Many birds can zip at high speed through a forest without crashing into trees. But, say MIT researchers, there's a top speed at which this is possible, and it's an important factor in designing drones.
Currently, most UAVs fly pretty slowly, particularly if navigating around obstacles, mainly so that they can stop within the field of view of their sensors.
"If I can only see up to five meters, I can only go up to a speed that allows me to stop within five meters, which is not very fast," says Emilio Frazzoli, an associate professor of aeronautics and astronautics at MIT.
If the northern goshawk, for example, used this strategy, it would travel a lot more slowly, says Frazzoli. Instead, he suggests, the bird likely gauges the density of trees and speeds past obstacles, knowing intuitively that, given a certain forest density, it can always find an opening through the trees.
Frazzoli points out that a similar intuition exists in downhill skiing.
"When you go skiing off the path, you don't ski in a way that you can always stop before the first tree you see," he says. "You ski and you see an opening, and then you trust that once you go there, you'll be able to see another opening and keep going."
Frazzoli and his team developed mathematical models of various forest densities, calculating the maximum speed possible in each environment.
And they found that, for any given forest density, there exists a critical speed above which there is no "infinite collision-free trajectory." - in other words, the bird is sure to crash. Below this speed, though, a bird has a good chance of flying without incident.
"If I fly slower than that critical speed, then there is a fair possibility that I will actually be able to fly forever, always avoiding the trees," Frazzoli says.
For UAVs, this means that no matter how good robots get at sensing and reacting to their environments, there will always be a maximum speed that they'll need to observe to make sure they survive.
The team's now checking to see whether this theory applies in nature, watching how birds fly through cluttered environments. So far, they say, their observations fit well with the theory.
Next, Frazzoli wants to see how close humans can come to such theoretical speed limits, and is developing a first-person flying game.
"What we want to do is have people play, and we'll just collect statistics," Frazzoli says. "And the question is, how close to the theoretical limit can we get?"