You might want to persevere with @parkinbots nspline library.
Specifically the example here: http://www.thingiverse.com/thing:1208001

I wrote some more about it here:

http://forum.openscad.org/Trying-to-incorporate-equation-based-shapes-td3116.html#a16625

Have a read and see if you can work out what's going on.

First thing to sort out is all the names are mangled.

• Rudolf uses linux and all uppercase letters are autoconverted to
  lowercase.
• Also the zip file contains mangled long hash style filenames.

Look inside each one and make sure you rename them to match the following
files(taken from the boat propeller file):
use <Naca4.scad>
use <Shortcuts.scad>
use <Naca_sweep.scad>
use <splines.scad>

You know its working when the boat propeller appears.
http://forum.openscad.org/file/n16812/2prv6mc.jpg

One of the hased files is the boat propeller scad file.
Inside you will see these things:

• module axis -
• module boat_prop with a function called gen_dat inside it
• A variable called A which holds the shapes
  and these global variables:
  N = 100;        // refinement
  R_axis = 100;  // radius of axle
  r_bore = 50;    // radius of axle bore
  size = .1;      // sizing factor
  slices = false;  // set true to view interpolation slices
  ty = 50;        // common y-distancing of blades

So what's going on and how do you control it:

1. if you set slices to true you will see the NACA airfoils spread out along
   a single one of the propeller blades. There are 4 but the last one is not
   visible as its scaled vey small.

2. The functoin gen_dat takes teh A variable and uses it to make teh data to
   build each blade.

3. The A variable holds the shape definition
   // core data to be interpolated by nSpline()
   //      transform data            airfoil nacaXYZZ
   //--------------------------------------------------
   A = [ // Tx    Ty  Tz  Rz  Ry  |  L    X    ZZ
   [0,    ty,  60, -25,  0,  300,  .12,  .1],
   [-50,  ty,  200, -50,  0,  600,  .2,  .08],
   [-100, ty,  500, -75,  0,  700,  .1,  .06],
   [00,  ty,  660, -82,  10,  10,  .01,  .1],
   ];
   As indicated in the comment the first 5 values translate and rotate the
   shape into the proper position.
   The second set of three values (L,X,ZZ) define teh airfoil shape.
   Rather than use the NACA number @parkinbot has put in the bvalues that are
   extracted from eth NACA number directly. (see more about NACA numbers in my
   post referenced at top of this post)

So

• you can change these three values and get different airfoils.
• you can use different Translate, roatte values to get different overall
  blade shape
• you can alter the shl value (270 degrees) to get other numbers of
  blades...

Good luck

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A simpler approach letting you design your own fan would be:
http://www.thingiverse.com/thing:1016047

@Neon22: Thanks for exploring and explaining the design. I know it is a bit
sophisticated, but it lets you do so much. Meanwhile I have corrected the
upper/lowercase mismatch. Sorry for that. Please let me know if there is
some problem left.

Rudolf

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@parkinbot its a very nice layout you have.
Its the sort of thing that would be good to see slightly simplified and
formalised for OpenSCAD2 maybe.

The idea of defining a structure containing shapes and transforms is useful.
Also the trick in splines of splining across however many items you put in
the array is very clever.

So if you want a three point spline or 100 points - its the same function
applied the same way.

I like your http://www.thingiverse.com/thing:1016047 but I'd humbly suggest:

• some explanation of Repeller vs Propeller. One is not quite the inverse of
  the other (still need to negate twist etc. so perhaps explain what it is
  reversing. Maybe its just CCW vs CW.
  (https://en.wikipedia.org/wiki/Repeller suggests this is unusual usage of
  the term)

• I would not have used the same name with just a change of case for two
  modules - even though one only calls the other. I'd humbly suggest using a
  different name. Like in your other airfoil code you used airfoil_data() and
  airfoil().
  Maybe Propeller is the internal module and Prop_CW and Prop_CCW are the
  upper level modules ??

• Only users of your shortcuts will relise the D() examples are differencing
  the propeller from a Ring and that this code is to round the propeller tips.
  Its obvious once you know, but on initial examination its not clear. Nice
  trick BTW.

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