Both, conical helix and conical spiral, have a continuously decreasing radius.
How can I visualize the difference between the two? No need for equations. Any explanation in 3D plots would be of great help. On the internet, both the plots appear to be same.
A conical helix need not meet at a point and can have a minimum( or maximum i suppose) radius, whereas a conical spiral must start at a point. A conical helix will reverse the direction of change in radius once it passes the origin too, kind of like a stint frayed outward at both ends.