Unruled Notebook

Take a strip of paper that is about five times longer in length than in width. Hold one of it near your lips, while the rest of the paper strip sags down and forms a convex surface. Blow over this “convex” surface and what you would observe, contrary to what one would expect, is the paper raising up from its original convex sagged down position!

The air that you blow flows over the paper surface and tends to stick to it even along the convex surface that curves away (down) from the general horizontal direction of the initial air flow from your lip. This is counter-intuitive to what one expects, that the air could flow in its original direction of flow from the lip tangential to the convex sagging down surface of the paper strip.

That is, what in fluid mechanics parlance known as, the Coanda effect in operation for you. [named after the Romanian mathematics professor and inventor Henri Coanda - wiki link]

Another instant of this effect we can demonstrate in our kitchen faucet. Observe the picture shown below [thanks to Lakshmi, for taking the pictures in one of her usual somnambulist session, courtesy me].

In the left picture, a spoon is kept nearby to the water stream flowing out of the faucet in the expected downward direction A, as marked. When the spoon is touched on the water surface however, unexpectedly water instead of following its original course along direction A, follows now the back surface of the spoon curved away in direction B.

Here is the crucial thing: If you use a plastic spoon (light weight) instead of the one shown in the picture and hang it loose enough in your fingers and do this experiment, while the water changes course to direction B, you will feel the tug in your fingers and the spoon being drawn towards the water jet.

This is akin to the the effect that we observe when sitting on a canoe in a river and trying to pull towards us a friend wading nearby in the water using a connecting rope. Both the friend and our canoe would move to meet each other. In other words, in the above water faucet example, the convex spoon surface is “lifted” towards the water jet. In the earlier paper strip experiment the convex sagging paper strip is “lifted up” towards the air jet from your lip that attaches itself to the top surface of the strip.

Check the video taken at my home kitchen faucet yesterday [thanks again to Lakshmi]

[googlevideo]http://video.google.com/videoplay?docid=-6832142747419944229&hl=en[/googlevideo]

Just taking this Coanda effect as an experimental fact, we now shall see this effect in action in a slightly larger scale demonstrated in the ongoing Shaastra 2007 the tech-fest of IIT Madras. The aircraft is called the Yu Fly project built indigenous by the IITM UG students with the guidance of professors.

In the picture below you can see the student activity center with the Coanda effect airplane at the center.

A gist of some details of this project from the Yu Fly project page is

This particular model uses special light weight foam called Depron for the body of the craft. It is a quadrotor (consisting of 4 individual units working together) powered by state of the art, brushless DC motors, and lightweight Lithium-polymer batteries.

The advantages of such an aircraft are many. In addition to its greater energy efficiency, it is also far more mechanically simple, compared to conventional VTOLs. The quadrotor configuration gives it superior stability, and hence better control. The energy efficiency of the craft makes it a part of the underlying Energy Conservation theme at Shaastra 2007.

How exactly Coanda effect is used in this Yu Fly aircraft?

Below is a picture with a closer look of the Yu Fly aircraft.

Looking at one out of the four modules, air, as marked in the picture, is sucked into it from the top using the mounted propellers. The air flow instead of following a radial exit path out of the module, due to Coanda effect, follows the curved path of the bottom blob of the module along the curved path marked in red arrow. The flow of high speed air over this blob generates a “lift” of the blob in the normal direction, marked with yellow arrow. Owing to the symmetry of this lift in each of the module, all four modules coupled in the quadrator configuration lifts directly up.

The aircraft in operation is shown in the picture below.

Check this forty second video (hosted at Google Videos by me) for the actual demonstration.

[googlevideo]http://video.google.com/videoplay?docid=-6832142747419944229&hl=en[/googlevideo]

The picture below lists some of the Professors and UG students involved in this project. Hats off to all of them for their efforts. Given the fact that each trial run that crashes the aircraft would involve about Rs. 200,000, I hear the aircraft was built with very less damage cost, even though the guys played with it as if it were a real-life “video” game.

Now for a quick technical explanation of the Coanda effect. In the water over the spoon experiment, imagine initially the original water jet coming out of the tap to drag along with it a portion of air (the secondary fluid) that immediately surrounds it. Until this arrangement reaches the curved away surface of the spoon, the symmetry is intact. Near the spoon side, the air that is entrained by the liquid water flow needs to “squeeze through” causing a local low pressure. The higher pressure on the other side of the liquid jet (side away from the spoon) causes the jet to get pushed and stick to the spoon surface. This “push” of the liquid jet is “reacted” by the spoon with a “pull” and it is drawn towards the jet and gets stuck.This Coanda effect is also explained as a phenomenon of continuance of attachment of the viscous boundary layer of the flowing fluid over a curved surface.

References on the web

1. Coanda Effect: Understanding Why Wings Work by Prof. Jef Ruskin. A very good explanation of what exactly is lift. The example of the canoe used in this post is taken from his essay. A must read of all of us interested in flying airplanes – business class or otherwise.
2. Wiki pages on Coanda Effect and Henri Coanda – not as great as the first reference; The page on Henri Coanda doesn’t yet carry a picture of him, but carries a picture of his aircraft
3. This page from Thermofluids, UK, gives a short explanation and a rare picture of Henri Coanda
4. Coanda Saucer – a video of Coanda Effect in action in a dome shaped aircraft, similar to one of the modules in the Yu Fly.
5. Shaastra 2007 page for Yu Fly project – contains some more pictures.

Update: The street where Coanda was born is named after him. Thanks to Prof. Andrew Rees – who took the picture on his last visit to Romania – for sharing the picture of that street sign with us. Click on the picture for a larger image.