When you build a wing with dihedral, the root ribs need to be leaned inward at a specific angle to give the wings the correct dihedral angle. This is easy to do with a dihedral template, which you can make from a piece of paper. All you need to know is the length of one wing panel, and the amount each wing panel will be raised from the building board. This can be tricky, so make sure you get this part right. Let’s say your plan calls for a 72 inch wingspan and 2.5 inches of dihedral at each tip. In this case our numbers would be 36 inches and 2.5 inches. If your plan says 48 inch span and dihedral 4 inches at one tip with the other wing panel flat on the table, then that’s a 24 inch wing panel. We want to imagine the wing sitting on the table like a V, so we have 2 inches at each tip. If you’re building an 84 inch plane with a 12 inch center section and 4 inches of dihedral at each tip, it gets a bit complicated. You have to make two dihedral joints where the panels join the center section. Imagine one joint sitting on the table like a V with the root ribs standing vertical. The center section is angled up on one side, and the wing panel is angled up on the other. This cuts your 4 inch dihedral at the tip to 2 inches. So your necessary data for our formula is 36 inch panel and 2 inch dihedral. If you find any of this confusing, send me an email and I’ll walk you through it.
Now you need a sheet of paper, a ruler, a pencil and an X-Acto knife. We’re going to make a scale model of your root rib angle on the lower left corner of the paper. Let’s use the example with a 48 inch span and 4 inches of dihedral at one tip. The numbers we’re looking for are 24 inches and 2 inches. Use a handy scaling unit that will fit on a sheet of paper. I have a ruler with centimeters on one side, so I just count centimeters. Measure from the lower left corner of the paper 2 cm to the right and make a mark on the lower edge. Measure from the corner 24 cm upwards and make a mark on the left edge.
Now place the ruler on these two points and cut with an X-Acto knife.
Discard the sliver of paper you cut off.
The angle you have created on the corner of the sheet is the angle your root ribs need to lean inwards. To make it easier to see which way to use the template, cut the other corner off so it’s round on that edge. That way you’ll never use the wrong corner by mistake.
Place one edge of this paper flat on the table, and position the rib with the other edge.
If you are working with numbers that are just too big for a sheet of paper, such as a 96 inch wingspan and 5 inches at each tip, you’ll need to scale things down even further. You can’t measure 48 cm on a piece of paper because the paper isn’t that big. Just divide it by some convenient number, but make sure you divide your dihedral number also. You started with 48 and 5, so divide by 2 and get 24 and 2.5. This is small enough to fit on a sheet of paper.