A method for manufacturing a spar of a blade with the aid of tapes of composite materials, said spar comprising transversely at least one arrangement (10, 15), each arrangement (10, 15) being for winding around a root axis (AX1, AX2) of said spar, characterised in that: - during a preparation step, one male mandrel (20, 30) is defined per arrangement (10, 15) to be manufactured, each mandrel (20, 30) comprising a lower-surface working surface (21, 31) and an upper-surface working surface (22, 32) in a U-shape, each working surface comprising: • a root zone provided with a root wall (23, 33) in a C-shape forming a base (231, 331) of said U-shape, • a connecting zone having two twisted connecting walls (24, 25, 34, 35) connected respectively to two ends (232, 233, 332, 333) of said base (231,331), • a working zone having two working walls (26, 27, 36, 37) connected respectively to the two connecting walls (24, 25, 34, 35), - during a manufacturing step, each arrangement is subdivided into a lower-surface sub-assembly and an upper-surface sub-assembly comprising at least one hank, each hank comprising a plurality of segments of tapes in a U-shape stacked one on another, - during said manufacturing step, said lower-surface sub-assembly and said upper-surface sub-assembly of an arrangement (10, 15) are manufactured on one and the same mandrel (20, 30) which is adapted to this arrangement (10, 15) and outside a mould for manufacturing said blade, said lower-surface sub-assembly and said upper-surface sub-assembly of an arrangement (10, 15) being manufactured by stacking segments of tape on said mandrel by a fibre placement method with the aid of a placement head, said placement head winding each segment of tape of the lower-surface sub-assembly around a root axis (AX1, AX2) by applying said segment over the lower-surface working surface (21, 31), said placement head winding each segment of tape of the upper-surface sub-assembly around a root axis (AX1, AX2) by laying said segment on the upper-surface working surface, - during a heating step, each arrangement (10, 15) is heated on its mandrel (20, 30). - during a preparation step, one male mandrel (20, 30) is defined per arrangement (10, 15) to be manufactured, each mandrel (20, 30) comprising a lower-surface working surface (21, 31) and an upper-surface working surface (22, 32) in a U-shape, each working surface comprising: • a root zone provided with a root wall (23, 33) in a C-shape forming a base (231, 331) of said U-shape, • a connecting zone having two twisted connecting walls (24, 25, 34, 35) connected respectively to two ends (232, 233, 332, 333) of said base (231,331), • a working zone having two working walls (26, 27, 36, 37) connected respectively to the two connecting walls (24, 25, 34, 35), • a root zone provided with a root wall (23, 33) in a C-shape forming a base (231, 331) of said U-shape, • a connecting zone having two twisted connecting walls (24, 25, 34, 35) connected respectively to two ends (232, 233, 332, 333) of said base (231,331), • a working zone having two working walls (26, 27, 36, 37) connected respectively to the two connecting walls (24, 25, 34, 35), - during a manufacturing step, each arrangement is subdivided into a lower-surface sub-assembly and an upper-surface sub-assembly comprising at least one hank, each hank comprising a plurality of segments of tapes in a U-shape stacked one on another, - during said manufacturing step, said lower-surface sub-assembly and said upper-surface sub-assembly of an arrangement (10, 15) are manufactured on one and the same mandrel (20, 30) which is adapted to this arrangement (10, 15) and outside a mould for manufacturing said blade, said lower-surface sub-assembly and said upper-surface sub-assembly of an arrangement (10, 15) being manufactured by stacking segments of tape on said mandrel by a fibre placement method with the aid of a placement head, said placement head winding each segment of tape of the lower-surface sub-assembly around a root axis (AX1, AX2) by applying said segment over the lower-surface working surface (21, 31), said placement head winding each segment of tape of the upper-surface sub-assembly around a root axis (AX1, AX2) by laying said segment on the upper-surface working surface, - during a heating step, each arrangement (10, 15) is heated on its mandrel (20, 30).