My XL plans Page 37A with a plans change date of 9-9-12 have a note added."One Side Only" Ribs #3, #5, #7, #9. This numbering does not count the second rib next to Rib #1 or the rib next to the Tip Rib (plan only counts 11 ribs per wing). If you count every built-up rib starting at the root, the compression struts go on the 4th, 6th, 8th, and 10th ribs. The wing drawings on page 36 & 37 show single compression struts on the root side. I'm sure the only penalty for double compression struts is the weight. I wonder how much 16 pieces of 1/4" x 3/4" x 29" weigh?