A collapsible structure comprising a non-rigid helicoidal sheet braced by light-weight members according to one of several methods. The structure is used in the construction of a wind turbine, wherein a flexible fabric sheet (21) is reinforced by wires (42) sewn into its edges, and may be further reinforced by ribs or battens (30) sewn crosswise into the sheet. One end of the turbine is cnnected via a swivel (24) to a fixed mast (22) while the other end is conneced to the shaft (26) of an alternator. The electrical output of the alternator is connected through a rectifier/regulator (34) to a storage battery (38), thereby providing constant DC power. An alternative method of bracing employs ribs (30) along with counterhelical cables (64).