Many disc shaped VTOL aircraft designs use the Coanda effect to generate the vertical thrust to lift the aircraft. The cool thing about it, is the not-using of rotary wings or jets. No part of the propulsion system has to stick out of the body. This looks great, but it may also be important for being less vulnerable in certain circumstances, like being in space, flying supersonic, or flying through a city.
The Coanda effect is useful for vertical take off and landing, but will be distorted when the speed relative to the ambient air increases. It only works within the atmosphere (and under water). A disc shaped body can generate thrust for lift, but never as efficient as wings do (although a disc might work better than wings at extremely high altetudes).
It's not clear to me how energy efficient the Coanda effect is relative to a rotary wing or duckted fan. If a lot of ambient air is sucked into the airflow, increasing the mass, then it might be pretty good.
The coanda effect is describing how an airstream gets pushed against a surface, even when the surface is curved away from the direction of flow. No: it doesn't really stick to a surface, that's only how it looks like. The airpressure between the airstream and surface is lower IF the surface is curved away from the flow. The "fast air has less pressure" is a false statement, because there is no such thing as "fast air". Speed is relative.
The coanda effect can be used to:
- Make air flow outside a disc body.
- Adding ambient air to the airstream, thus adding weight and improving efficiency.
The ambient air is pulling the airstream, and the airstream is pulling the ambient air. This causes lower air pressure inside and around the airstream. But soon will this pulling and pushing cause turbulence.
When blowing an airstream close to a solid surface, the interaction of the airstream causes a drop of air pressure inbetween the airstream and the surface. The ambient air at the other sides of the airstream and surface pushes the two together.
Remember the power of sea level airpressure: about 1 KG/CM2
A curved surface causes a continuing acceleration of the airstream, and thus a continuing area of low pressure between the airstream and the surface.
The airstream though, should not be bent beyond 90º. More than 90º would cause negative thrust. But going beyond 90º can be useful if the goal is to gather all air under the disc, building an area of higher air pressure.
DRAG & TURBULENCE
Main trouble of the coanda effect is the airstream becoming turbulent and detaching from the surface, like how a wing stalls. Drag causes turbulence, drag from the surface and from the ambient air. It's a goal to drag as much as possible ambient air into the airstream, but the drag caused by the difference in velocity between the airstream and the surface is just a loss of energy. If the airstream gets turbulent and stops following the curved surface, there's no more low airpressure, no more thrust.
BOUNDARY LAYER CONTROL
An airstream can be "glued" to a surface by boundary layer control, using suction or acceleration.
It literary sucks the airstream towards the curved surface, even when it got turbulent!
A wing full of holes can deliver that sucktion, if the air inside the wing is pumped out fast. Extreme low pressure can be reached this way, on top of the surface.
Boundary layer suction is complex because the amount and position of optimal suction varies. If the airstream is stronger than the suction, and no vaulve is in place, then the air is sucked out instead of in, and that can cause the airstream to detach from the curved surface, turn from laminair into turbulent.
When using a flat surface and a lot of distance between the airstream and the surface, there is no airstream-surface drag. When sucking away the air inbetween, a great area of low air pressure can arise, meaning: thrusts.
The air sucked can become the airstream, but a closed loop would obviously not produce any thrust. The airstream is very powerful at first, and able to suck in many times it's weigh of ambient air into the stream. If enlarged 6 times, sucking in 1 time still 5 times untouched to produce thrust.
Adding "fast air" is easier. The added air accelerats the boundary layer and the airstream already out there. The whole airstream could be the result of many small jets plus a lot of ambient air.
A fan that blows air, produces thrust in the opposite direction.
An airfoil that directs the airstream downwards under the airfoil, will be pushed up and away from the fan.
I don't have real numbers from a coanda test bench, so my guess is that the total amount of thrust is upwards and about equal to the original thrust directly from the fan.
An airfoil that directs the airstream downwards over the airfoil, using the "Coanda effect", will also be pushed up and away from the fan.
My guess is that very the total force will again be about equal to the direct thrust.
The airfoil adds a lot of weight, so direct thrust would probably be more efficient.
ADDING AMBIENT AIR
Adding a lot of ambient air and thus extra weight into the airstream, can improve the thrusting power and energy efficiency significantly. So, the goal is to entrain as much as possible ambient air into the airstream. The area where the airstream and the ambient air are in contact needs to be a very large surface. Working near the edge of the disc is therefore most effective.
An air amplifier, air knife / air curtain, are products that can amplify flows up to about 30 times the input air consumption rate. The input air is of very high pressure though (1.5 - 7 Bar).
LESS DISTANCE, LESS DRAG
A radial airstream, flowing from the center to the edge, loses volume and energy really fast. If the airstream would, for example, start at 30 cm from the center at 4 cm thick, it would be ((30*2)*3.1416)*4 = 753.98 square centimeters.
If the edge of the disc would be at 200 cm, and for now no air resistance, the airstream would at the edge only be 753.98 / ((200*2)*3.1416) = 0.6 centimeters thick.
But in reality there is resistance, which makes the airstream turbulent and discontent from the airfoil pretty soon, losing thrust. I guess this must have plagued the first Astro Kinetics design. The last Astro Kinetics design starts its airstream almost at the edge.
As an airstream flows next to a surface, they interact, which will cause the airstream to become turbulent. One solution is boundary layer control, but a more simple solution is to keep the distance short, so that the air near the surface has enough velocity, all the way.
OPTIMIZING THE COANDA EFFECT
IMPORTANTCE OF GEOMETRY in optimizing the Coanda effect is demonstrated by these shadow photographs. At left is the flow of an undeflected jet of air. With a curved surface outside the slit (middle) the air tends to follow the surface, but with distortions. A step at the exit of the slit (right) produced a better flow but not an ideal one because the air impinged on the surface.
FURTHER MODIFICATIONS showed (left) that too high a step can cause the flow to separate from the surface. With a grooved surface (middle) the flow was undistorted. A well-balanced geometry (right) proved an ideal flow that was tangent to the surface.
"Step near slit proved to be crucial in maintaining good lift"
"The small step at the very lip of the slit is important because it turns the flow from the slit into an eddy, or vortex, which in this [circular] device rings the entire nozzle and therefore forms what is called a ring vortex. It is the vortex—or rather the low pressure generated within it— that causes the stream from the slit to bend and thus follow the contour of the shoulder. The curving of the stream around the shoulder produces a force directly radially outward; this force, tending to pull air away from the shoulder surface, apparently accounts for the relative vacuum, or suction, next to the surface. To maintain the suction, which prevents separation of the stream from the surface and causes the flow to accelerate, the stream must hug the shoulder surface closely without actually impinging on it or adhering to it.
Our experiments showed clearly that the creation of a stable Coanda effect depends on the appropriate adjustment of many factors: the diameter of the slit, the strength of the jet, the depth of the shoulder contour. Particularly crucial is the ratio of the slit diameter to the diameter of the nozzle as a whole. If the slit is too wide in relation to the shoulder breadth, the stream will tend to detach itself from the surface; if it is too narrow, the stream will tend to stick to the surface. Also important is the texture of the surface: a properly roughened surface helps to prevent distortion of the stream flow. In short, it takes a complex set of arrangements to produce a useful Coanda effect. It is no wonder that most investigators of the effect had found it to be an elusive phenomenon.
(source: "Applications of the Coanda Effect" by Imants Reba and Assistant Edward Wohlthausen, which appeared in the June 1966 edition of Scientific American.)
So, according to these tests by Imants Reba it's more efficient to create a "vacuum cell" than to blow the jet of air attached to a curved surface!
Similar to a Spay diffuser, a which one can blow and spay varnish on a drawing.
That makes sense, because the difference in speed between the jet and the solid surface is high, thus causing turbulence and all.
A coanda vacuum cell would be something like this:
Though beware of the air impinging (hitting) on the lower surface. Maybe an automatically adjusting lower surface flap can be made.
Another way to prevent friction between the jet of air and the surface, is to move the surface equally fast. Blowing the jet for example over a fast spinning cylinder. The jet of air drives the cylinder, so no extra motor is needed.
Something like this:
An ordinary propulsion jet pushes a mass of air away, to get an equal but opposite push, with which it can force an aircraft through the air. The rule for efficiency here is: the larger the air's mass, the more efficient the propulsion is. That is why modern subsonic airliners have jet engines with a very high bypass ratio. Turbine driven fans is what they have become.
A Coanda thruster is something different. Its goal is to lower the atmospheric pressure at the side where it wants to go, and let the atmospheric pressure push the craft there. When lowering the atmospheric pressure with a jet of air, the rule is not "the more mass the better", because a large air mass with little speed can't suck a cell vacuum. The opposite seems the efficient way to go here: the faster the drive jet goes, the more suction you'll get, up to a point because I don't think a supersonic jet of air doesn't suck well. I'm just speculating here, have no test data to proof it right.