The future of aviation is bright … green. That was the environmentally-promising message delivered during the Green Aviation Technical Interchange Meeting (TIM), held recently at NASA Langley Research Centre.
More than 100 attendees from government, industry and academia listened as speakers discussed aeronautical research innovations that aim to lessen aircraft noise, reduce emissions, decrease fuel use, make gate-to-gate air travel more efficient, and boost safety beyond even today’s high levels.
“The goals NASA has outlined are ambitious. And that’s a good thing,” says Dr Naveed Hussain, Boeing Company vice-president of aeromechanical technology. “We at Boeing applaud it. What NASA is doing is very exciting, especially looking at the long-term view. In this business it’s hard to pivot every 18 or 24 months.”
The gathering included results generated by a series of technology demonstrations conducted by NASA’s Environmentally Responsible Aviation (ERA) Project, which was created in 2009 and completed research in 2015.
ERA’s eight demonstrations focused on aircraft drag reduction through innovative flow control concepts; weight reduction from advanced composite materials; fuel and noise reduction from advanced engines; emissions reductions from improved engine combustors; and fuel consumption and community noise reduction through innovative airframe and engine integration designs.
TIM speaker Robert Stoker, a senior manager in the Boeing Company’s Environmental Performance Group, iterated results from Boeing’s ecoDemonstrator program, which uses flight testing to accelerate new technologies that can lessen emissions and noise, and improve airlines’ operational efficiency, among other environmental goals.
Proven ecoDemonstrator technologies and processes may be incorporated into existing Boeing production models, made available for in-service fleets, and applied to new airplane-development programs industrywide.
Three NASA experiments were flight tested during the most recent ecoDemonstrator program. Stoker noted two of those studies – one involving creation of smoother, or “laminar” flow across an airplane tail, and the other a series of coatings to prevent the buildup of insect residue on wing surfaces – showed great promise in drag reduction, and decrease in fuel burn and overall emissions. In particular, he praised NASA’s laminar-flow approach as “work[ing] exceptionally well”.
Describing the Federal Aviation Administration’s (FAA) Continuous Lower Energy Emissions and Noise Program (CLEEN), FAA CLEEN program manager Levent Ileri and program engineer Arthur Orton charted progress in several technologies worked on collaboratively by the aviation community that will likely be introduced into use by 2026.
These include advanced composite materials structures specifically for entire aircraft fuselages, more efficient wing designs, leading edge protective coatings for turbofan blades to reduce emissions and fuel burn, low-emissions engine combustors, and advanced flight management systems directly integrated with engine controls.
Presenters also focused on the need to rethink the future of airplane design from tip to tail. Boeing’s Hussain suggested the most promising approach could be the blended, or hybrid wing body (HWB) aircraft. He characterised the concept as “a breakthrough in energy efficiency and the operational benefits that accompany it. This is a configuration that, if we look over the tree tops, it looks like an answer.”
In April 2013, a NASA-Boeing project team completed the last of 122 test flights of an 8,5% scale model of the remotely piloted, manta-ray-shaped HWB research aircraft known as the X-48. Studies encompassed six years and the evaluations of three versions: the A, B and C variants.
Although manufacturers have yet to announce any HWB commercial aircraft, Rick Hooker, Skunk Works programme manager for Lockheed Martin’s HWB project, and Andrew Wick, Lockheed Martin senior aeronautical engineer, said that such designs could be adapted for next-generation military cargo aircraft or refueling tankers. Lockheed has partnered with the U.S Air Force Research Laboratory (AFRL) to develop a next-generation strategic airlifter.
Hooker and Wick cited one Lockheed HWB concept that, if scaled to full size, would weigh 18% less and use 70% less fuel than the C-17, the US Air Force’s current troop/cargo transport workhorse.
Taking a longer look out, aircraft may incorporate a number of design elements inspired by nature. Dr. Sridhar Kota, founder of FlexSys, said that “flexible, bio-inspired machines are the future of [aeronautical] engineering.”
A series of studies conducted at NASA’s Armstrong Flight Research Center have focused on a FlexSys patented design of shape-changing flaps made from composite materials that form continuous bendable surfaces.
The work, begun under the ERA project, is known as the Adaptive Compliant Trailing Edge, or ACTE, experimental flight research project. ACTE is a  joint effort between NASA and the AFRL to determine if advanced flexible trailing-edge wing flaps can both improve aircraft aerodynamic efficiency and reduce airport-area noise generated during takeoffs and landings.
The experiment is being carried out on a modified Gulfstream III (G-III) business aircraft that has been converted into an aerodynamics research test bed, with both of the G-III’s conventional 19-foot-long aluminum flaps replaced by the FlexSys bendable flaps.
If aircraft can eventually mimic bird flight, at least in part, it’s possible they one day could be almost as quiet. TIM presenters cited progress in aircraft noise-reduction improvements that incorporate new design approaches to slats, flaps, engine-nacelle liners and airframe modifications that would quiet sound frequencies by deflection and/or shielding.
Hand-in-hand with noise-reduction research are experiments that aim to significantly decrease fuel use. Even putting aside crucial environmental imperatives, speakers at the TIM noted that the biggest financial outlay for both military and civilian aircraft operators remains the price of fuel. Reducing those costs wouldn’t just help to curtail emissions, but would also boost the bottom line.
Striking improvements to aircraft fuel-burn efficiencies could be in the offing. As Speaker Jack Schirra, manager of materials and process engineering for jet engine manufacturer Pratt & Whitney, put it, “We’re entering a new era of engine architecture. It’s a new era of propulsion.” In the near term that translates into work on much more efficient designs, including ultra-high bypass and geared turbofan engine concepts.
A more difficult challenge is fuel itself. The FAA is working to enable US use of 1-billion gallons per year of “drop-in” sustainable alternative jet fuels by 2018. Though they are created from renewable sources, drop-in fuels mimic the chemistry of petroleum jet fuel and can be used in today’s aircraft and engines without modification, while providing the same levels of performance and safety as today’s petroleum-derived jet fuel.
In studies known as Alternative Fuel Effects on Contrails and Cruise Emissions, or ACCESS, NASA researchers found that a 50-50 blend of kerosene jet fuel (JP-8) and a renewable alternative fuel produced from camelina plant oil performed just as well as an all-JP-8 formulation, while decreasing aircraft particle emissions 50% both on the ground and in the air.
Biodiesel, a fuel comprised of one or more oils made from canola, algae, jatropha, salicornia, palm, and tallow, among others, has the potential to reduce carbon dioxide emissions by as much as 90% over a period of years. And it may prove affordable. According to Boeing’s Robert Stoker, “green diesel” is very close to price parity and, significantly, the US’s diesel infrastructure already in place could handle ramped-up demand.
Infrastructure capacity is also a serious issue, according to Dr Jim Hileman, the FAA’s  chief scientist and technical advisor for environment and energy. Producing affordable but more environmentally friendly fuels would require a comprehensive “greenhouse gas accounting,” from extraction, to refinery transport, to fuel refinement, transport to aircraft, and, finally, engine combustion itself. “We need to develop a systems approach that accounts for interdependencies among environmental impacts,” he says.
Tantalising but tough is the prospect of replacing fossil fuels completely. Hybrid electric propulsion would be an interim step, with onboard battery storage enabling substantial liquid-fuel decreases. Longer-term, distributed electric concepts, including electric-drive technologies, may eventually prove feasible. Flight-weight electrical drives are now possible; but it is fully superconducting electric motors that may, 20 to 30 years hence, make all-electric, large-scale civil transports practical.
A new chapter in industry-government collaboration may open if the NASA proposal known as New Aviation Horizons (NAH) is approved by Congress. The NAH initiative would be a decade-long effort to design, build and fly a variety of flight demonstration vehicles, experimental aircraft called “X-planes.” Incorporated into X-plane designs will be a variety of green-aviation technologies from the ERA project, that, if adopted by the aviation community, could save the airline industry $255-billion during the first 25 years after being put into service.
These 21st century X-planes would typically be about half-scale of a production aircraft, although some may be smaller or larger, and are likely to be piloted. Design and build would take several years, with vehicles going to flight starting around 2020, depending on funding. The 10-year plan also includes major field tests in collaboration with airlines, airports, and the FAA.
“The NASA X-plane program is a great idea,” says Pratt & Whitney’s Epstein. “It gets people thinking about different approaches. That’s as much a valuable contribution as some of the technology maturation.”
TIM presenters praised NASA Aeronautics’ approach to a longer-term budget request that, by outlining funding requirements beyond just the next fiscal year, makes the case for amped-up research monies that will spur aviation transformation. Should the 10-year outlay be approved, the boost would come as very good news, according to Joseph Doychak, program manager for jet noise reduction at the Office of Naval Research. The proposed increase “is exactly what the nation needs,” he said. “We’re in this together, across all aviation.”
Green innovations couldn’t come at a more auspicious time, given the projected growth in air travel within the next 20 years. By 2034, according to a summary of figures compiled from statistics collected by the International Air Transport Association, the Air Transport Action Group and Boeing, the number of passenger trips will more than double, from 3,3-billion to 7-billion. In addition, the number of jobs connected to the aviation industry is projected to soar by roughly 85%, from 58-million to 105-million.
To accommodate demand, the global new-aircraft fleet will need to balloon to more than 36 000 vehicles: a market alone that would represent a $5-trillion investment. Most of the growth in airplane numbers will come in Asia, followed by North America and Europe.
Still, no green-aviation advances are assured of automatic adoption. Even if a specific technology proves its mettle, successfully meeting all milestones, and proves to be reliable and robust, innovation has to demonstrate its economic worth over time before it is incorporated industry-wide.
“We don’t want to be stuck with a product no one is willing to write a check for,” said Boeing’s Hussain. “We try to create a positive business case. And we have to balance all of these competing requirements.”
Helping to analyse tradeoffs is an aeronautics “dashboard” developed for NASA by the Georgia Tech Aerospace Systems Design Laboratory. Vehicle concepts from large to small were modeled, and in excess of 80 specific technologies assessed for their interrelationships.
Dr Dimitri Mavris, Georgia Tech’s Boeing professor of advanced aerospace systems analysis, said that the dashboard enables policymakers to gauge system impacts depending on what concepts are adopted and what designs are chosen. Also taken into consideration are how changes affect airports and eventually ripple throughout an entire air fleet.
Mavris cautioned that switching over from legacy to the latest is not a simple matter. There is a sizeable lag between technology introduction and widespread incorporation. The production-line shift to new aircraft requires an average of four years to complete, and years more can pass before new aircraft comprise the majority of a given airline’s operational fleet.
Pratt & Whitney’s Epstein noted that the value proposition remains front and center for aeronautical firms. “Companies have products to sell,” he said. “The industry today is much more risk-adverse. Refinements have to be less risky.”
Do something that companies find valuable, Epstein asserted – say, develop one or more technologies that would quiet airplane noise so much that takeoffs and landings could be routinely scheduled very late at night and into very early morning – and the potential for additional revenue would likely prove irresistible.

Pictured: A green aviation technology called Active Flow Control that could lead to smaller vertical tails, reducing drag and fuel use, was flight tested on Boeing’s ecoDemonstrator 757.
Credits: Boeing / John D Parker