BUZZ KILLER
Drone noise annoys. To soften it, a group of Johns Hopkins researchers turned to the rotor design of a true Renaissance man.
Written by Michael Abrams
The boundary of vortices around a helical screw-type propeller.
FOR HOMEOWNERS, PICNICKERS, AND PRIVACY ENTHUSIASTS, the buzz of a drone is an annoyance. For the military, that sound is worse—it’s a dead giveaway. “Even a small drone makes a lot of noise,” said Rajat Mittal, a professor of mechanical engineering at Johns Hopkins University in Baltimore. “Once you start thinking about delivering a 10-pound Amazon delivery or something, you can imagine the kind of noise that these drones are going to make.”
With a grant from the Army Research Office, Mittal and a Johns Hopkins team that included Jung-Hee Seo, an associate research professor of mechanical engineering, and Suryansh Prakhar, a mechanical engineering doctoral student, set out to mitigate the whine of these quadcopters.
They first turned to airborne products of evolution for inspiration. Flying insects of roughly the same size can make wildly different amounts of noise. The mosquito, for instance, is famous for its annoying buzz, to the point of having children’s books written about it.
“But from a same-size fruit fly, you will not be able to hear the noise from their wings,” Mittal said. “The wings of fruit flies have a much larger area than the wings of, for instance, mosquitoes, and because they have larger area wings, they actually flap them at a lower frequency.”
Mittal and his team conjectured that if they could increase the area of a drone’s rotor, they would generate more lift per revolution per minute (RPM), allowing for a reduction in RPM and ultimately slashing noise as well. So, for the purposes of noise reduction, they assumed that the larger the rotor, the better.
This insight led them to the design of a Renaissance artist and polymath.
Sometime in the 1480s, Leonardo da Vinci sketched out a concept for a flying contraption in his notebooks. His “aerial screw” looks like one-and-a-half turns of a corkscrew—essentially an uninterrupted spiral rotor.
A model based on a 1480s sketch of an aerial screw by Leonardo da Vinci.
“It dawned on us that [the da Vinci rotor] is the ultimate large area rotor that one can think of—the area is continuous, it’s not individual blades.”
– Rajat Mittal, professor of mechanical engineering, Johns Hopkins
“It dawned on us that this is the ultimate large area rotor that one can think of—the area is continuous, it’s not individual blades,” Mittal said. “If our hypothesis is correct, then the da Vinci rotor should produce less noise.”
To prove it, the researchers decided to make a 3D model of the screw and run it through a few virtual paces. Thankfully, that didn’t require extrapolating from the tiny centuries-old sketch drawn by the painter/scientist/engineer/architect. Researchers at the University of Maryland had already created a da Vinci aerial-screw-powered drone two years before, so the Johns Hopkins team used its design to create its own for simulation.
Running the simulations was data heavy business. “Each of these simulations runs on about 256 parallel processors—and they run for many days,” Mittal said. “If I were to just kind of back extrapolate Moore’s law to 15 years ago, it would probably take many months to run a single simulation.”
In the end, all that processing proved the team’s hunch correct. They showed that for the same amount of lift generated by a two-bladed rotor, they are able to rotate the da Vinci rotor at a much lower RPM. “Because of that and because of the continuous distribution of the blade area, we are able to reduce the overall acoustic noise for a given amount of lift,” Mittal said.
What’s more, the extra area required no additional power. “It turns out that this change in the shape of the rotor seemingly gives us both a reduction in the noise and a reduction in the power consumption,” he said. That’s a crucial assist for an application where battery weight is so limited.
The noise of a conventional two-bladed rotor comes from the turbulence spilling off the tips of the rotors. In simulation, the aerial screw created much less chaos, turning the air in long “smoothish fingers.” But that’s not to say that the da Vinci corkscrew is a piece of unimprovable perfection—for noise or for lift. The dimensions and materials of an aerial screw rotor are sure to be tweaked in the future for greater efficiency and noise reduction.
This animation depicts the boundary of swirling air currents (vortices) around a propeller, color-coding each vortex based on measurements to visualize which flows generate the most upward lift force keeping the propeller aloft, enabling detailed study of propeller physics.
Even so, it’s remarkable that the Renaissance design, out of the box, showed an improvement over contemporary ones. In fact, da Vinci seems to have understood that a single turn of a corkscrew would not produce enough lift and that a multiple-turn corkscrew would end up pushing too much air down on itself. “He was kind of brilliant,” Mittal said. “He doesn't have five corkscrews or half a corkscrew—he has one full circle and a half. I think intuitively he might have understood that too few is not good and too many is not good either.”
The next step is to refine the model to try to further reduce the sound the rotor makes. The team is currently working with the Indian Institute of Technology Delhi to explore using a larger overall rotor area and lower RPM.
The end product will be a corkscrew that can replace the rotors of any drone. “We don’t want to rebuild an entire drone,” Mittal said. “All we want to do is replace the rotors of an existing drone, because then we don’t have to worry about controls.”
After that, the drones of the world will be able to do what they do a little more surreptitiously.
Michael Abrams is a technology writer in Westfield, N.J.
© 2025 The American Society of Mechanical Engineers. All rights reserved.