Aircraft makers are banking on a raft of emerging technologies to make their next-generation airliners quieter, more fuel efficient and lower on emissions.
In the U.S., the research effort is spearheaded by's Environmentally Responsible Aviation (ERA) program, which is exploring a suite of airframe and propulsion technologies that could be ripe for full-scale development around 2020 and entry into service five years later. Given the growing environmental pressures on aviation in the 21st century, the ERA goals are suitably ambitious for potential products aimed 15 or so years into the future.
Although the ERA targets are individually challenging, what sets them apart from many previousresearch projects is that they are expected to be met simultaneously and without compromise. “It's actually a very difficult thing to do as these usually trade off against each other,” says Craig Nickol, ERA vehicle systems integration element lead. They include a fuel-burn reduction of 50% relative to current state-of-the-art aircraft, a 75% cut in oxides of nitrogen (NOx) emissions below the current standard, and aircraft noise 42 db below the 's Stage 4 certification level.
“It's quite a challenge, and people are asking how we are going to do this,” says ERA Project Manager Fayette Collier. “We've done some experiments that lead us to believe that although this is difficult, we think it is achievable.”
The targets are tough because, NASA believes, without major improvements the growth of air transport will be actively impeded by environmental and related cost concerns. From a cost perspective, cutting the fuel burden makes better business sense with each jump in oil price. U.S. commercial carriers alone spent $59 billion on fuel in 2008 when oil was at roughly $3 per gallon. Together with theburning 4.6 billion gal., this usage also pumped 250 million tons of carbon dioxide (CO2) into the atmosphere, while noise continued to be seen as a restraining factor on growth of the U.S. National Airspace System.
Formulated in 2009, and kicked off in fiscal 2010, ERA is approaching the midpoint of Phase 1 and managers are sketching out plans for a second phase that will take the program through fiscal 2015. Coming at a time when fuel prices and environmental concerns are combining to accelerate new commercial engine and airframe projects, the ERA initiative appears to be well timed for maximum impact. “We're trying to look at those technologies that have a high payoff and, no matter whether the platform is a  737, [ ] , [Boeing] 777 or regional jet replacement, we believe the portfolio covers it,” explains Collier.
“The program has a significant impact on the 737/A320 replacement market as well as the next widebody replacement, irrespective of whether they are conventional tube-and-wing or more advanced configurations. It also depends on the timing; but if the market opportunity moves to 2025-30, it gives us a chance to [include] more advanced configurations where we can bring in more significant gains in terms of environmental metrics,” says Collier.
For drag reduction, NASA and Boeing will test an advanced fluidic control system that could increase rudder effectiveness, allowing vertical tail size, and therefore drag, to be decreased. Under ERA, the active flow control (AFC) study will evaluate the use of pressurized jets in place of a conventional mechanical actuator to help control the rudder. The concept—which works by using the jets to alter local airflow and pressure gradients to control surface movement—has previously been tested on experimental aircraft such as the Bell XV-15.
The AFC study comes as Boeing starts development of a drag-reducing hybrid laminar flow-control system for the stretched, and could eventually become a complementary feature should it prove effective. Active and laminar flow control are NASA's two main areas of investigation for reducing skin friction, which accounts for an estimated 48% of the drag on current airliner designs.
“The intent is to increase its technology readiness level [TRL] so that it could be applied to any generic widebody or single-aisle hinged rudder,” says Tony Washburn, NASA's ERA chief technologist. The AFC, in the form of fluidic oscillating jets or synthetic jets, will be located near the rudder hinge line and operated only during takeoff and landing, or in the event of an engine out. Applied to operations in the most critical conditions, it would be sized for airspeeds of 100-150 kt., ±15 deg. of sideslip and ±30 deg. of rudder. “Part of what the study will do will determine the limits of the vertical rudder size reduction,” Washburn adds.
Noise reduction is being tackled across three fronts: airframe, propulsion and combined propulsion-airframe aeroacoustics. Under ERA, NASA is working with Gulfstream to test methods for mitigating airframe noise, particularly from flaps and slats on the wing and from the landing gear. Mehdi Khorrami, lead project investigator atResearch Center and ERA noise-reduction element lead, says flight tests are being conducted using a specially instrumented G550 to gather detailed noise-source data.
Noise mitigation technologies being tested include a continuous mold-line flap and a faired main landing gear, both of which will be tested on an 18% scale half-model at Langley. Further flight tests of the G550 will gather steady-state surface pressures on the wing and flaps, as well as unsteady pressures on the flap edges, nose and main landing gear. Acoustic tests of the “most promising noise-reduction technologies” will be undertaken on the 18% model; follow-on fly-over noise tests of the selected devices will be conducted at NASA's, says Khorrami.
Fuel-burn reduction is being addressed via laminar flow control, improved engine performance and weight reduction. NASA's ERA program is funding flight tests of a wing glove with a natural laminar airflow airfoil on a modified Dryden Flight Research Center-based Gulfstream III. Micron-size discrete roughness elements, pioneered by William Saric of Texas A&M University, will be placed on the glove for passive laminar cross-flow control during flight trials starting around 2013.
Dramatic weight savings are expected from the Boeing-developed PRSEUS (pultruded rod-stitched efficient unitized structure) lightweight composite concept. This consists of carbon-epoxy panels infused using high temperature and vacuum pressure, with no autoclave required. Under the manufacturing process, the composite frames and stringers are stitched to the skin to produce a fail-safe structure. After stitching, pre-cured carbon-fiber rods are fitted through pockets in the stringers, locking the structure together and creating a self-supporting preform.
Because no autoclave is required, larger parts can be produced, which increases the attractiveness of the concept, says Collier. “It could be a game changer, and that's what we want to prove—whether it is or is not.” PRSEUS is “broadly applicable to fuselages of any shape and wings. It is lightweight, damage tolerant and built with fewer parts,” he adds. The frames and stringers provide continuous load paths and the nylon stitching stops cracks.
The process has been used in limited areas so far forgear doors and other fairings; but “so far, everything has been done in terms of unidirectional loading,” says Dawn Jegley, NASA ERA structures lead. “Now we're going to look at combined loads.” Testing of a 127 X 75 X 90-in. curved panel is set for mid-2011. The biggest test for the material comes later this year when a 30-ft.-wide section of double-deck hybrid wing-body (HWB) airframe, incorporating a PRSEUS shell and floor structure, will be tested in the combined-loads facility at Langley.
Chi Lee, ERA combustor task lead, is studying fuel-flexible combustor technology aimed at cutting NOx by 25% below CAEP6 international levels. “Emissions during landing and takeoff affect local air quality, and above 3,000 ft. they account for 92% of total ozone. We think that improvements here [at higher altitudes] could result in as much as a 60% reduction in cruise emissions. The problem is that the NOx emissions increase as overall pressure ratio increases, particularly above 50:1, so it is a tremendous challenge for us.”
Lee's team is studying balanced combustor designs withand Pratt & Whitney, as well as a NASA configuration with an improved fuel-air mixer and a lean direct-injection combustion system. “Every time we improve fuel mixing, the NOx drops,” he observes. The plan is to progress from tests of a single injector flame tube to multi-injector sector combustor trials in 2012-13, with a full annular combustor test aimed at substantial NOx reductions of up to 80% by 2015.
The combustor group is focused on injector design, active combustion and advanced liners to improve emissions. Further refinement of existing air blast or atomizer injectors may be possible, they believe, with further characterization of combustion processes using alternate and regular jet fuel planned over the next two years. For example, fuel produced through the Fischer-Tropsch process “provides more opportunities for emissions reductions because it has no aromatics, no sulfur and its distillation profile is different. That means it can vaporize quicker than jet fuel, and the viscosity is smaller; this, in turn, means droplet size is 10-20% smaller,” says Lee.
Active combustor concepts under study are aimed at devices that will carefully control combustion instability and incorporate an intelligent fuel/air management system, he adds. Work on advanced liners is focused on silicon carbide fiber-reinforced silicon carbide matrix ceramic composites (Sic/Sic CMCs). “We need 80% of air in front of the combustor to get fuel/air mixing going and that's going to come from the combustor liner,” says Lee. “CMC liners will reduce combustor cooling air requirements.”
Other engine efficiency improvements, meanwhile, are being tackled through core and propulsor studies aimed at a 50% reduction in fuel burn. Described as “very aggressive” targets by James Heidmann, ERA propulsion technology project engineer, the core research effort is challenged with the age-old conundrum of how to improve thermal and propulsive efficiency without adversely affecting emissions. The key, says Heidmann, is the combined use of improved materials and cooling methods, such as film cooling, to boost turbine performance without generating more NOx.
The turbine film-cooling experiment looks at the fundamental heat transfer and flow field. “The small size of features makes it very difficult to get data, so we've scaled it up and tested various hole shapes using a plexiglass model for much more detailed results. Now we're transitioning to direct measurements with an infrared camera,” Heidmann says. Testing is also underway of an “anti-vortex” film-cooling concept in which a series of auxiliary holes produce counter-vorticity to promote jet attachment and reduce its velocity.
The ERA project includes studies with General Electric of a highly loaded three-stage axial compressor with swept rotors and stators at NASA Glenn Research Center's W7 turbine test site. The trials are aimed at building up a database of transonic performance as part of efforts to improve matching of highly loaded compressor blade rows to increase overall pressure ratios. Work is also under way with GE on a highly loaded high- and low-pressure turbine in the nearby W6 single-spool turbine facility. The HP turbine is a reduced shock design, while the LP turbine features a flow-controlled stator and a contoured endwall.
NASA is also experimenting with active-control plasma actuators as a potential way of improving the efficiency of LP turbines. The solid-state dielectric barrier discharge electrodes are mounted perpendicular to the flow and impart control via an oscillating wall jet, while others mounted parallel to the flow provide control by generating vortices. “We place them upstream or on a separation point on a blade. It produces velocity and forces close to the wall that give you more control authority than you'd expect,” says Heidmann.
There are also two CMC-related experiments in ERA, one of which is an uncooled high-pressure turbine vane. Heidmann says the approach is to demonstrate different manufacturing techniques and explore design issues. A pre-preg lay-up of a CMC turbine vane made from a high-temperature resistant material called Hi-Nicalon SiC/SiC, and another using a hybrid approach involving chemical vapor-infiltration and melt-infiltration is planned in 2011. “Right now we're looking at a temperature capability of 2,400F for this generation, but next year we're looking at raising that to 2,700F.”
NASA's ERA effort includes follow-on research into the next steps for ultra-high-bypass (UHB) ratio engines, including potential derivatives of Pratt & Whitney'sgeared turbofan (GTF), now in development for service entry in 2013. “The GTF pushes the state of the art; but for the 2020 timeframe we need to push the state of the art even further,” says Chris Hughes, UHB engine technology sub-element lead. A 22-in.-dia. scaled second-generation GTF is being investigated collaboratively by NASA and Pratt in Glenn's 9 X 15-ft. wind tunnel, and the two are expected to join with the FAA for an engine demonstration as part of the administration's Cleen (continuous lower energy, emissions and noise) program in 2014.
Work to integrate larger-diameter UHB engines on single-aisle aircraft such as the 737 successor has involved tests of a powered half-span model in the 11-ft.wind tunnel. The tests will help explore the performance tradeoffs of various engine-airframe combinations, allowing optimization of transonic wing-body shapes. These include nacelles beneath a high wing or over the wing nacelles, as well as designs incorporating boundary-layer ingestion. “As there are more issues the larger the nacelle gets, the wind-tunnel tests will be used to validate the shape we will come up with,” says Steve Smith, vehicle systems integration element lead.
The baseline test model is based on awith an 88-in.-dia. fan, versus the 61-in. diameter of the current -7B engine, with the pylon located at 34% span. The test assesses 161 design variables, including seven wing sections, says Smith. “Initial optimization eliminated a double shock on the inboard leading edge, and a shock in the large ‘trench' between the engine and fuselage,” he adds. Although lift is transferring to the nacelle and offloading the wing, “[overall] for now it looks like it's going to work pretty well, and we're starting to build a semi-span model that will have a new 15-18 bypass ratio [powered] test engine,” says Smith. “We will validate cruise performance in tests around November-December 2011,” he adds.
Beyond GTF, the efficiency benefits of a direct-drive open rotor are being actively pursued under a cooperative research effort involving NASA, GE and. Testing is focused on validating blade designs that minimize noise from a set of counter-rotating unshrouded rotors while retaining the 25-27% fuel-burn advantage of the UHB ratio concept.
Baseline aerodynamic and acoustic testing has been completed in the 9 X 15-ft. low-speed wind tunnel at Glenn Research Center. This involved the blade design from GE's Unducted Fan engine to provide a database for evaluation of improved blades intended to reduce noise. Follow-on tests were conducted on a further set of five different GE blades (12 forward, 10 aft), all modified from the baseline “historic” design of the 1980s. The research effort is past the halfway point and is now testing at cruise Mach numbers in Glenn's 8 X 6-ft. high-speed tunnel, and will include both aerodynamic performance and near-field unsteady pressure measurements.
From May to October, a further phase will focus on a second-generation GE blade design.
Undertaken jointly by NASA, GE and the FAA as part of the Cleen program, the tests will use both Glenn's 8 X 6-ft. and 9 X 15-ft. tunnels. An intense phase will see aerodynamic and acoustic evaluation of the next-generation design at both high- and low-speed tunnels at takeoff, approach and cruise conditions. NASA says tests to date have demonstrated a 8–10% noise reduction relative to the 1980s design; confirmation of predicted cruise efficiency improvements is still pending.
As early conclusions from ERA and other studies point to integrated airframe-propulsion concepts as being the only way to meet NASA's performance targets, an element of the program is focused on combined airframe-propulsion aeroacoustics. The goal of the effort is to reduce interaction effects directly, or to actively use integration of the engine and airframe to reduce net radiated noise.
System noise assessments have been made with an HWB and a 7:1 bypass ratio turbofan, as well as with an open rotor, with plans to complete the latter assessment by the end of 2011. Tests evaluated the effect of turbofan engines mounted from above on 777-like pylons versus ones mounted from below on “keel” pylons on an HWB. Different acoustic liner and chevron nozzle configurations were also evaluated. “We see a strong technical path to meeting the -42-dB goal,” says Russ Thomas, HWB community noise team leader. “It's the last 10 dB that's really tough,” he says, noting that the target can only be achieved by applying acoustic liners to crown pylons, adding “quiet” landing-gear technology and having a reduced approach speed. Open rotor tests will take place later this year in Boeing's 9 X 12-ft. low-speed aeroacoustic facility.
The highest profile element of ERA is the preferred system concepts competition in which the winners will identify advanced integrated vehicle and component technology concepts that could ultimately be flight tested as a demonstrator. Boeing,and won contracts, and up to two finalists will be selected by year-end.
Boeing is proposing a blended wing-body powered either by two GTFs or optionally by three General Electric/-X-based open rotors. Lockheed Martin's more radical-looking option is configured with a laminar flow, modified box-wing and tail-mounted engines. Northrop Grumman proposed a notional double-fuselage concept and says its final concept will “fall out of the study.”
The coming year will focus on five main tasks, says ERA chief engineer Mark Mangelsdorf. The first will be to evaluate a full-scale concept, starting with studies of how the contenders could best fit into the FAA's NextGen airspace plan. Task 2 will focus on meeting the preferred performance requirements, which include a 50,000-lb. payload and an 8,000-nm range for the passenger version. The freighter is required to carry a 100,000-lb. payload over a range of 6,500 nm.
Task 3 will sketch out a detailed 15-year technology maturation road map, while Task 4 will focus on the “long poles,” or the critical technology demonstrations for the second half of the ERA program in 2013-15. “We're also interested in technologies that apply to N+1—[these are] nearer term single-aisle concepts or retrofitable to current 737/regional jet-class models,” Mangelsdorf says. Task 5 covers the conceptual design of a subscale test vehicle that, if built, will be large enough “to demonstrate noise, emissions and fuel-burn goals,” he adds.
“In June we'll start seeing some refinement of the vehicle concept, and around September we'll have to get more maturity and start seeing the technology road maps that go with them,” says Collier. Up to two concepts may be downselected in December.