Coanda Effect

Coanda effect is the phenomena in which a jet flow attaches itself to a nearby surface and remains attached even when the surface curves away from the initial jet direction.

In free surroundings, a jet of fluid entrains and mixes with its surroundings as it flows away from a nozzle.

When a surface is brought close to the jet, this restricts the entrainment in that region. As flow accelerates to try balance the momentum transfer, a pressure difference across the jet results and the jet is deflected closer to the surface - eventually attaching to it.

Even if the surface is curved away from the initial direction, the jet tends to remain attached. This effect can be used to change the jet direction. In doing so, the rate at which the jet mixes is often significantly increased compared with that of an equivalent free jet.


The phenomena derives its name from a Romanian born aeronautical engineer - Henri Coanda.

In 1910, Coanda designed and built what was arguably the world's first jet propelled aircraft. The plane used a 4-cylinder piston engine to power a rotary compressor. The compressor exhaust entered two ring-shaped burning chambers located on each side of the fuselage. The gasoline engine's exhaust and additional fuel was also ported into the chambers. The combustion of this mixture exhausted from the chambers down the steel-sheeted plywood sides of the Coanda-1910 producing a thrust of 220 kgf.

He performed the first reactive flight on 16 December 1910 - which unfortunately ended in an accident. After the plane took off, Coanda observed that the flames and burned gases exhausted from the engine "hugged" the fuselage and the aircraft side rapidly caught fire. After studies which lasted more than 20 years, the phenomenon was recognized as a new aerodynamic effect: the Coanda effect.

Many devices use Coanda effect. One notable application of the Coanda effect is the NOTAR™ helicopter. NOTAR (No TAil Rotor) is the name of a helicopter system which replaces the use of a tail rotor. The system uses a fan inside the tailboom to build a high volume of low-pressure air, which exits through two slots and creates a boundary layer flow of air along the tailboom utilizing the Coanda effect. The use of directed air to provide anti-torque control had been tested as early as 1945 in the British Cierva W.9. There are three production helicopters that utilize the NOTAR system, all produced by MD Helicopters.

Coanda effect forms the basis of much of S & C Thermofluids' innovative in-house research programme. To date, successful devices and granted patents have included:

  • A fluidic Thrust Vector Control (TVC) nozzle
  • A compressed air powered ejector
  • A jet pump shower nozzle
  • A catalytic converter exhaust system
  • The high speed delivery of chocolate from a trough
  • Flow deflection devices

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S & C Thermofluids Ltd

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