Most kites use thin sheet material for their sails.
It is well known, however, that this is not the most efficient lifting surface, and aircraft wings invariably have a pear-drop cross section which causes the air passing over the top to travel further than the air passing underneath, so producing a reduction in air pressure above the wing with minimal turbulence. In an aircraft, this is done with rigid structures but these would be far too heavy to fly as a kite in moderate wind speeds. The solution is the parafoil kite, invented in 1963 by Domina Jalbert of Florida. An aerofoil shape is created as a structure inflated by wind entering vents in the front or underside. Normally, no spars are needed at all, although fibreglass rods or loops are sometimes used to help maintain the shape or to keep vents open, making it more reliable in cross-winds.