A prototype drone being tested by Nasa comes equipped with ten different engines in an attempt to make the technology's trademark buzz a little quieter.
You'd imagine that more engines would make a drone noisier, but Nasa is working on the principle that multiple small motors are quieter than a few large ones. Each operates at a slightly different speed, which stops the sounds combining into one loud noise.
The GL-10, which stands for 'greased lightning', has eight engines on the wings and two on the tail. It's made from carbon fibre, has a wingspan of three metres, and can take off vertically. The silencing technology means that it's inaudible when flying more than 30 metres overhead.
Mind your head
Project controller Mark Moore told New Scientist that the drone is targeted at package delivery applications, meaning it needs to be both safe and quiet. "That's exactly what distributed electric propulsion provides," he said.
But the technology would no doubt be interesting to the military too. Future plans include larger, manned aircraft using the same technology which would match the size of the US Army's missile-carrying Predator drones.