The present invention provides a propulsion system for an aircraft. The system includes one or more thrust producing portions, wherein the one or more thrust producing portions include one or more duct means. The duct means are at least partially formed or defined by two or more substantially parallel wall members. At least one flapping or waving wing member is provided, at least partially located or positioned substantially within the one or more duct means, wherein the flapping or waving motion of the at least one wing member creates thrust, enabling the aircraft to fly in use.