Answer: It’s spring-loaded.
A research team from UC Berkeley has developed a drone that can fold itself into a smaller size in midair, and then expand again just as quickly. This is because the four arms that hold its propellers are spring-loaded.
When the motors on the drone’s arms are off, the springs pull the hinged arms of the drone into a vertical position. However, the force exerted by the motors when they are turned on immediately overpowers the tension in the springs and snaps them out horizontally. They remain that way until the motors are deactivated, at which point they are pulled down in less than half a second, reducing the drone’s overall size by 50 percent.
Drones like this one could be useful for getting into or through tight spaces like windows. While a smaller drone would be able to do that without having to change its size, larger drones are more stable and can carry more weight and stay airborne longer.