How is an Aircraft Propulsion System Like a Stack of Pancakes?

At the Eagle Flight Research Center of Embry-Riddle Aeronautical University in Daytona Beach, a group of graduate and undergraduate students are working a problem that aeronautical engineers the world over would like to solve: how to make an airplane go far and fast without being a noisy, polluting gas guzzler. And when I say working a problem, I mean working a problem. Master’s degree candidate Tianyuan “Tony” Zhao, who graduated last year from Embry-Riddle with a bachelor’s degree in aerospace engineering and computational math, says the center emphasizes hands-on learning. “Like soldering circuits for test equipment,” he says. “We hadn’t learned how to do that in a classroom.”

The students are designing and building a hybrid electric-gas turbine engine as their contribution to reducing aircraft gas consumption, noise, and emissions. Eagle Center director Pat Anderson, a certified flight instructor and airplane-and-powerplant mechanic as well as a Ph.D. in mechanical engineering, says students at the university are aware of the environmental impact of flight because they’re around it all the time. “They see 70 airplanes being dispatched every day,” he says, referring to Embry-Riddle’s flight training programs. “Hybrid or electric propulsion is exciting to these students because its first practical use will be in flight instruction—right where they’re at with their buddy students.”

Victoria Li, a graduate student who concentrated on electrical engineering in her first two years of undergrad work before switching to aerospace engineering, is designing the battery management system for the electric engine. “There is not a single big battery that we can implement on this aircraft,” she says. “We’re using cylindrical lithium-ion batteries”—“similar in shape and size to AA batteries, but with more energy storage capacity,” Zhao chimes in—“and we have a couple thousand of them on board to power the airplane, so to manage a couple thousand batteries, we need a complicated battery management system.” The students hope to connect them in such a way as to increase the capacity of the whole system.

Milton Marwa came to the project because his first interest was propulsion, and he’d been studying gas turbine engines. Marwa remembers that one of his professors, teaching a class on advanced gas turbine theory, started off by saying, “The gas turbine has been around for over 100 years, and there’s nothing new about it.” Marwa continues, “He let us pick our own propulsion system to study. When I learned that this project would produce a hybrid engine, I was in.”

It’s early days for the hybrid engine project, and most of the work so far has been computation and research. “This borrows a little from Professor Anderson,” says Li, “but the purpose of our project is to change the aspect of looking at the problem. If you want good range, you’ll use gas. Our goal is to reduce noise and emissions.” What kind of engine can meet that goal and still have acceptable range?

Marwa knows the engine won’t be built by the time he graduates in December. “This is what keeps me up at night,” he says, making his colleagues laugh. “Do I keep at it, as a Ph.D. student, or take a job in industry where there is only a slim chance of doing similar work?”

Maybe he will find an opportunity at Boeing, where engineers, under a contract from NASA, are studying an airplane that would use gas turbines for power-hungry operations like takeoff and electric engines for cruise. Siemens and Airbus have also teamed up to develop hybrid engines. Both Boeing and Airbus are predicting hybrid power for airliners by about 2030.

“A lot of people are talking about [building a] serial hybrid,” says Marwa. “They’re going to have a gas turbine generating electricity to drive an electric motor and charge a battery, which can also power the electric motor. And the motor then runs a propeller. With all those moving parts, there is a stacking of inefficiencies. We haven’t seen anyone who is looking at it from that point of view: that stacking of inefficiencies. We want to create an architecture that has the least stacking of inefficiencies.”

Like pancakes. The students at Embry-Riddle’s Eagle Flight Research Center are aiming for a short stack.