Method of fabricating a ducted blanket for a rotor spar

In the fabrication of impervious fiberglass blankets for spars of rotor blades which are hollow and pressurized with a gas which passes through any flaw in the spar, resulting in an internal pressure drop to indicate the flaw, wherein the blankets are molded with slots on the inner side thereof forming venting ducts for said gases, the improvement whereby spar-to-blanket adhesion is enhanced, comprising, for vacuum forming between a base plate and a caul plate, preparing over a grid of spaced-apart strips a plurality of sheets of flexible fiberglass pre-impregnated with a thermosetting resin, the grid when removed forming the slots in the molded blanket, inserting between the grid and the base plate and between the fiberglass and the caul plate planar films of peelable material, under vacuum forming conditions heat setting the thermosetting resin, causing the blanket material to conform substantially to the grid so that the strips of the grid provide the spaced-apart slots on the inner side of the cured blanket material, and causing the planar film between the fiberglass and the caul plate to adhere to the top of the resulting blanket and the planar film between the fiberglass in the slots of the grid and the base plate to adhere to the bottom blanket surfaces, protecting said surfaces from mold release and from outside contamination until the blanket is bonded to the spar and internal rotor blade surfaces when the planar films, being sacrificial, are removed, thereby maximizing the bond between the spar and the blanket and between the blanket and rotor blade internal surfaces.