The Coanda Effect 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 by Albert Metral after Romanian inventor Henri Coanda who became interested in the phenomenon after it destroyed a prototype aircraft he had developed (the Coanda-1910).
It 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. Several aircraft have been built to take advantage of this effect by mounting jet engines 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, the Antonov An-72 Coaler.
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.
The Coanda effect is important in the understanding of an airfoil's lift.
If one holds the back of a spoon close to (but not touching) a stream of water running freely out of a tap (faucet), the stream of water will deflect from the vertical in order to run over the back of the spoon. This is the Coanda Effect in action.