In China, for example, they might rely on used cooking oil, while in Australia they could turn to eucalyptus.
Like some of today’s cars, tomorrow’s aircraft could also gain efficiency through hybrid engines. In 2011, the first hybrid electric plane—developed by Siemens, Diamond Aircraft and Airbus—took off in Vienna, Austria.
Flying in a Material World
Burnett wants to go beyond today’s composites to materials than can perform multiple functions. If the skin of the airplane is going to contain carbon nanotubes to make it stronger, he says, why not also take advantage of the nanotubes’ conductivity and use the skin to transmit data from, say, the engine to the cockpit, rather than having bundles of wires running between bulkheads? Instead of bolting an antenna to the aircraft to communicate with the ground, why not use the body of the plane itself as an antenna? “How do I make one piece of the airplane do multiple things?” Burnett asks, focusing, as always, on reducing the vehicle’s weight. “If each item can do two things, I can carry half as many items.”
Nancy Sottos, a materials scientist at the University of Illinois at Urbana-Champaign, even envisions airplane skin that can heal itself, like wounded human skin. “When cracks occur to the structure, they’re usually very small [and] inside the structure. You can’t really see them or repair them at that point,” she says. Nonetheless, “That’s when we want to repair them.”
Sottos proposes lacing the composite with microcapsules containing a healing compound. Anything causing a crack would break the capsules, releasing that compound. Just what the compound is depends on the makeup of the composite. “If you’re trying to heal an epoxy part, your healing chemical should be epoxy based,” she explains. Though the system is generally aimed at repairing microscopic, internal damage, she’s used it to repair a hole as big as 9 millimeters.
In another design, a network of tiny tubes carries the self-healing compound, much like the blood vessels that run through the body. The healing can work repeatedly, meaning that parts can be replaced less often. “You’ll increase the lifetime, increase the safety,” says Sottos, though she expects it will take several more years to develop such skins.
Today and Tomorrow’s 3-D
One advanced technology that’s already improving planes is 3-D printing. Pratt & Whitney has been incorporating more 3-D printed parts into its engines. The company makes its parts with powders of nickel or titanium alloys, which are heated by a laser or an electron beam under computer control to produce a desired shape.
Rather than, say, pouring molten metal into a mold and then removing any excess material, 3-D printing allows an engineer to add one layer of material on top of another to get the desired part. The advantage of that, explains Pratt & Whitney engineer Lynn Gambill, is that you can base the shape of a part on what it needs to do, and not on what path the molten metal has to take through the mold, or on what kind of tool can reach inside the part to mill or cut it, so you reduce the amount of material used and, again, the weight. “The part looks different, but the function’s the same,” she says.
As 3-D printing develops the ability to print multiple materials into one part, engineers might be able to even build parts with, for example, internal wiring. “It does change the way you think about what a design needs to look like,” Gambill says.
Further into the future, airplanes may start to look very different. Airbus, for instance, imagines personalized pods within the cabins that can leave the plane after it lands to drive or fly passengers all the way to their homes. NASA is experimenting with scramjets, rocket-propelled craft that touch the edge of space and can make the flight from New York to Tokyo in two hours. Even technologies currently in development, though, are sure to make air travel more efficient, pleasant and environmentally friendly in the near future.