Blown flaps, blown wing or jet flaps are powered aerodynamic high-lift devices used on the wings of certain aircraft to improve their low-speed flight characteristics. They use air blown through nozzles to shape the airflow over the rear edge of the wing, directing the flow downward to increase the lift coefficient. There are a variety of methods to achieve this airflow, most of which use jet exhaust or high-pressure air bled off of a jet engine's compressor and then redirected to follow the line of trailing-edge flaps.
Blown flaps may refer specifically to those systems that use internal ductwork within the wing to direct the airflow, or more broadly to systems like upper surface blowing or nozzle systems on conventional underwing engine that direct air through the flaps. Blown flaps are one solution among a broader category known as powered lift, which also includes various boundary layer control systems, systems using directed prop wash, and circulation control wings.
Internal blown flaps were used on some land and carrier-based fast jets in the 1960s, including the Lockheed F-104, Blackburn Buccaneer and certain versions of the Mikoyan-Gurevich MiG-21. They generally fell from favour because they imposed a significant maintenance overhead in keeping the ductwork clean and various valve systems working properly, along with the disadvantage that an engine failure reduced lift in precisely the situation where it is most desired. The concept reappeared in the form of upper and lower blowing in several transport aircraft, both turboprop and turbofan.