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DEVELOPABLE HELICOID
Other names : torse helicoid, helical torse, open evolvent helicoid. 
Cartesian parametrization:
(where a is the radius of base circle of the helix and its vertical
shift is 2pb).
. Curvature lines: the generatrices and the level curves. 
The developable helicoid is the surface generated
by the tangents to a circular
helix.
Figure showing the helix and its halftangents. 
View from below 
It is a surface of equal
slope (like any surface generated by the tangents to a helix), and
it is the only helicoid
to be developable.
Being generated by a line of a rolling plane without
sliding on a cylinder of revolution, it is a molding
surface.
The level curves (sections by horizontal planes) are involute of circles; they are the involutes of the cuspidal edge (which is a helix) as well as the involutes of the cylinder that contains the helix. 
The parametrization of the section by the plane y = 0 is: ; the developable helicoid is therefore the result of the helical movement of this curve along its axis: 
Modeling of the developable helicoid by Robert March,
professor at the Paris school of architecture, who also proposes the following
concrete production:
take a tinfoil roll, unwind a length of foil equal to its width, form a right triangle by folding diagonally, rewind, and unwind again while keeping the foil tight; the diagonal generates the developable helicoid and its tip a beautiful involute of a circle. 

Entrance staircase of the Louvre by the Pyramid.
Of course, the stairs generate a right helicoid but the surface under the staircase (the intrados) is a portion of a developable helicoid. Notice indeed that the visible outline of the surface is composed of linear generatrices (cf. red lines). 
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© Robert FERRÉOL 2017