I saw a guy on YouTube who had made an aluminium helix that changed radius at the top and went down inside the original helix to join up again at the base. When it spins you get an up down optical illusion.
I tried to make it using formula spline - disaster
I tried joining 2 helix primitives using FFD to flatten the ends - another disaster
Eventually I used a tracer and some XPresso math. Here's the wip -