THE COANDA EFFECT Ref. Jef Raskin's Webpage Coanda Effect: Understanding Why Wings Work

If a stream of water is flowing along a solid surface which is curved slightly away from the stream, the water will tend to follow the surface. This is an example of the Coanda effect and is easily demonstrated by holding the back of a spoon vertically under a thin stream of water from a faucet. If you hold the spoon so that it can swing, you will feel it being pulled toward the stream of water. The effect has limits: if you use a sphere instead of a spoon, you will find that the water will only follow a part of the way around. Further, if the surface is too sharply curved, the water will not follow but will just bend a bit and break away from the surface.

The Coanda effect works with any of our usual fluids, such as air at usual temperatures, pressures, and speeds. I make these qualifications because (to give a few examples) liquid helium, gasses at extremes of low or high pressure or temperature, and fluids at supersonic speeds often behave rather differently. Fortunately, we don't have to worry about all of those extremes with model planes.

A stream of air, such as what you'd get if you blow through a straw, goes in a straight line
A stream of air alongside a straight surface still goes in a straight line
A stream of air alongside a curved surface tends to follow the curvature of the surface. Seems natural enough.
Strangely, a stream of air alongside a curved surface that bends away from it still tends to follow the curvature of the surface. This is the Coanda effect.

Another thing we don't have to wonder about is why the Coanda effect works; we can take it as an experimental fact. But I hope your curiosity is unsatisfied on this point and that you will seek further.

Coanda and Flight

Many scientists have recently begun using the COANDA EFFECT to at least partially explain how planes fly. Ref .

For a long time many people believed (and many people still do) that LIFT during flight is achieved due to something called the Bernoulli Effect. This theory suggest that air moving across a wing moves more quickly over the top than underneath. This creates an area of lower pressure on top of the wing in comparison to the underside of the wing. Thus, less pressure pushes down on the wing and more pressure pushes up and consequently LIFTS the craft into the air…

However, many scientists disagree with this explanation! Is it time to say good bye to Bernoulli's principle while speaking about a lift of wing? My interpretation can be found here.

Some scientists have suggested recently that due to the shape of a planes wing, air moving along it due to the COANDA EFFECT will be deflected downwards as it leaves the wing and thus push the craft up into the air (due to NEWTONS THIRD LAW OF MOTION) and consequently assist with LIFT

What is the Coanda effect?

Displayed above is an image of a girl and a few soldiers. Soldiers are marching forward in straight line. Each soldier holds hand of his neighbor. Suddenly outsider soldier caches a hand of a girl standing on a sidewalk.	See what happens. This is simplest explanation of Coanda effect. Note: it is assumed, that "soldiers" are some fluid elements, not single molecules.

Very simply, the COANDA (or ‘wall attachment’) EFFECT is the tendency for a moving fluid (either liquid or gas) to attach itself to a surface and flow along it.

One way of explaining this effect is to understand that as a fluid moves across a surface, and certain amount of friction (called skin friction) occurs between the two surfaces (friction is that force that slows down or prevents two surfaces from moving across each other). This friction tends to slow down the fluid as it moves across the other surface. This resistance to the flow of the fluid will then pull the fluid towards the other surface, making it stick to it… even as it bends around corners! Ref .

Information courtesy of Saulius Pakalnis, Research Support Technologies (researchsupporttechnologies.com)