The Coandă effect (IPA: ['kwandə]), also known as "boundary layer attachment", is the tendency of a stream of fluid to stay attached to a convex surface, rather than follow a straight line in its original direction. The principle was named after Romanian inventor Henri Coandă, who was the first to understand the practical importance of the phenomenon for aircraft development. He made the discovery during experiments with his Coandă-1910 aircraft, which is the first aircraft based on an early type of jet engine.
Henri Coandă obtained, in 1934, in France, a patent for a "Method and apparatus for deviation of a fluid into another fluid". What is today known as the Coandă effect was described by its discoverer as the "Deviation of a plan jet of a fluid that penetrates another fluid in the vicinity of a convex wall."
The Coandă effect has important applications in various high-lift devices on aircraft, where air moving over the wing can be "bent down" towards the ground using flaps and a jet blowing over a curved surface. The flow from high speed jet produces enhanced lift through turbulent mixing that does not occur above a normal wing. It was first implemented in a practical sense during the U.S. Air Force's AMST project. Several aircraft, notably the Boeing YC-14 (the first modern type to exploit the effect), have been built to take advantage of this effect, by mounting turbofans on the top of wing to provide high-speed air even at low flying speeds, but to date only one aircraft has gone into production using this system to a major degree, the Antonov An-72 'Coaler'. The McDonnell Douglas YC-15 and its successor, the Boeing C-17 Globemaster III, also employ the effect, though to a less substantial degree.
An important practical use of the Coandă effect is for inclined hydropower screens, which separate debris, fish etc otherwise in the input flow to the turbines. Due to the slope, the debris falls from the screens without mechanical clearing, and due to the wires of the screen optimisng the Coandă effect, the water flows though the screen to the penstock leading the water to the turbine.
Closely following the work of Coandă on applications of his research, and in particular the work on Aerodina Lenticulara, John Frost of Avro Canada also spent considerable time researching the effect, leading to a series of "inside out" hovercraft-like aircraft where the air exited in a ring around the outside of the aircraft and was directed by being "attached" to a flap-like ring. This is as opposed to a traditional hovercraft design, in which the air is blown into a central area, the plenum, and directed down with the use of a fabric "skirt". Only one of Frost's designs was ever built, the Avrocar.
