There are already a number of hand-powered helicopters entered in the Make It Fly contest (like this, and this, and this, and this and this), so why add another? Well, I was intrigued by these new toroidal rotors (all the cool kids are using them), and I wanted to push the limit on how light I could make one. This has a 5"/127mm diameter rotor and tips the scales at just about 4.5g, including the integrated twisty-stick. Every part of it except for the hub is printed with 0.5mm single-wall extrusions.

I actually started with a 3.5g version, but ran into an interesting trade-off involving drag, stored kinetic energy, and weight and moment of inertia. Adding to the moment of inertia increases the amount of energy that is stored to overcome the rotor drag and keep it spinning faster for longer. Increasing the moment of inertia requires adding weight somewhere (ideally at the outer rim), but adding too much weight ruins the lift to weight ratio. For this design, adding about a gram of weight to the outer part of the 3 toroidal blades seemed to be about the sweet spot. (you can see this in the thickened sections of the rotor.)

And because it is so light and flexible, it's actually quite sturdy, bouncing as it lands. And that's even with it printed in PLA.

Printing
Printing single wall extrusions, especially with steep overhangs, can be a little tricky, and some slicers handle it better than others. I used PrusaSlicer with the Arachne perimeter generator. The only problem I ran into when printing was that with the default retraction setting of 3.2mm for my Mini+, small gaps were left where the rotor arc section joined. (With a single wall extrusion, that just leaves the sides completely unconnected.) Reducing the retraction to 1.6mm seemed to solve the problem. I've provided 3mf and GCODE files, along with the STL so you can see exactly the settings I used.