Propeller, Repeller, Wings - construction kit

#Propeller
Realistic propeller, repeller (e.g. used for wind turbines) and wing designs lead to highly organic shapes. They are usually constructed by skinning a sequence of airfoil-shaped ribs, with each rib being designed with an individual airfoil profile and having its dedicated position in 3D space.
Using OpenSCAD to model such an object can be quite challenging and requires the use of several advanced programming techniques and in particular an efficient description which I present and explain in this post.
The example code given shows the construction of two typical propellers, a Graupner 33/20 cm E-prop (i.e. diameter 33cm and advance 20cm per revolution) and a blade pair for Mavic pro mini 11/6.6 cm.
Just for fun I extended the design scheme (i.e. the description format and interpreter) to be able to do a *loop propeller* according to the invention of Rudolf Bannasch. This kind of propeller is supposed to have a high efficiency and to run extremly silent. I don't know,