I saw this [YouTube clip][1] for a light weight drone, under a few ounces, with very impressive maneuverability. They Have used styrofoam for frame and taped the rotors and battery and gyro chip to it.  

It seems like the most complex part of these drones, gyro and balance and navigation, is the most easy to make and cheapest part!  

Can somebody explain to me how the gyro feeds the controls of rotors? Does it have any sense of right side up, gravity?


  [1]: https://youtu.be/5XinbTcQAAc