airfoil profile geometry plotting

I have a little issue with calculating coordinates. Given are airfoil profiles in two lists with the following exemplary coordinates:

Example:

x_Coordinates = [1,   0.9,   0.7,   0.5,   0.3,   0.1, 0.0, ...] 
y_Coordinates = [0, -0.02, -0.06, -0.08, -0.10, -0.05, 0.0, ...]

diagram 1:

The only known things about the profile are the lists above and the following facts:

the first coordinate is always the trailing edge, in the example above at (x=1, y=0)
the coordinates always run on the bottom/underside to the leading edge, in the example above at (0,0) and from there back to the trailing edge
the profile is not normalized and it can exist in a rotated form

Now I want to determine

the leading edge and
the camber line.

Until now, I have always used the smallest x-coodinate as the leading edge. However, this would not work in the following exemplary profile, since the smallest x-coordinate is located on the upper surface of the profile.

diagram 2:

Does anybody have an idea, how I could easily calculate/determine this data?

edit

one full sample array data

(1.0, 0.95, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4, 0.3, 0.25, 0.2, 0.15, 0.1, 0.075, 0.05, 0.025, 0.0125, 0.005, 0.0, 0.005, 0.0125, 0.025, 0.05, 0.075, 0.1, 0.15, 0.2, 0.25, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.95, 1.0)
(0.00095, 0.00605, 0.01086, 0.01967, 0.02748, 0.03423, 0.03971, 0.04352, 0.04501, 0.04456, 0.04303, 0.04009, 0.03512, 0.0315, 0.02666, 0.01961, 0.0142, 0.0089, 0.0, -0.0089, -0.0142, -0.01961, -0.02666, -0.0315, -0.03512, -0.04009, -0.04303, -0.04456, -0.04501, -0.04352, -0.03971, -0.03423, -0.02748, -0.01967, -0.01086, -0.00605, -0.00095)

Well it was quite a few years I do something with wings.

I do not have any skewed wings data as on your image the closest thing I found was this:

leading edge not correct for nontrivial wings

just find point where the sign of dx is flipping and compute
```
dx(i)=x(i)-x(i-1)
```
then mark zones where dx is positive or negative and find the middle between them (usually dx==0 for that zone). Mark the edge point as ix1
camber line

for precise geometry you will need intersections of normals casted from each side so:
- start on outline point i
- cast normal inside wing
- search opposite side.
- find point, It's normal intersect the opposite normal and divide both normals to the same distance
This is doable but with insane complexity
approximate camber line

less precise way but much much faster so:
1. start on outline point i
2. find closest point to it on the opposite side
3. compute midpoint between them and store it as inaccurate axis0 points. Do this for all points i=(0-ix1) (Red line)
4. do the same but start from opposite side store as axis1 (dark red)
5. when done then just find the average between axis0,axis1
This can be done in the same way result is blue axis polyline

C++ source:

    List<double> pnt;   // outline 2D pnts = {x0,y0,x1,y1,x2,y2,...}
    List<double> axis;  // axis line 2D pnts = {x0,y0,x1,y1,x2,y2,...}
    int ix0,ix1;        // edge points

void compute()
    {
    int i,i0,i1;
    double d,dd;
    double *p0,*p1,*p2;
    double x0,x1,y0,y1;
    List<double> axis0,axis1;

    // find leading edge point
    ix0=0; ix1=0;
    for (p0=pnt.dat,p1=p0+2,p2=p1+2,i=2;i<pnt.num;i+=2,p0=p1,p1=p2,p2+=2)
     if ((p1[0]-p0[0])*(p2[0]-p1[0])<=0.0) { ix1=i; break; }
    // find axis0: midpoint of i0=(0-ix1) i1=find closest from (ix1,pnt.num)
    for (i0=2,i1=pnt.num-2;i0<ix1-2;i0+=2)
        {
        x0=pnt[i0+0];
        y0=pnt[i0+1];
        for (d=-1.0,i=i1;i>ix1+2;i-=2)
            {
            x1=pnt[i+0];
            y1=pnt[i+1];
            dd=((x1-x0)*(x1-x0))+((y1-y0)*(y1-y0));
            if ((d<0.0)||(dd<=d)) { i1=i; d=dd; }
            }
        if (d>=0.0)
            {
            x1=pnt[i1+0];
            y1=pnt[i1+1];
            axis0.add(0.5*(x0+x1));
            axis0.add(0.5*(y0+y1));
            }
        }
    // find axis1: midpoint of i0=(ix1,pnt.num) i1=find closest from (0,ix1)
    for (i1=2,i0=pnt.num-2;i0>ix1+2;i0-=2)
        {
        x0=pnt[i0+0];
        y0=pnt[i0+1];
        for (d=-1.0,i=i1;i<ix1-2;i+=2)
            {
            x1=pnt[i+0];
            y1=pnt[i+1];
            dd=((x1-x0)*(x1-x0))+((y1-y0)*(y1-y0));
            if ((d<0.0)||(dd<=d)) { i1=i; d=dd; }
            }
        if (d>=0.0)
            {
            x1=pnt[i1+0];
            y1=pnt[i1+1];
            axis1.add(0.5*(x0+x1));
            axis1.add(0.5*(y0+y1));
            }
        }
    // find axis: midpoint of i0=<0-axis0.num) i1=find closest from <0-axis1.num)
    axis.add(pnt[ix0+0]);
    axis.add(pnt[ix0+1]);
    for (i0=0,i1=0;i0<axis0.num;i0+=2)
        {
        x0=axis0[i0+0];
        y0=axis0[i0+1];
        for (d=-1.0,i=i1;i<axis1.num;i+=2)
            {
            x1=axis1[i+0];
            y1=axis1[i+1];
            dd=((x1-x0)*(x1-x0))+((y1-y0)*(y1-y0));
            if ((d<0.0)||(dd<=d)) { i1=i; d=dd; }
            }
        if (d>=0.0)
            {
            x1=axis1[i1+0];
            y1=axis1[i1+1];
            axis.add(0.5*(x0+x1));
            axis.add(0.5*(y0+y1));
            }
        }
    axis.add(pnt[ix1+0]);
    axis.add(pnt[ix1+1]);
    }

List<double> xxx; is just mine dynamic list template the same as double xxx[];
xxx.add(5); adds 5 to end of the list
xxx[7] access array element
xxx.num is the actual used size of the array
xxx.reset() clears the array and set xxx.num=0

[edit1] correct leading edge point

Have an insane thought about this to find the edge point on the run plus some code tweaking and the outcome is good enough for me :) so first some explaining:

algorithm for axis stays the same but instead of ix1 bound use only points that was not yet used ... Also count only valid closest points (on the opposite side) if none found stop (top image case). From this point find the most far point from last axis point this is the leading edge point.

This approach has much much accurate output (axis0,axis1 are closer together)

Now the C++ code:

void compute()
    {
    int i,i0,i1,ii,n=4;
    double d,dd;
    double x0,x1,y0,y1;
    List<double> axis0,axis1;
    ix0=0; ix1=0;

    // find axis0: midpoint of i0=(0-ix1) i1=find closest from (ix1,pnt.num)
    for (i0=0,i1=pnt.num-2;i0+n<i1;i0+=2)
        {
        x0=pnt[i0+0];
        y0=pnt[i0+1];
        i=i1+n; if (i>pnt.num-2) i=pnt.num-2; ii=i1;
        for (d=-1.0;i>i0+n;i-=2)
            {
            x1=pnt[i+0];
            y1=pnt[i+1];
            dd=((x1-x0)*(x1-x0))+((y1-y0)*(y1-y0));
            if ((d<0.0)||((dd<=d)&&(dd>1e-10))) { i1=i; d=dd; }
            if ((d>=0.0)&&(dd>d)) break;
            }
        if (d>=0.0)
            {
            if (i1-i0<=n+2) { i1=ii; break; } // stop if non valid closest point found
            x1=pnt[i1+0];
            y1=pnt[i1+1];
            axis0.add(0.5*(x0+x1));
            axis0.add(0.5*(y0+y1));
            }
        }
    // find leading edge point (the farest point from last found axis point)
    x0=axis0[axis0.num-2];
    y0=axis0[axis0.num-1];
    for (d=0.0,i=i0;i<=i1;i+=2)
        {
        x1=pnt[i+0];
        y1=pnt[i+1];
        dd=((x1-x0)*(x1-x0))+((y1-y0)*(y1-y0));
        if (dd>d) { ix1=i; d=dd; }
        }
    axis0.add(pnt[ix1+0]);
    axis0.add(pnt[ix1+1]);

    // find axis1: midpoint of i0=(ix1,pnt.num) i1=find closest from (0,ix1)
    for (i1=0,i0=pnt.num-2;i0+n>i1;i0-=2)
        {
        x0=pnt[i0+0];
        y0=pnt[i0+1];
        i=i1-n; if (i<0) i=0; ii=i1;
        for (d=-1.0;i<i0-n;i+=2)
            {
            x1=pnt[i+0];
            y1=pnt[i+1];
            dd=((x1-x0)*(x1-x0))+((y1-y0)*(y1-y0));
            if ((d<0.0)||((dd<=d)&&(dd>1e-10))) { i1=i; d=dd; }
            if ((d>=0.0)&&(dd>d)) break;
            }
        if (d>=0.0)
            {
            if (i0-i1<=n+2) { i1=ii; break; } // stop if non valid closest point found
            x1=pnt[i1+0];
            y1=pnt[i1+1];
            axis1.add(0.5*(x0+x1));
            axis1.add(0.5*(y0+y1));
            }
        }
    // find leading edge point (the farest point from last found axis point)
    x0=axis1[axis1.num-2];
    y0=axis1[axis1.num-1];
    for (d=0.0,i=i1;i<=i0;i+=2)
        {
        x1=pnt[i+0];
        y1=pnt[i+1];
        dd=((x1-x0)*(x1-x0))+((y1-y0)*(y1-y0));
        if (dd>d) { ix1=i; d=dd; }
        }
    axis1.add(pnt[ix1+0]);
    axis1.add(pnt[ix1+1]);

    // find axis: midpoint of i0=<0-axis0.num) i1=find closest from <0-axis1.num)
    for (i0=0,i1=0;i0<axis0.num;i0+=2)
        {
        x0=axis0[i0+0];
        y0=axis0[i0+1];
        for (d=-1.0,i=i1;i<axis1.num;i+=2)
            {
            x1=axis1[i+0];
            y1=axis1[i+1];
            dd=((x1-x0)*(x1-x0))+((y1-y0)*(y1-y0));
            if ((d<0.0)||(dd<=d)) { i1=i; d=dd; }
            }
        if (d>=0.0)
            {
            x1=axis1[i1+0];
            y1=axis1[i1+1];
            axis.add(0.5*(x0+x1));
            axis.add(0.5*(y0+y1));
            }
        }
    }

constant n=4 is just for safety overlapped search for closest points it should be a fraction of pnt.num. Sometimes the closest point is before the last found closest point this depends on the curvature of booth sides. Too big n will cause slowdowns and if n>pnt.num/4 it could also invalidate output.

If too small then for smaller radius of curvature will lower the accuracy this approach is dependent on sufficient point coverage. If the wing is sampled with too low point count it can lead to inaccuracy. The source code is 3 times almost the same thing you can chose which ix1 to remember (from first or second search) they are neighboring points

test profile:

1.000000 0.000000
0.990000 0.006719
0.980000 0.013307
0.970000 0.019757
0.960000 0.026064
0.950000 0.032223
0.940000 0.038228
0.930000 0.044075
0.920000 0.049759
0.910000 0.055276
0.900000 0.060623
0.890000 0.065795
0.880000 0.070790
0.870000 0.075604
0.860000 0.080234
0.850000 0.084678
0.840000 0.088935
0.830000 0.093001
0.820000 0.096876
0.810000 0.100558
0.800000 0.104046
0.790000 0.107339
0.780000 0.110438
0.770000 0.113342
0.760000 0.116051
0.750000 0.118566
0.740000 0.120887
0.730000 0.123016
0.720000 0.124954
0.710000 0.126702
0.700000 0.128262
0.690000 0.129637
0.680000 0.130829
0.670000 0.131839
0.660000 0.132672
0.650000 0.133331
0.640000 0.133818
0.630000 0.134137
0.620000 0.134292
0.610000 0.134287
0.600000 0.134127
0.590000 0.133815
0.580000 0.133356
0.570000 0.132755
0.560000 0.132016
0.550000 0.131146
0.540000 0.130148
0.530000 0.129030
0.520000 0.127795
0.510000 0.126450
0.500000 0.125000
0.490000 0.123452
0.480000 0.121811
0.470000 0.120083
0.460000 0.118275
0.450000 0.116392
0.440000 0.114441
0.430000 0.112429
0.420000 0.110361
0.410000 0.108244
0.400000 0.106085
0.390000 0.103889
0.380000 0.101663
0.370000 0.099414
0.360000 0.097148
0.350000 0.094870
0.340000 0.092589
0.330000 0.090309
0.320000 0.088037
0.310000 0.085779
0.300000 0.083541
0.290000 0.081329
0.280000 0.079149
0.270000 0.077006
0.260000 0.074906
0.250000 0.072855
0.240000 0.070858
0.230000 0.068920
0.220000 0.067047
0.210000 0.065242
0.113262 0.047023
0.110002 0.042718
0.106385 0.038580
0.102428 0.034615
0.098146 0.030832
0.093556 0.027239
0.088673 0.023844
0.083516 0.020652
0.078101 0.017670
0.072448 0.014904
0.066574 0.012361
0.060499 0.010044
0.054241 0.007958
0.047820 0.006108
0.041256 0.004497
0.034569 0.003129
0.027779 0.002005
0.020907 0.001129
0.013972 0.000502
0.006997 0.000126
0.000000 0.000000
0.000000 0.000000
-0.003997 0.000126
-0.007972 0.000502
-0.011907 0.001129
-0.015779 0.002005
-0.019569 0.003129
-0.023256 0.004497
-0.026820 0.006108
-0.030241 0.007958
-0.033499 0.010044
-0.036574 0.012361
-0.039448 0.014904
-0.042101 0.017670
-0.044516 0.020652
-0.046673 0.023844
-0.048556 0.027239
-0.050146 0.030832
-0.051428 0.034615
-0.052385 0.038580
-0.053002 0.042718
-0.053262 0.047023
-0.053153 0.051484
-0.052659 0.056093
-0.051768 0.060841
-0.050467 0.065717
-0.048744 0.070711
-0.046588 0.075813
-0.043988 0.081012
-0.040935 0.086297
-0.037420 0.091658
-0.033435 0.097082
-0.028972 0.102558
-0.024025 0.108074
-0.018589 0.113618
-0.012657 0.119178
-0.006228 0.124741
0.000704 0.130295
0.008139 0.135828
0.016079 0.141326
0.024525 0.146777
0.033475 0.152169
0.042930 0.157488
0.052885 0.162722
0.063339 0.167858
0.074287 0.172883
0.085723 0.177784
0.097643 0.182549
0.110038 0.187166
0.122902 0.191621
0.136226 0.195903
0.150000 0.200000
0.164214 0.203899
0.178856 0.207590
0.193914 0.211059
0.209376 0.214297
0.225227 0.217291
0.241453 0.220032
0.258039 0.222509
0.274968 0.224711
0.292223 0.226629
0.309787 0.228254
0.327641 0.229575
0.345766 0.230585
0.364142 0.231274
0.382749 0.231636
0.401566 0.231662
0.420570 0.231345
0.439740 0.230679
0.459054 0.229657
0.478486 0.228274
0.498015 0.226525
0.517615 0.224404
0.537262 0.221908
0.556930 0.219032
0.576595 0.215775
0.596231 0.212132
0.615811 0.208102
0.635310 0.203684
0.654700 0.198876
0.673956 0.193679
0.693050 0.188091
0.711955 0.182115
0.730644 0.175751
0.749091 0.169002
0.767268 0.161869
0.785149 0.154357
0.802706 0.146468
0.819913 0.138207
0.836742 0.129580
0.853169 0.120591
0.869166 0.111246
0.884707 0.101553
0.899768 0.091518
0.914322 0.081149
0.928345 0.070455
0.941813 0.059445
0.954701 0.048128
0.966987 0.036514
0.978646 0.024614
0.989658 0.012439
1.000000 0.000000