Plotting individual ribs for a tapered or elliptical wing used to be a long tedious process. We say "used to be," because now, when you want to plot those ten or fifteen different ribs you won't have to draw an individual graph, divide up odd size lines, read coordinates and estimate the position of points, again and again. Our new method of plotting looks somewhat odd, but is actually based upon the older method and is absolutely correct, mathematically. Although our new method takes only one-fifth the time, the results are just as, if not more, accurate.

The procedure is to first draw a chart (like that shown in the drawing) which is kept for permanent use; then by using this chart we can draw any size airfoil in about three minutes. A separate chart must be made for every different airfoil but it takes only about one hour to draw one, which isn't long, considering you can use it for years. That one hour investment will pay tremendous dividends in time saved later on.

The chart shown is for R.A.F. 32 airfoils of any size up to 10"; gasoleers who use larger size airfoils should make large charts. The chart should be made on a good piece of paper. (Bristol board is excellent for the purpose.) In the upper left draw a 10" airfoil of whatever particular airfoil you want. The 10" is chosen because it is easy to plot even by the regular method, using an engineer's scale. Readers not familiar with the regular method of plotting will find an explanation on the drawing. Now from point B (the 100% point of the airfoil) draw a line about 12" long at 60 degrees to the horizontal. From point A (the end of this line) draw lines to the intersection of the 10" chord line and the vertical station lines used in laying out the 10" airfoil. Call these base lines. Then draw lines from A to the points of the airfoil itself. Call these contour lines. Also draw a horizontal line through A. That's all there is to the chart and it is ready to use.

Suppose you want to draw a 6 7/8" airfoil. To do this, tack a piece of tracing paper over the chart and measure off 6 7/8" from point A along the horizontal line. From this point draw a line parallel to AB (i.e., 60 degrees to the horizontal). Where this line hits the 0% base line draw a horizontal line. This line is the chord line for the 6 7/8" airfoil. The next step shown in Fig. 2, is to draw vertical lines through the intersection points of the 6 7/8" chord line and the base lines. Then as in Fig. 3, mark the intersection points of these vertical lines with the contour lines. Connect these points with a French curve and you have a 6 7/8" airfoil. Simple, isn't it?

Here are a few further hints in using the chart: To draw a complete set of ribs for an elliptical, tapered or other shaped wing just repeat the above procedure for each size rib needed. They can all be drawn on one sheet of tracing paper, the paper being shifted after each plotting so the ribs don't overlap.

To make templates, paste the tracing paper to some heavy paper or thin plywood and trim to shape.