A stiffened skin panel of an aircraft that includes an aircraft skin panel of composite material and a composite stringer consolidated with the aircraft skin panel. The stringer has a pair of stringer sides, each extending from a stringer top to a stringer leg. The stringer sides may each extend through a wide radius, smooth continuous curve to a stringer leg. The wide radius, smooth continuous curve may be proximate the base of the stringer side in a region where the stringer side transitions to the stringer leg.