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. It was first implemented in a practical sense during the U.S. Air Force's AMST project. Several aircraft, notably the Boeing YC-14, 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'. The McDonnell Douglas YC-15 and its successor, the Boeing C-17 Globemaster III also employ the effect, though to a less substantial degree.

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.

Demonstration

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.

This demonstration is the combination of the Venturi effect and the Coanda effect. The Venturi effect would cause a drop in pressure between the spoon and the stream of water which would then be drawn towards the spoon. Once the surface of the stream hits the spoon the Coanda effect keeps it running over the convex surface.

See also

• Aerodynamics
• Fluid dynamics
• Boundary layer

External links

• Coanda Effect: Understanding Why Wings Work
• Coanda Effect example