Common spar assembly for use in nonfoldable and foldable proprotor blades

1. A method of constructing a foldable spar assembly, comprising:

providing a common spar, comprising:

a tip comprising a most outboard portion of the spar;

a root comprising a most inboard portion of the spar;

a main section disposed inboard relative to the tip that is comprised exclusively of curved surfaces; and

a middle section disposed inboard relative to the main section, the middle section comprising a first set of opposing flat outer surfaces and a second set of opposing flat outer surface situated laterally between and substantially perpendicular to the first set of opposing flat outer surfaces;

wherein the first set of opposing flat outer surfaces are configured to accommodate bolt holes for a nonfoldable spar assembly;

wherein the second set of opposing flat outer surface are configured to accommodate tang holes for a foldable spar assembly; and

joining the main section and the middle section using a transition section, the transition section comprising an outboard interface cross-sectional shape and an inboard cross-sectional shape that is different than the outboard interface cross-sectional shape;

drilling holes through the second set of opposing flat outer surfaces, wherein the holes are coaxially aligned; and

removing the root and a majority of the first set of opposing flat outer surfaces.

2. The method of claim 1 , further comprising:

forming the second set of opposing flat outer surfaces into blade tangs.

3. The method of claim 2 , wherein the blade tangs extend inboard and outboard of the holes.