We fondly remember the Twin Star as a popular ARF that was most commonly powered by .25 engines, and sometimes engines as big as .40 or .45, but RCM originally published this plan as the Twin Star 15, an aerobatic sport plane for two .15 to .19 engines.
On the plan you will see a 1/4 x 1/4 balsa stringer at the top of the fuselage side, just below where the windows would be. The plan shows the bulkheads without notches, so the stringer must be cut into pieces that go between the bulkheads. My bulkheads have notches in them, so you put a full length, uncut stringer on the fuselage side, then add the bulkheads. If you have any questions, just ask.
2 x 15-25