Making tapered ribs like the picture above can be done easily and accurate with a little planning.
The first thing I do is cut a piece of material about 1-2 inches longer than needed. Then notch the end that fits into the spar channel. After that mark the actual length that you need.
Now we need to find the centerline of the rib. I used a scrap of RS-17 and a piece of RS-8. One side of the RS-8 is 5/8″ the exact distance to the center of an RS-17. Line up the pieces of wood and draw the centerline on both sides of your length mark.
Next cut a small piece of RS-15 and mark the centers. The RS-15 is mounted at an angle as noted in the rib detail on drawing 10. Using the RS-15 piece mark the top, bottom and glue angle on your RS-17 rib.
Now we need to draw the tapper. The plans seem to indicate that you should make a pattern like the rib detail on drawing 10. The issue with that idea is each rib is a different length and different angle and would result in a non uniform tapper. To create a consistent tapper no matter the angle or length you need to start the tapper at the same relative position. I picked 1/3 of the length for the start point. I measured from the front of rib to the length like made earlier, calculate what 1/3 of that is and measured that amount from front of rib and made a mark. This is where the taper will start. Draw a straight line from that mark to your RS-15 marks.
Sorry I forgot to take a picture of this, I will try to remember to take one and update this post when I build the elevator.
Before you cut out the rib draw a little tab on the bottom of the rib, this will support the back of the rib as you glue things together. The top of this tab will support the RS-15 when you glue it on so it is important to accurately cut the top of the tab to match the bottom of the RS-15 line drawn earlier. When you cut it out it will look something like this.
With the RS-15 in place the tapper continues smoothly.
With a little sanding you can round of the start of the tapper and end up with an awesome rib.
After you get them all cut its time to assemble. It takes some time to fine tune each rib so everything is perfect. Once everything fits mix up some epoxy and glue.
I put a board over the front spar and ribs, before they tapper, and stacked some bricks letting it dry overnight.
Once dry I cut the tabs off, did a little sanding and it looks something like this.