Spiral Sweep Tangents

[myname-random]

Is there a way to control the tangents used in a spiral_sweep?

When a spiral sweep has the same internal diameter at the top and the bottom, the segments of the sweep will sit flush to a cylinder with the same ID. If the top and bottom diameters are different, then the segments are angled inwards instead of matching the vertical slope. They will not sit flush on a cylinder with a matching top and bottom ID.

I think in a linear path scenario this could be addressed by providing tangents to modify how it sits. Is there something similar for spirals or am I just looking at this incorrectly?

(I'm not sure if I'm explaining this well, but here is an example designed to illustrate the difference.)

include <BOSL2/std.scad>

tHeight = 50; sHeight = 7.5; sDepth = 1;
// Cylinder
idTop=75; idBottom=75;
// Funnel
//idTop=75; idBottom=50;

color("red", 0.75) spiral_sweep(
    poly =
        move(v=[sDepth/2, sHeight/2, 0], 
            p=zrot(-90, 
                p=trapezoid(h=sDepth, w1=sHeight, w2=sHeight/3) ) ), 
    h=tHeight, d1=idBottom, d2=idTop, turns=.5);

tube(h=tHeight, od1=idBottom, od2=idTop, wall=1);

On a cylinder, you would see the whole spiral, and on a funnel, you would only see a partial.