Abstract: Hollow filament wound spar structure is provided adapted for rotational hub mounting under centrifugal and bending loads. The structure comprises an integrally wound, filament and resin composite spar of a predetermined wall thickness, a fitting integrally wound therein, the fitting defining rotational hub fastening means and comprising a plug having a mechanical engagement groove circumferentially of its longitudinal axis into which the opposing spar wall portion is deflected responsive to winding fabrication of the spar wall on the fitting. Belt means additionally are provided opposite the groove locally peripherally confining the deflected spar wall portion in mechanical engagement reinforcing relation, against relative dislodgement of the spar wall portion from the plug groove in the mounted condition of the spar structure, under centrifugal and bending loads.