The rotors of a typical quad-rotor drone have very low inertia, and fixed pitch. They'll come to a stop almost immediately if the power fails.
Oh yeah, fixed pitch. I completely forgot about that.

And in more detail:
The reason quadcopters work so well using such cheap parts is the same reason they autorotate only slightly better than a brick.

The fixed pitch rotors are far cheaper and mechanically simpler than the mechanism a conventional helicopter uses. However, in order to make a fixed patch aircraft hover so well, you need to be able to respond to changes in roll/pitch/yaw extremely rapidly. So you need extremely low inertia rotor blades, relative to the inertia of the quadrocopter body and the torque from the motors.

This low inertia means if you lose power, they will slow to a speed at which they provide negligible thrust very rapidly, and the quadcopter crashes like a brick.

This is also why large quadcopters don't use longer blades, but instead become octocopters and larger.