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All about Coanda

WHAT ABOUT COANDA?

Strictly in the name of science, borrow or purchase a beach ball---one of those big, floaty, plastic-pool-toy, infatable balls. Your next science experiment begins now. Predict what will happen if you throw the ball forward with a lot of backspin (the bottom of the ball is moving the same direction as the ball is traveling). Don't be afraid. I got this wrong on the first try.

First, a little background. The COANDA EFFECT is the tendency for air (or a fluid) to follow the surface that it contacts. The classic demonstration is water being deflected off-course by the bottom of a spoon. If you've never played with a spoon in stream of water like this, please do so now or take a look at Figure 18, It's possible to dramatically alter the path of the water with the bottom surface of the spoon. Go ahead. I'll wait. Water or air following the surface of the spoon, wing, or beach ball may be the key to understanding lift. Did you make a guess about the beach ball? My first guess, which is wrong, was that the ball would curve to the foor. I reasoned that the bottom of the ball was moving faster through the air, so, like Bernoulli said, faster air means lower pressure and that ball should move that way. Wrong answer. Correct answer: The airfow is trying to stick to the surface of the ball. This is all a bit complicated, so take a look at Figure 19 on the next page. Tiny gobs of air stick to the surface and collide with the oncoming air, slowing the flow on the bottom of the ball. The top of the ball is moving the same direction as the airflow, helping the air go faster and hold onto the surface longer. The net effect is lower pressure on the top and air deflected downward. The ball curves upward! This is how a curve ball works in baseball. The ball moves away from the side of the ball moving toward the target. Golf balls curve for the same reason. I wouldn't explain this to an adult during a series of hooking golf shots, though. Obviously flight is complicated, and I said at the beginning of this chapter, we're not sure why stuff fies. It's likely a mix of all the ideas here and elsewhere. This is a classic case of looking at the facts at hand, developing a working theory, and testing. The idea that the facts should fit into multiple theories shouldn't bother people familiar with the scientific method. This is how science works. Let the best theory win!

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