Source: 2009 Q4 Beartracks, Glenn Patterson
The fuselage must be sitting square and level on the table with the tailpost dead vertical. The rudder also has to be assembled accurately for this method to work. The string is a fine 50lb. braided fishing line in the plumb bob and for the baseline string. Fishing line can take a lot of tension which helps with accuracy.
First step is to roughly install the vertical stabilizer with one tack weld on the bottom of the leading edge tube. Place the tack on the center facing towards the front of the fuselage. This will allow sideways movement of the stabilizer. Tack the top so T25 rib runs parallel to rudder and so that the assembly looks fairly straight front to back.
Tie a “Baseline” string line to the fuselage and wrap it around the tailpost & the base of the leading edge tube to also be tied tight to the fuselage. The string needs to be on the same side of both tubes and tied tight. Slide a piece of 1/8×1/2 flatbar under the string at the leading edge tube and set it so that it is flat with the string. The tailpost is a 7/8” tube and so the 1/8” shim added to the leading edge tube makes the string line parallel to the vertical stabilizer center line running through the tailpost and the lead edge tube. Similar to these illustrations.
Now take a piece of 1/8” aluminum flatbar and hold it against the top of the leading edge tube with the plumb bob string so that the flatbar is flat against the tube and not held off by any welds. This is shown in the diagram on the next page. The top of the fin is adjusted until the string lines just touch.
The strings must barely touch to be accurate because if the plumb bob line leans heavy into the baseline string then the accuracy is lost. The top of the Vertical Stabilizer (VS) leading edge tube is the exact center line required for the vertical fin when the strings just touch. Stand back and eye ball the fin as the leading edge tube should be centered on the tailpost.
Slide the plumb bob halfway down the leading edge tube and check that the two strings just touch. This will make sure that the center of the VS tube is on center or needs to be adjusted to take out any twist in the leading edge tube. It the string lines just make contact at the top and mid point then the VS is located over the centerline from the tailpost to the offset point of the leading edge tube. Stand back and visually check that the front tube of the VS fin runs vertical with the tailpost. If the fuselage is set up straight then the fin should be exactly where it needs to be without any guesswork. Check the top of fin and midpoint after each adjustment until the fin is in the right place and aligned. Then start tacking welding the vertical fin to the tailpost and keep checking. Once the top & middle of the vertical fin is set it is more accurate than the eye. Check the fit ups with the rudder as well as the fin has to flow into the rudder.
Some may argue that the top is off the width of the string line but that is only a few thousandths of an inch. A person can compensate for that by adding a shim equal to the line thickness under the plumb bob string.