Ever see someone demonstrate lift by blowing over a piece of paper? While this is fun and interesting, the usual explanation given is misleading by claiming it’s an effect of the Bernoulli principle — the correlation between air speed and pressure, as speed increases pressure decreases. Many folks were taught this is how planes fly, and they pass this “wisdom” along. But they’re fooling themselves, and you.
Aerodynamic engineers have know the correct explanation of lift for decades, yet the misconception of Bernoulli principle lift lives on in the public imagination. However, it is not how an airfoil generates lift. That’s a whole different kettle of fish. Read either How Planes Can Fly for a breif overview or See How it Flies to get the complete skinny on lift.
By the way, the illustrations here are not true diagrams, but visual aids. Fluid behavior is a complicated business which can seem like organised chaos, what with tumbling and swirling, i.e., turbulence. We’re not going to get into these details, but concentrate on simplified basics.
What does this correlation between speed and pressure mean? Here’s a way to imagine it. If you’ve ever watched a road race you know the cars slow down and bunch up in tight turns. Speeding up out of the turn they get stretched out. In the picture, a race official with a stopwatch at the turn times the cars as they pass, they are one second apart, though close together. A second official times them on the straightaway, where they are also one second apart, though with several car-lengths between them.
The same number of cars will pass each official in the same amount of time, but the faster they go the more space there is between them. This is what the Bernoulli principle is about, only with liquids and gasses, not cars. The faster air flows the more it gets stretched out so there’s less air in the same volume, it’s less dense.
As speed increases pressure decreases, and vice-versa. Higher speed means more force in the direction of flow. I mean, you’ll take a bigger hit from a fast car on the straightaway than a slow car in the turn. This higher speed has an effect, too. After all, trees get blown over in a wind storm. Is that because of a change of pressure or the force of the airflow? Let’s return to wings and airflow and such.
Paper Lift: Let’s have a look at a the lift demonstration mentioned in the opening and see what it’s about. Take a piece of stiff paper and curve it like a wing. Hold it up under your mouth and blow on it. It lifts! (This works similarly with a flimsy sheet of paper because gravity pulls down the far end creating a curve.)
The common explanation is the fast airstream over top has lower pressure (Bernoulli principle) which sucks the paper up. Problem is, this paper lift really demonstrates the Coandă effect rather than the Bernoulli principle.
You can easily demonstrate the Coandă effect for yourself with a tablespoon and faucet. These are conveniently often found together in the kitchen, no need for highly technical lab equipment. Follow the steps you see below. Gases behave pretty much like liquids, so when you see the water behaving strangely with the spoon, that’s what the air does with the curved paper.
Dangle the spoon as shown next to the stream coming from the tap. I say dangle because you want to hold it loosely enough so it can swing back and forth a bit. (It helps to attach a piece of tape at the handle end to act as a hinge.) Move the spoon up to the edge of the stream so it barely touches. When you do the water will flow around the bowl of the spoon and off the bottom deflected to the side and the spoon will move into the stream.
Just as water flowing around the spoon’s curved surface draws it into the stream, air blown over the curved paper is what causes the lift in that common paper lift demonstration. Imagine the stream of water turned 90 degrees and you can see we have the same situation as in the paper lift, only with water you can see instead of air which you can’t.
Notice how unlike a wing in flight it is, flow on one side only. The Coandă effect only works in specific conditions where an isolated jet of fluid (or air) flows across a surface, a situation which is usually man-made. You don’t find it much in nature and doesn’t happen over a wing, either. Just so you know, there is no Coandă lift on an airfoil.
The Coandă effect is a ballancing act between many factors, among them speed, pressure, molecular attraction, and a centrifugal effect if the surface is curved. How exactly it all works is unimportant to understanding the demonstrations we’ll examine. But do the spoon test and you’ll see that it works. I can’t explain gravity either, but we both know it’s real enough.
This effect is named for Romanian air pioneer Henri Coandă, an interesting character in his own right. Read about his jet plane built in 1910. If you are still curious how the Coandă effect works you might read this explanation.
Clashing Balloons: Hang two balloons from strings an inch or so apart. Blow a stream of air between them and they move together. Now, I show a wind sprite in the picture, but if one is not around do it yourself.
Railroad Bottles: Set a pair of empty soda bottles about an inch or so apart on a counter or table. Blow a stream of air between them and they move together.
The explanation usually given for both phenomena is by speeding up the air between the balloons and the bottles you reduce the pressure in the airstream as per the Bernoulli principle. The lower pressure pulls the objects into the stream so they come together. Unfortunately, that’s not quite the way it works. Notice you’re blowing a steam across a curved surface. If you suspect the Coandă effect is involved, you suspect correctly.
The airflow across the surface of the round objects in both cases give you a Coandă effect pulling them together. Once again, just as with the paper lift, the Bernoulli effect alone is not doing the trick as advertised. Again, this does not demonstrate how a wing generates lift either.
Cone and Ball Levitation: Put a light ball in an upside down funnel or cone and blow through the small end, or otherwise get some fast airflow through it. The ball will not be blown out, but will levitate in the cone as shown.
The explanation sometimes given for this phenomenon is the Bernoulli principle creates a low pressure zone in the cone so the ball doesn’t fall out. But it’s not so simple as that. In this instance we’ve added a force to reckon with, the stiff cone barrier around the flow.
One thing you need to know before we proceed. When you blow a stream of air, like out of a hair dryer, you don’t get a distinct tube of air punched through the surrounding air. Fluid behavior is rarely so simple. Instead, you get something more along the lines of a fast river constantly eroding a sandy riverbed. It carries what it’s rubbing against along with it.
Something similar happens with airflow, the ambient air besides the flow gets swept along where they intermingle, which is entrainment of fluid. This creates a sort-of cone of air, fast in the center, fanning out and gradually slower along side. Air not only flows in the hair dryer’s inlet and out through the blower tube, but also is drawn in from surrounding areas and swept along.
Returning to the cone business, as the airstream exits the tube into the small end of the cone you get the same entrainment of air as explained above. New air streams toward the space where the air is being blown out forming a ring vortex shaped like a donut around the fast airstream in the center. The ball levitates in this vortex as if it were flying rather than being held up by the Bernoulli principle.
One important aspect of this effect is a ball is the only shape that works. You have to have a ball so the airflow around it is smooth so not to disrupt the vortex which is what’s doing the trick.
Ball Levitation: Take a hair dryer blowing straight up and introduce a ping-pong ball into the airstream where it will float. Tilt the blow dryer up to about 45 degrees, the ball doesn’t fall out but “levitates” in the airstream. (I’ve also seen it work with a vollyball and a strong leaf blower.)
The commonly given explanation for this phenomenon is the Bernoulli principle creates a low pressure zone in the fast flowing airstream compared to the air along side so the ball doesn’t fall out. However, it’s not that simple. What we have here is what’s referred to as parachuting. How a parachute works seems obvious and simple, but it’s not.
Air fills the inside of a parachute in effect turning it into a half ball of air with a fabric skin on top. The falling parachute “scoops” out an area of low pressure over top. (You might like to know a parachute doesn’t fill out from increased pressure below, but because of decreased pressure above.) As with anything moving through a gas, a vortex forms and the air from below races up along the curved fabric towards the lower pressure over top creating Coandă and Bernoulli effect lift.
The air rolls along the fabric in a ring vortex not unlike the one in the cone from above. These cannot last forever (for reasons too boring to get into) and are periodically shed in what’s called periodic vortex shedding aptly enough. To keep it simple let’s just say a parachute not only has some air resistance underneath, it has lift over the top, which is parachuting.
Not being only a half ball but a whole one, a ping-pong ball in an upward blowing stream is similar, but different. (As Yogi Berra’s son once said, “Our similarities are different.”) Because of a ball’s aerodynamic shape, being round on the bottom in any position, there’s less air resistance underneath than with the half ball parachute. Compare the difference between a ball and a wad of paper. The ball hovers about 3 inches from the blower end, the paper wad is blown about two feet up and out. The wad displays no tendency to stay in the stream at all, a good hint pressure differences alone aren’t the full story.
The paper wad gets more upward push from the airstream because the irregular shape inhibits flow and traps more air underneath. On a ball the air slips around the sides and so there’s less upward push and equal flow and pressure all around keeping it centered in the stream. In fact, a ball is about the only shape that will float this way in a vertical airstream.
When you tilt the stream and get the dramatic “levitation” effect, the ball doesn’t stay in the center of the stream, it hangs down with the upper part in the faster center (A) and the bottom in the slower section of entrained air (B). The ball falls to a spot with different airspeeds and pressure above and below where it has enough parachuting lift to stay up.
After you tilt the stream past a certain angle you reach the tipping point and the ball falls out of the airstream. If you’ve ever done any house framing you’ll understand the tipping point concept. Once you get past 45 degrees upright it gets easier and easier to tip up your wall as more of the weight is transferred down vertically through the wall rather than on you. One way to think of it, a vertical slab is a wall and a horizontal one is a roof. At 45 degrees it’s half wall and half roof, that’s the tipping point.
Something like this happens to the levitating ball. The more you tilt the more the flow pushes horizontally than vertically, so you’re relying on the lift above more and more until you reach the tipping point. The ball falls out because gravity overcomes the decreasing upward push. (I told you it wasn’t all that simple.)
Only a ball shape works for this demonstration. A wad of paper or puff of cotton of the same size an weight simply don’t work. That’s because it’s not just pressure differences, a ball creates parachuting lift and flies.
What’s sometimes omitted in the misleading explanations is the confounding forces besides a pressure reduction as per the Bernoulli principle. There is also an increase in dynamic pressure. Plus, every bit of aparatus like a funnel, a curved shape, or something redirecting the flow will have an impact. You need to look at the whole picture.
You can always count on increased air speed to reduce pressure as described by the Bernoulli principle. However, once it encounters a new force, like a wall or you sitting there being cooled off, additional principles also apply. How a fan cools you is another can of worms we won’t go into. But I will say this, it has nothing to do with the Bernoulli principle or the Coandă effect. At least, I’m pretty sure it doesn’t. I’ll look into it later.
© Terry Colon, 2007
Special thanks to fluid dynamics engineer Terry Day whose invaluable assistance, and patience, helped me understand it. His site:
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