Since one spar cap is in tension and the other is in compression, what you have is something like an earthquake fault line between the spar caps. They would like to move relative to each other in a sliding fashion and that is what shear webbing was invented to resist. The highest load on shear webbing is at the midpoint between the two spar caps. My patient Uncle and I examined the shear web loads on my 1/6 scale Cub spar design and found that 1/64 ply was more than adequate for a spar
of
those dimensions and loadings (total depth 1 inch, width 3/8 inch, spar cap
thickness 1/8 inch). If your spar dimensions or loadings go up by a factor
of more than two or three you might want to go to 1/32 ply for the shear
webbing. Shear loads are constant from root to tip. If your spar at the
wing center section is solid top to bottom, along the center 10% of span
(approx.)it will help resist shear loads.
For the program to be valid the spar must not be allowed to buckle as you
look at it in cross section from the wingtip. The wing ribs do this job so
they should not be too far apart. If the spar can buckle it will fail at a
far lower loading. I usually don't go past 3 inches on rib spacing unless
the wing chord is more than 15 inches.
I use this program to gain confidence in the strength of my wing designs. It is a starting point, and should be followed up by actual failure tests of your wood and design, especially if you are cutting close to the margin on weight and load factor. Before testing the sample run the program a few times, using a load factor of one and various weights until you arrive at a 100 percent rating in the design requirement statement. Your sample spar can be about three feet long and should be supported in the middle.Load the sample evenly from tip to tip until it fails. The weight the failure occurs at should be close to the 100 percent rating weight predicted by the program. A variance will most likely be due to 9000 not being the correct stress for your wood sample. You might want to lower this value if your samples consistently fail at too low a value, or you can look for some better samples of wood. Another thing that will throw off the result is if your sample is not exactly the size you are putting into the program. Your measurements should be within 1/64 of an inch, especially on small sizes.
To test for errors in the program after you type it, do a test run using the values from the sample run which are for the 1/6 scale Cub spar. Remember, testing is central to the design process. Calculations are only the starting point, and have to be compared to the real world by careful testing.