The Brink: Uber and Amazon are among those exploring the potential uses of drones and vertical lift air vehicles in urban environments—why is NASA interested in them?

Grace: NASA’s role in the aeronautics field in general is to enable new technologies, it’s that way with space, as well. And so their role here is to help enable this new aircraft technology. The NASA that people think about is the space stuff—moon and Mars programs, the International Space Station, satellites looking back at the Earth to study the weather. But I work with the other side of NASA, which studies and develops helicopters, sonic transport, next generation clean engines, and air transportation in a very broad sense. This project has nothing to do with the space side of NASA, it’s all on the aeronautics side.

The Brink: Where do you come in, what’s your area of expertise?

Grace: I’m an aeroacoustician, which means that I study flow noise. In my field, researchers study jet noise, airframe noise, surface and underwater vehicle noise, car noise—noise from anything in motion. I’ve done a lot of research over the years on turbofan noise—the front end of aircraft engines. A while ago, it became pretty clear to me that small drones needed attention. Around 2014, I got my son a small drone and we’d fly it around the neighborhood. At the time, I also had past students who were working with Wing and [Boeing’s] Aurora Flight Sciences, developing small drones for military surveillance, package delivery, all these things. Surveillance applications require low noise, and the drone my son was flying around wasn’t particularly low noise. I was also thinking, “If a bunch of these drones are delivering daily in my neighborhood, we’re gonna go nuts.” I just knew that noise was an issue and this all led me to work with colleagues from Aurora and Virginia Tech to acquire some of the first noise measurements of a small quadrotor.

That’s where it all started. But now we’re way beyond thinking about drone delivery—that’s happening, and I’m not saying it’s perfectly quiet—and we have a new challenge: personned eVTOLs, electric vertical take-off and landing vehicles. And if those are going to be flying in our urban environments, there’s a lot we need to know about how they will perform and their potential noise. It’s a huge challenge and a fun set of problems to work on.

The Brink: What’s the mission or goal of your NASA-funded project?

Grace: We know what helicopters do—there’s been a lot of research on helicopters. When you move to the distributed electric propulsion systems, you have multiple rotors and that means more interactions between the flows on the rotors. And the rotors are different: they’re smaller, more blades than on a helicopter rotor, they’re actuated differently, so there’s just new dynamics that come into play. Now, you take that and then you run it in an urban environment. You’ve walked through city streets, you know that as you pass through openings between buildings, you get disturbances, wind tunnel effects, we call them. There’s flow coming around buildings, there’s flow over the tops of buildings. How do these multirotor vehicles respond to those unsteady disturbances? How can the vehicle stay in a nice forward flight from a performance point of view? And what noise will be created by the interaction with the disturbances and by the actuation required to fly through the disturbances? This performance and noise response of multirotor vehicles to disturbances is what we’re going to be quantifying.

The other idea here is that we should be able to address path planning for these systems. Once we understand, “Okay, don’t come up against this kind of flow, because you’re going to make more noise,” we can pick a path of quieter operation. A goal is to develop methods to enable us to optimize the way the multirotors will operate in an urban environment.

The Brink: Where will you test new theories—in cities, wind tunnels, on computers?

Grace: I personally do theoretical and computational modeling. The experiments will be done in a facility at Virginia Tech, called the Stability Wind Tunnel, and it has the capability to introduce disturbances and also measure noise, which a lot of wind tunnels are not set up to do. Our industry partner Joby has been taking measurements of their full-scale vehicle and some of that data will inform this project.

The Brink: Some drone delivery experiments, like Amazon’s, seem to have stuttered, but do you think we’ll see more drones and rideshare aircraft crowding our skies soon?

Grace: It’s happening and it’s going to happen. Wing is delivering in Australia—and the complaints have come about the swarm of bees noise. For the eVTOL, personned movement around urban settings, I think before we get true interior urban air traffic, we’re going to get short-haul flights from the edge of one city to another. That’s definitely going to happen soon—2025 is Joby’s goal. The idea of using the tops of buildings and parking garages as vertiports and really getting from vertiport to vertiport within a city, I think that’s going to take a little longer, because there’s a lot of logistics. What I do know is that when the logistics are sorted out, if the vehicles create too much noise, all of the effort will be for naught, because community complaints will halt their adoption. We hope this research helps pave the way for these new air vehicles.