a kite having separate forward and aft sails , each arched about a common longitudinal kite axis and each sail disposed concave downward in flight . the concave shape of the sail funnels the wind through the kite . lift is derived from the central portions of the sail while the lateral portions of the sail , which are nearly vertical and are parallel to the wind direction , provide lateral stability . the proportional lift and weight balance provided by the aft sail , in combination with the bridle arrangement , provide longitudinal stability .