Extrude-Scale by Function: Propeller

WilliamAAdams suggested, quite rightly, that the various extrusion parameters I've been doodling with could be controlled by functions... and I pointed out that the thing being extruded could be any 2D shape at all.

As a semi-practical application of these two ideas, this is a four-blade propeller made by extruding the union of a square and four rectangles while scaling that union according to the cosine of the extrusion height (with some twist thrown in, of course).

The layers are 1/4 mm