Tiny Vibrations Beneath Aircraft Skin Could Cut Turbulence and Fuel Use

At cruising speed, a passenger jet can travel around 640 mph while its wings push through turbulent air along their surfaces, a drag-inducing effect that makes flight less efficient and burns more fuel. Mahmoud I. Hussein, a professor in the Ann and H.J. Smead Department of Aerospace Engineering Sciences at the University of Colorado Boulder, is exploring a different way to control that airflow: engineered microscopic vibrations generated by synthetic materials beneath the surface, rather than changing the aircraft's external shape. A commercial aircraft can burn more than 10,000 gallons of jet fuel on a single cross-country flight, so even modest efficiency gains could mean substantial savings for airlines.

In papers published in Physical Review X and Proceedings of the Royal Society A, Hussein and colleagues describe two advances that move the concept, called phononic subsurfaces, closer to practical use. "The prevailing paradigm since the beginning of aviation is to control drag by only shaping the vehicle," Hussein said. "Now we have a new concept to influence surface drag using materials that can dynamically interact with the airflow." The approach relies on phonons, tiny vibrations within a material rather than motion of an entire structure; Hussein introduced phononic subsurfaces, engineered materials placed below a surface to passively control such vibrations, in 2015, but earlier designs worked at only a single frequency.

The first advance, which Hussein calls "super resonance," uses coiled phononic subsurfaces to extend their effect across a broad range of frequencies, matching the many frequencies found in real turbulence. The second, "scatterless interference," arranges multiple phononic subsurfaces in a grid so their effect reaches further downstream across a larger surface, such as a wing or fuselage. "These two problems, downstream control and broadband control, have been the key limitations of the technology since its introduction over a decade ago," Hussein said. "We've resolved both."

The current results are computational, but the concept is no longer purely theoretical: research groups around the world have built working physical prototypes and are moving toward wind-tunnel tests. "With phononic subsurfaces, a wing or a fuselage can retain its shape and smoothness and remain passive, while the material beneath it is engineered to allow interaction with the flow in a highly targeted way," Hussein said. The ongoing research, which also examines hypersonic flows, is supported by a $7.5 million, five-year grant from the Department of Defense's Office of Naval Research.