Drone technology has come a long way over the last decade. The scope of its applications extends far beyond military missions and toy hobby use. Drones are now being used every day around the world in countless capacities. They are being used to teach children in schools, capture cinematic films, sell real estate, discover archaeological treasures, explore the oceans, rescue lost people, suppress fires, provide tactical awareness, and even deliver a burger and fries. The list goes on, with new uses for drones being discovered daily.

However, the sound of a drone’s propellers has been found to be very irritating. As these devices are being flown closer to and around people, this noise becomes an increasing concern. In 2016, Andrew Christian and Randolph Cabell of NASA’s Langley Research Center in Hampton, VA, were tasked with investigating how people perceive the noise pollution created by drones. Their findings were published in 2017 in an article titled Initial Investigation into the Psychoacoustic Properties of Small Unmanned Aerial System Noise. For their research, Christian and Cabell had individuals listen to recordings of drones in various settings, comparing them to typical neighborhood sounds like delivery trucks. The results showed that people were far more annoyed by the acoustics of the drones than by the sound of trucks.

Drone developers are continuously looking for ways to make drones quieter. This is exactly what Dr. Thomas Sebastian, a senior staff member of the Structural and Thermal-Fluids Engineering Group at MIT’s Lincoln Laboratory, set out to do. Dr. Sebastian explained that the idea to create a quieter drone propeller came to him while developing a quiet propulsion system for a fixed-wing aircraft. He needed to compare the acoustic technology of the fixed wing aircraft to a standard drone, which led him to realize just how loud in fact the propellers on a drone could be.

He was inspired by some of the aerodynamic ideas presented by engineers in the early 1900s, including wings shaped like rings. “So I wondered what it would look like if you took a ring wing and turned something like that into a propeller,” Dr. Sebastian said. Working with one of his interns, Dr. Sebastian developed the concept of a toroidal-shaped propeller. His intern got to work designing several versions of the ring-shaped propeller, each fabricated using a 3D printer from the MIT Technology Innovation Laboratory. The various toroidal propellers were then tested for aerodynamics and acoustics.

The result was a propeller that appears as though its two opposing tips are bent toward each other, fusing to create a figure-eight ring shape. “The key thing that we thought was making the propeller quieter was the fact that you are now distributing the vortices that are being generated by the propeller across the whole shape of it, instead of just the tip,” he explained. Dr. Sebastian explained that distributing the vortices make acoustics “effectively dissipate faster in the atmosphere, that the tip vortex doesn’t propagate as far, so you are less likely to hear it.” Because the propellers have rounded, closed edges instead of free hanging edges, they cause less air disturbance and are therefore quieter.

Not only was Dr. Sebastian’s toroidal drone propeller quieter, but it also earned him the prestigious R&D 100 Award. “Established in 1963, the R&D 100 Awards is the only S&T (science and technology) awards competition that recognizes new commercial products, technologies and materials for their technological significance that are available for sale or license,” the organization’s website states. “The R&D 100 Awards program identifies and celebrates the top 100 revolutionary technologies of the past year.”

As demand for drones continues to grow, innovations like Dr. Sebastian’s quiet toroidal propellers could have a huge effect on the drone industry. “Gaining the R&D 100 Award was a little surreal,” Dr. Sebastian said. “We all have this sense that, you know, the things that we’re working on could potentially have some greater impact.” He envisions toroidal propellers on drones being used for everything from covert military operations to reducing noise pollution from delivery drones. The sky is the limit—only now, it could be a little quieter.

Previous Drone News: