Very good trying to explain that, I understand what you are saying, but I do it all the time too, for the last 20 years..........went from pen and ink to CAD.
I can think of no other way to put it into words. Recently I have been putting NA T-6/SNJ drawings into CAD, but the problems I have seen is knowing what they mean in some areas of the original drawings. You really have to understand the basics of drawing conventions and then think really hard about how they did it back then.
I also enjoy taking a part or shape and doing it backwards so to speak. I draw and plot out a grid. Then take the part/shape and lay it down on the grid, trace it out and then measure the X and Y coordinates and "draw" into CAD. Kinda fun stuff.
Cvairwerks wrote:
V: The ordinate system is pretty easy. Think back to junior high math and learning to plot curves. The basic layout is that one column gives you the distance along the chord from the datum and the other tells you how far from the chord the surface line is at that point. If the airfoil is symetrical, the table will only give you one pair of numbers. If it's not a symetrical shape, there will be a third column of numbers that apply to the other surface. The chord dimension will either be in a decimal number or a percentage. You will have to figure that out from the drawing. Also verify if the tables are using the airfoil centerline or MAC for the reference.
Once you have the data plotted, its just drawing the best fit, smoothest curve between all the points.
There are numerous books on aircraft lofting, but also look at lofting and working with tables of offsets from boats.
If I remember correctly "Theory of Wing Sections" has some good info on how this all works and a bunch of good data in the referenyou have your CAD routine, it's easy to draw using copy and offsets. Draw a horizontal centerline. Draw a vertical centerline. Making the assumption that the the nose point is the datum, and that your data is in decimal form, copy the vertical line and place it offset to the right by the distance given in the table for the second point on the rib. You've now done half the locating for the point. Repeat the process for all of the vertical dimensions. Repeat the process for the other column, but use the horizontal line for the offsets. Repeat the offset above and below the centerline.
As you work across the airfoil, trim your verticals and horiozontals so that you end up with stairstepping across the drawing. Draw a curve via intersection points using the pick feature so that you go through the intersection, and make it a polyline. Connect the top and bottom surfaces, get rid of your construction lines and you've electonically lofted the rib surface. Now all you have to do is calculate the bend allowance and reductions and flange width and add the appropriate offset to the profile, and you have a basic sheetmetal pattern.
You can do the same thing by hand drawing, but the curve fitting will have to be done with french curves and flexible curves. It's a bit harder and more tedious to do it by hand and get it right. It's also terribly time consuming too. I did a fairly simple transonic wind tunnel model for a research program and even doing the drawings at 20 times size, I had over 200 manhours in the basic model of a wing section. By the time you add the instrumentation drawings, I belive I was over 300 hours on those 10 "E" sized pages.
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(Another CAD guy) 
