diff -r 000000000000 -r 15eac37578b7 libs/Threading/Naca4.scad
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/libs/Threading/Naca4.scad	Wed Sep 12 12:48:28 2018 +0200
@@ -0,0 +1,55 @@
+// Naca4.scad - library for parametric airfoils of 4 digit NACA series
+// Code: Rudolf Huttary, Berlin 
+// June 2015
+// commercial use prohibited
+
+
+// general use: for more examples refer to sampler.scad
+// naca = naca digits or 3el vector  (default = 12 or [0, 0, .12])
+// L    = chord length [mm]      (default= 100)
+// N    = # sample points        (default= 81)
+// h    = height [mm]            (default= 1)
+// open = close at the thin end? (default = true)
+// two equivalent example calls
+ airfoil(naca = 2408, L = 60, N=1001, h = 30, open = false); 
+// airfoil(naca = [.2, .4, .32], L = 60, N=1001, h = 30, open = false); 
+
+module help()
+{
+  echo(str("\n\nList of signatures in lib:\n=================\n", 
+  "module airfoil(naca=2412, L = 100, N = 81, h = 1, open = false) - renders airfoil object\n", 
+  "module airfoil(naca=[.2, .4, .12], L = 100, N = 81, h = 1, open = false) - renders airfoil object using percentage for camber,  camber distance and thicknes\n", 
+  "function airfoil_data(naca=12, L = 100, N = 81, open = false)\n",
+  "=================\n")); 
+}
+
+help(); 
+// this is the object
+module airfoil(naca=12, L = 100, N = 81, h = 1, open = false)
+{
+  linear_extrude(height = h)
+  polygon(points = airfoil_data(naca, L, N, open)); 
+}
+
+// this is the main function providing the airfoil data
+function airfoil_data(naca=12, L = 100, N = 81, open = false) = 
+  let(Na = len(naca)!=3?NACA(naca):naca)
+  let(A = [.2969, -0.126, -.3516, .2843, open?-0.1015:-0.1036])
+  [for (b=[-180:360/(N):179.99]) 
+    let (x = (1-cos(b))/2)  
+    let(yt = sign(b)*Na[2]/.2*(A*[sqrt(x), x, x*x, x*x*x, x*x*x*x])) 
+    Na[0]==0?L*[x, yt]:L*camber(x, yt, Na[0], Na[1], sign(b))];  
+
+// helper functions
+function NACA(naca) = 
+  let (M = floor(naca/1000))
+  let (P = floor((naca-M*1000)/100)) 
+  [M/100, P/10, floor(naca-M*1000-P*100)/100];  
+
+function camber(x, y, M, P, upper) = 
+  let(yc = (x<P)?M/P/P*(2*P*x-x*x): M/(1-P)/(1-P)*(1 - 2*P + 2*P*x -x*x))
+  let(dy = (x<P)?2*M/P/P*(P-x):2*M/(1-P)/(1-P)*(P-x))
+  let(th = atan(dy))
+  [upper ? x - y*sin(th):x + y*sin(th), upper ? yc + y*cos(th):yc - y*cos(th)];
+
+