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Research and teaching on sustainable aviation takes flight in AA department

To minimize the environmental impact of increased air travel, Stanford aeronautical engineers have begun a wide-ranging research and teaching program.

Noon on a typically sunny day in Palo Alto would seem to offer idyllic conditions for an Aeronautics and Astronautics (AA) student to lunch at a picnic table and watch airplanes soar through the sky. Instead, 20 graduate students are packed like coach passengers into a room in Building 80 of the Main Quad, listening to AA Professor Juan Alonso talk about how irritating airplanes can be. In this first ever class, “AA 260: Sustainable Aviation,” the day’s lecture is on the noise pollution produced around airports, which is one of the key environmental effects of flight.

“Noise starts to become annoying at 65 to 70 decibels and you start to feel pain at around 120-130,” Alonso says, as he puts the measurement of noise levels into the context of human and animal comfort (55 db is normal conversation; 140 db is a rocket launch).

Alonso’s ultimate intent is not to give the students a few more numbers to memorize, and it certainly isn’t to squelch their passion for airplanes. Instead his goal is to teach a generation of aeronautical engineers that their ingenuity is needed to help the environment absorb a projected tripling of air travel in the next 30 years. To that end, he and AA Professor Ilan Kroo have not only initiated this class, but also the department at large has launched a sustainable aviation research program.

“Sustainable aviation is the concept of being able to accommodate the dramatic projected increase in air transportation demand over the next 20-30 years while not increasing the impact on the environment,” Kroo says.

This goal of mitigating noise and air pollution is not unique to Stanford  — NASA, for example, has announced and set goals along these lines  — but Kroo argues that the University is capable of creating more than its share of innovations to make aviation sustainable. Not only does the highly-ranked department have a strong track record and expertise in many subdisciplines that are relevant to the challenge, he says, but also the multidisciplinary culture that can bring them all together to make real progress.

Take, for example, the idea of allowing planes to fly in formation like geese, which maximize their efficiency by using the wake of the birds ahead to push themselves along.

“In order to enable things like formation flight we need to understand precision navigation, we need autonomous control, and we need fluid mechanics,” he says. “We are used to working in that mode. Stanford’s AA department is well situated, and then Stanford in general is well-suited to this because of the interaction in engineering among the various departments.”

Environmental effects

There are several ways that airplanes affect the atmosphere and climate. Like all vehicles, they emit pollutants. Calculations suggest that airplanes account for 2 to 4 percent of human-generated carbon dioxide (CO2) emissions. But airplanes also emit other greenhouse gases such as nitrous oxides and water vapor. They do so, Kroo adds, at altitudes where the gases may linger in the atmosphere much longer and react in different ways to increase the impact of greenhouse gases.

Out of both economic and environmental concern, engineers have made tremendous strides over many decades in improving airplane fuel efficiency. A traveler will use less fuel per mile in a modern plane than in a typical car, Kroo says. But more aerodynamic designs, new fuels, lighter structural materials, and better engines can have a huge impact yet.

“We believe that we can reduce the energy requirements of air travel by a third to a half in the next 20-30 years,” says Kroo, who like Alonso and Professor Antony Jameson, is a well-known leader in the computational aerodynamics and design fields. “There are lots of possibilities.”

The ideas under study range from nudging familiar commercial aircraft designs closer to the slender, long-winged design of sailplanes, to more radical ideas such as merging the fuselage and wings into a continuous, stingray shape, a configuration known as a blended wing. Kroo has been studying the latter design and believes the fuel savings from it could approach 25 percent.

“It’s still quite new so there are some stability and control issues, structures issues and aerodynamics questions,” he says. “These are all things that we hope in this program to address.”

Unhappy contrails

Another area that AA and mechanical engineering Professor Sanjiva Lele has begun to investigate is water vapor contrails, those long white streaks that planes sometimes carve across the sky. Although the phenomenon is not well understood, they are believed to contribute to the amount of heat trapped in the atmosphere because, like clouds, they can reflect energy back to earth. Water vapor emissions might also spur cloud formation.

Lele and Civil and Environmental Engineering Professor Mark Jacobson have begun working together on a study of the environmental impact of contrail formation.

Scientists already know that contrails form under certain conditions within a specific range of altitudes. That leads to a potential solution: Satellite-based environmental monitoring technology integrated with individual airplanes and air traffic control systems, could allow pilots to make subtle altitude or flight-path adjustments that avoid contrail-forming conditions.

In both the contrail-avoidance and formation flight scenarios, the need for pilots to have an exquisitely precise sense of where they are plays perfectly into the hands of AA Professor Per Enge. He directs the Stanford Center for Position Navigation and Time which focuses on augmenting satellite navigation systems, like the Global Positioning System, to make them more precise and reliable.

Lele, meanwhile, is also an expert on computer modeling of aircraft turbulence and aerodynamics. That makes him a leading researcher on the physics that underlie not only contrails, but also noise. That research is also part of the sustainable aviation program, because, as Professor Alonso told the students, hundreds of thousands of U.S. residents and untold numbers of animals live in areas where aircraft noise levels are annoying or worse.

That problem would surely get a lot worse if travelers used the same aviation technology three times as often, but it could also get a lot better if clever engineers figure out ways to make planes quieter. That’s the charge for a new generation of students, and a new generation of research.