|Commercial aircraft have the potential to become dramatically cleaner and quieter in the next 25 years, but manufacturers will have to decide how far they want to push technology, and airlines must decide how much they are willing to pay for efficiency. The potential for improvement is outlined in studies completed for NASA by four design teams led by Boeing, General Electric and Cessna, the Massachusetts Institute of Technology and Northrop Grumman. The teams defined configurations to meet NASA’s aggressive environmental goals for an “N+3-generation” subsonic airliner entering service in 2030-35 (N is today’s aircraft). They then developed road maps to mature the required technologies. In parallel, Boeing and Lockheed Martin conducted similar studies into N+3 supersonic transports, defining configurations and developing technology road maps for a quiet, efficient high-speed airliner (see p. 43). The intent of all these studies is to help NASA shape subsonic and supersonic research within its resurgent aeronautics program. “The studies had three objectives,” says Ruben Del Rosario, principal investigator for NASA’s subsonic fixed-wing project. “Define a future scenario for 2030-35, develop advanced aircraft concepts for that scenario, and assess technologies and provide road maps for their development. Overall, the studies provide a clear path for research in aerodynamics, propulsion and materials.” NASA’s subsonic N+3 goals include reductions exceeding 70% in fuel burn, 75% in emissions and 71 dB. in noise relative to today’s aircraft. The teams struggled to meet all the goals in one design. Some proposed new aircraft and propulsion concepts. One got close to the goals with a conventional configuration, through the synergistic application of new technologies, but it cautions such an aircraft would be expensive. “Our N+3 strategy is to simultaneously address all the goals. Clearly, from the results, they fell short of addressing them all simultaneously,” says Del Rosario.
“Everybody was told they are stretch goals, but not beyond the realm of the doable in the time frame,” says Rich Wahls, NASA’s subsonic fixed-wing project scientist. “We tried to push them beyond their threshold of comfort. I think they had fun with the studies, which went beyond what would normally be allowed.” Although free to define their own scenarios, three chose a Boeing 737-like mission. “They did gravitate toward smaller domestic vehicles,” he says. MIT also studied a Boeing 777-size long-range aircraft, while the GE/Cessna team focused on a small aircraft for point-to-point regional service.
There are common themes in the studies, such as cruising slower and higher than today’s aircraft to reduce drag and save fuel. They settled at Mach 0.70-0.75 and 45,000 ft., compared with Mach 0.78-0.80 and around 35,000 ft. today, deciding the impact on air traffic would be manageable. They also gravitated to higher bypass-ratio engines with smaller cores, deciding the benefits of open rotors remain unproven. Hybrid wing-body designs also did not fare well because of empty space around the passenger cabin at smaller sizes.
Boeing’s Subsonic Ultra Green Aircraft Research (Sugar) team, which included GE and the Georgia Institute of Technology, selected a 737-class 150-seat aircraft and developed a series of increasingly advanced concepts as it strived to meet the N+3 goals. Nicknamed Refined Sugar, its conventional 2030 configuration has reduced sweep, laminar flow and advanced turbofan engines and achieves 44% lower fuel burn and 16 dB. less noise. Nitrogen-oxide (NOx) emissions are 42% of current limits set by the International Civil Aircraft Organization Committee on Aviation Environmental Protection (CAEP/6). These figures are well short of NASA’s goals.
This led to Sugar High, a configuration with a long-span, strut-braced wing for higher aerodynamic efficiency. The design also taps additional technologies expected to become available in the N+3 time frame, including more efficient turbofan or open-rotor engines. The result is reductions of 39-46% in fuel burn, 22 dB. in noise and NOx at 28% of CAEP/6. And this configuration comes with a caution—the uncertainty in estimating the weight of the long, thin wing.
Still short of NASA’s goals, the team turned to Sugar Volt, Sugar High with electric propulsion. They studied battery-only and fuel-cell/gas-turbine hybrids, but selected a turbine-electric hybrid in which a high-bypass turbofan runs on jet fuel and/or batteries. Short missions are flown on electric power, while for longer flights jet fuel takes over. The result is a fuel-burn reduction exceeding 63%, close the NASA’s 70% target, but at the expense of carrying 20,900 lb. of batteries.
The Boeing team also studied a hybrid wing-body (HWB) configuration, Sugar Ray, with the emphasis on reducing noise through airframe shielding of the advanced turbofans. Noise is reduced by 37 dB., the lowest of all the Sugar configurations studied, but still well short of NASA’s N+3 goal.
In contrast, Northrop Grumman’s team, whose other members were Rolls-Royce, Sensis Corp., Spirit AeroSystems and Tufts University, came within a hair’s breadth of achieving NASA’s fuel-burn, noise, emissions and field-length goals with an outwardly conventional configuration. The team selected a 120-seat aircraft capable of operating from reliever airports with 5,000-ft. runways to provide additional capacity.
“We demonstrated you can achieve some pretty dramatic improvements if you apply reasonable technology and allow them to cascade one on top of the other,” says Sam Bruner, configuration design manager at Northrop Grumman Aerospace Systems. “It’s not revolutionary in appearance but has very revolutionary improvements in fuel burn, noise and NOx through the contributions of many things.”
Northrop Grumman’s N+3 configuration achieves a 69.6-dB. reduction in noise, 63.5% in fuel burn and NOx emissions 90.6% below CAEP/6—almost meeting or slightly beating NASA’s targets. Technologies include an advanced three-shaft turbofan, swept-wing laminar flow, large integrated composite structures, active aeroservoelastic control, carbon-nanotube electrical cables, and advanced acoustic treatments.
“The advanced-technology, three-shaft engine contributes quite a lot in reduced fuel consumption and jet velocities,” says Bruner. “When you reduce fuel burn, you reduce aircraft weight, reduce noise, and things snowball.” Flying at higher altitude requires lower wing loading, which improves field performance without elaborate high-lift devices, and eliminating leading-edge slats reduces noise, he says.
“To our advantage, NASA specifically said cost was not a consideration,” says Bruner. “This will be a slightly more expensive aircraft, with bigger engines for a 45,000-ft. cruise and bigger wings for a 5,000-ft. field length.” Manufacturers and airlines will have to decide the trade between cost and efficiency and the environment, he adds. Despite the dramatic improvements projected, Northrop Grumman’s team used “fairly conservative technologies” in its configuration. “We drew a firm line at TRL [technology readiness level] 6 by 2025,” Bruner says.
MIT’s approach was more outside the box. The team, which included Aerodyne Research, Aurora Flight Sciences and Pratt &amp; Whitney, selected 180-seat domestic and 350-seat international airliners and proposed two unconventional configurations. The 737-sized D8 Series has a “double-bubble” lifting fuselage, slender low-sweep wing and three turbofans atop the aft fuselage. The 777-sized H3 Series is a hybrid wing-body with embedded, distributed propulsion.
MIT says the D8.5, with an advanced composite airframe, ultra-high-bypass engines and laminar-flow wing, narrowly beats NASA’s 70% fuel-saving target, exceeds the NOx goal with an 87% reduction from CAEP/6, but falls slightly short on noise, coming in 60 dB. lower. All the D8 configurations could operate from 5,000-ft. runways. The largest part of these improvements is attributed to the basic configuration.
“The study allowed some clean-sheet thinking,” says Jim Hileman, principal research engineer at MIT. “It’s a challenge to get real improvements at 737 size. The D8 design captures aspects of the hybrid wing-body—like the lifting fuselage—but without all the unused white space.” The D8’s fuselage cross-section is like two conventional “tubes” placed side by side to provide twin-aisle seating. This allows the aircraft to have a lifting nose and reduces the size of the wing and tail.
Another key design feature is the location of the engines—embedded in the rear fuselage where they ingest the boundary layer over the upper fuselage, improving propulsive efficiency, and between the twin vertical tails, shielding noise. The turbofan engines for the D8.5 have a bypass ratio of 20, compared with five for the 737’s CFM56s, but their constrained size requires small high-performance cores.
“It was a big surprise that we met the goals at 737 size,” says Hileman. “We assumed that if we made it big enough, the HWB would do it, but the 777-size HWB did not get there.” MIT also looked at turbine-electric hybrid propulsion. “We thought that would be a big winner, but it did not turn out to be that beneficial.”
The GE/Cessna team took a quite different approach, focusing on a small aircraft for point-to-point travel between small airports. The team developed several scenarios in which a 20-seat airliner would connect community airports to hub satellites or connect satellites to avoid major hubs. They developed a baseline concept, the B-20, a three-abreast, 20-seat twinjet based on current Cessna Citation technology.
Subsequently, the team developed a series of configurations to meet the N+3 goals. This culminated in a 20-seat twin-turboprop concept, the AR-20, with four-abreast seating; laminar flow over the forward half of the fuselage, outer sections of the high wing and the T-tail; and an ultra-efficient low-NOx turboprop with an ultra-quiet eight-blade propeller. Results are reductions of 69% in fuel burn, 75% in NOx and 54 dB. in noise from the baseline B-20—meeting or approaching NASA goals.
All four subsonic N+3 teams have submitted proposals for a follow-on Phase 2, to begin in Fiscal 2011, under which key technologies selected from their Phase 1 road maps would undergo further modeling and testing. “It was up to the companies to propose which areas and justify why certain work needed to be done first,” says Del Rosario.
Lack of funds means there is no Phase 2 planned for the supersonic N+3 studies, but both Boeing and Lockheed Martin will wind-tunnel-test models of their 2020-time frame N+2 designs. “From my perspective, the N+3 goals are good goals to have. But we now have some other things to start thinking about,” says Del Rosario. “Our goals tie fuel burn into CO2 reduction, but we’ve learned there are other things we can do to tackle CO2 that don’t necessarily transfer to fuel-burn reductions.”
“The goals served their purpose. They are not unattainable,” says Wahls. “But instead of talking fuel burn, it needs to be energy consumption so we can compare different types of propulsion using different energy sources.”
Bruner says Northrop Grumman’s study showed the goals to be valid, adding, “We will be energy-constrained globally by 2030, so anything we can do to reduce fuel burn will help conserve resources.”
Photo Credit: GE/Cessna