Alternate fuselage leveling

[Jim]

Hi,

...."You can then measure the distance of the components as from the Datum as needed."

Another purpose for leveling is for damage repair or inspection. A central chalk line is certainly necessary to determine the width and position of the lower longerons as well as the gear attach fittings. a common line will also allow plumbing down from the wing spar attachments and their positions compared to the lower fittings and distances from the chalk line. The engine mount bolt holes should also align symmetrically with the other established mounting positions. I'd guess the wing attach fittings would be what to compare everything else to. The lower longerons couldn't be trusted to build or repair up from without prior confirmation they are properly positioned. A racking of the lower fuselage is common after a ground loop.

I would go with a line originating from the centered tail spring fuselage mount, the Tri Pacer had a tie down or some such thing under there, snapped to mid point between the upper engine mount bolts. Working from what's welded in already will allow for establishing, or confirming the basics first. Hanging a plumb in the door won't help much if the whole fuselage bottom is off center for some reason or other.

[Gilbert Pierce]

Since the hole you use to hang the plumb bob is offset from the center line of the aircraft about 20” you are leveling the aircraft laterally and longitudinally when the plumb bob is centered over the punch mark on the outboard end of the seat rail.
If the plumb bob is left or right of the punch mark you are not level horizontally. If the plumb is is in front of or behind the punch mark you are off longitudinally.
Since the rigging instructions require the plumb bob to be centered over the punch mark that in essence says you must be level laterally.