This one is for the kids...although...
To have this chopper working I needed six ball-bearings 3x7x2.5.. I also needed a smooth rod 3mm diameter, 4 batterie-holders, and a switch. 2 rubber bands. Also 2 small gears. Electromotor runs on 1.5 to 4.5 volt. I have it running at 3V, which gives me about 800rpm for the main rotor. Batteries used are 1.5 AA size.
All bolts and nuts are M3.