The Coanda Effect has come to mean attachment of a flow to a surface beyond where we "expect" it to remain attached - but this is a scientifically meaningless view.

The classic example is that of fluid pouring out of a bottle or teapot. If the fluid does not pour straight out but flows around the lip, reversing direction before finally succumbing to gravity and detaching, this effect of "sticking to the surface" has come to be called the "Coanda Effect".

A more interesting example that anyone can perform for themselves is to allow a fine continuous stream of water to issue from the kitchen tap.  Bring the rounded pad of your first finger towards the stream until it just touches.  With the finger barely touching the flow, the stream of water will be diverted around the finger and spray out horizontally with some force.  The horizontal distance the water is projected is remarkable.

The best introductory article available on the subject is still  "Applications of the Coanda Effect" by Imants Reba, which appeared in the June 1966 edition of Scientific American.

This diagram shows a Coanda Thruster tested by Reba.  Air is ejected from a plenum at the front of the body.  A small step is inset into the surface of the body which causes the ejected air jet to attach to the surface and flow around it towards the upper surface.  A sheet of attached air called a Coanda Jet flows towards the back of the thruster. 

In so doing, it entrains by suction up to 20 times as much air from the surrounding atmosphere as is in the jet itself.  A shroud placed around the body increases the suction on the surrounding air even more.

Air pressure on the front of the thruster is therefore reduced by the entrainment suction so the body moves forward.  In addition, the afterbody made of flat angled segments causes the attached Coanda jet or sheet to exert a positive pressure on the rear of the thruster and so further increase thrust.

We therefore have exactly the opposite situation to a normal airfoil moving through air:  instead of positive air pressure on the front and negative pressure on the rear, creating drag, we have Negative Drag - i.e. Thrust.

This shows a model levitating device (hovercraft) tested by Reba.  The body of the device is made of short flat surfaces, creating a so called Coanda Surface; high pressure air ejected from an annular slot on the top of the device flows around and down to wards the bottom, entraining the surrounding air as it does so and creating a partial vacuum on the upper surface - a lower pressure region.  Lift is therefore produced.  As with the thruster above, ambient air pressure is increased below the device to increase lift.

Hydrofoil and Submarine Propulsion
Reba tested a model hydrofoil using a shrouded Coanda thruster.  The entrainment of the surrounding water to produce thrust results in very little wake or noise.  Reba felt that a hydrofoil so equipped could reach speeds of 80 knots.  At about the same time in 1962, Stine at the Huyck Corporation worked with Henri Coanda to build a similar device using ejected steam for submarine propulsion.

This concept has recently been re-invented by Australian Alan Burns and developed by Pursuit Dynamics in the UK.  A 20 cm long "underwater jet engine" that injects steam from an annular slot into an internal Coanda nozzle is said to produce 30 HP output.  (See New Scientist, 1/3/03, P19).

Allied Signal patented an internal Coanda nozzle using similar principles in the late 90s.  One application is to eliminate the  back pressure from the exhaust of an internal combustion engine, but to instead suck the combustion products out and so improve efficiency.

Circulation Control Wing
Circulation Control Wing technology is one of the most important potential applications of the Coanda Effect.

The objective is to replace the lift devices on the leading and trailing edges of a wing by use of Coanda Surfaces and slot blowing instead.

The diagram above is from  AIAA 93-0644.

The first known use of this "blown flaps" concept was on the prototype Boeing 707.  Boundary Layer blowing was successfully used on the Hawker Siddeley Buccaneer to improve STOL performance for aircraft carrier operations.  The supersonic TSR2 also incorporated blown flaps, allowing the 90,000lb delta winged aircraft with a wingspan of only 37 feet to achieve an approach speed of 130 knots:  the Concorde type droop nose was also eliminated since a much flatter non alpha lift approach could be made.

In the late 1970s, Robert Englar tested a modified Grumman A6 Intruder fitted with a prototype CCW system.  The  aircraft was able to fly at less than 60 knots and take off or land in less than 600 feet without catapult or arrestor assistance.

Studies showed that a Boeing 737-100 weighing 105,000 lbs fitted with CCW and no headwind would take 3000 feet to clear a 50ft obstacle on take off (Sea Level, ISA); the normal distance is 5000 feet.  Landing roll with no headwind  would be 750 feet, compared to 2000 feet for the conventional configuration.  A lightly loaded 737 (65,000 lbs) with a 20 knot headwind would land in 300 feet with CCW lift devices.

CCW allows lift to be produced at Zero Degrees Angle of Attack.  Coefficient of Lift (C_L) of 8 at alpha = 0 was achieved in tests.

CCW has been criticised for requiring extra APU or engine capacity to supply the bleed air to drive the slot blowing,

According to Englar, a pressure differential of 13.8 psi at Sea Level ISA is sufficient to produce a Coanda Jet velocity equal to the speed of sound.  Normal airfoils achieve a C_P of between -1.0 and -2.0; the most efficient airfoil possible, the cylinder, has a C_P of -3.0.  CCW equipped wings have achieved C_P of between -50 and -60, i.e. the suction on the upper wing surface is 50 times the freestream dynamic pressure.

Cross feed can be used between the plena in each wing to maintain flap blowing in case of an engine failure.

Engine and APU manufacturers are studying a next generation of APUs that would supply all the bleed air for pneumatic systems, leaving the engines dedicated for thrust alone.  APU reliability will have to improve greatly before this is achievable.

The X Wing Helicopter
The conventional helicopter is limited to a maximum forward speed of about 200 knots; above that speed the decreasing relative velocity of the leading edge of the retreating blade with respect to the freestream air flow leads to unsustainable loss of lift and lift asymmetry between the forward going and retreating blades; if the helicopter tried to fly even faster, the oncoming airstream would impinge the trailing edge of the retreating blade causing even more loss of lift.

Numerous designs have been put forward since the 1950s for Compound Helicopters to help solve this.  The compound helicopter adds a wing and thrust engines to transition to fixed wing mode and unload the rotor at high speed.

Despite the great advantages in efficiency, payload and range that this would bring, no compound helicopter has been commercially developed.  Piasecki Aircraft have been trying to commercialise this technology since 1955 and flew their first Vectored Thrust Ducted Propellor (VTDP) designs in the 1960s:  in 2002 they were awarded a contract to convert an SH-60 to VTDP mode but this apparently was cancelled due to disagreements over flight test rules. 

Another solution is to use CCW technology.  In this application, the helicopter rotor blades are constructed as Coanda surfaces.  A plenum and spanwise blowing slot run the length of the trailing and leading edges, with a symmetrical cross section to the rotor blade.  Air is blown onto the trailing edge of the forward going blades.  As the blades retreat, slot blowing is swapped onto the leading edge of the blade, which is now a trailing edge with respect to the forward motion of the helicopter.  This maintains lift on the retreating blades and theoretically will allow forward flight speeds of 400 knots in pure helicopter mode.  Above that speed, fixed wing mode is more efficient so the rotor is locked into place to create an "X Wing".  Differential slot blowing is used in the transition phase to eliminate differential lift as the rotor slows down.

NASA Ames developed a fully functioning flight test vehicle in 1979 to test this X Wing concept (right):  apparently it never flew due to engine bearing problems.  It is more than just a compound helicopter with a wide chord rotor:  the circulation control rotor can maintain helicopter mode lift at high forward speed and use differential blowing for lateral control.

US Patent 4,626,171 "Rotor Blade Construction for Circulation Control Aircraft" was then issued to United Technologies (Sikorsky) in 1986 for the same technology.

The latest embodiment of this idea - the Canard Rotor Wing - is a stopped rotor design that does not as far as we know use circulation control or slot blowing to increase maximum forward speed in helicopter mode, or to control the transition from helicopter to stopped rotor mode.

A search through the patent literature shows that quite a number of stopped rotor X Wing configurations have been developed, such as

• US Patent 4,711,415 to Northrop for "X Wing helicopter Scout Attack Configuration"
• US Patent 5,405,104 for "Stopped Rotor Aircraft Utilising a Flipped Airfoil X Wing"
• US Patent 4,573,871 to the US Army for "X Wing Aircraft Circulation Control"
• US Patent 3,794,273 to Teledyne Ryan for "VTOL Rotor Wing Drone Aircraft".

VTOL craft development has remained completely unchanged since the 1930s with the exception of the NOTAR No Tail Rotor technology by McDonnell Douglas Helicopters. This technology eliminates the danger and noise of the tail rotor and provides impressive performance improvements.

Drag Reduction Spoilers for HGVs
A straightforward application of the Coanda Effect has been the development by Robert Englar of spoilers for Heavy Goods Vehicles. These could reduce fuel consumption by up to 15% by reducing the negative pressure region formed behind the trailer of a typical square edged haulage vehicle.

Other Applications

Some other applications and technologies that have been developed using the Coanda Effect include:

• The Gurney Flap
• Dual Cavitating Hydrofoil Thrusters
  
• Drag elimination and aerodynamic braking systems for automobiles
• The NOTAR helicopter anti-torque system
• The Flettner Rotor Wing
• The Kline Fogleman Wing
• Wing in Ground Effect Vehicles
• The Carr Internal Wing (or Channel Wing) Aircraft

References

Further Information

Further information can be found in Meridian International's 1997 "Advanced Propulsion Systems Study".  An updated version will be coming out later in 2005.