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Science
by Jerry Oltion

WHY DO KITES FLY?

 

Summer is the perfect season to fly kites. The Sun is high in the sky, pouring heat onto the ground, which creates thermals in the air over that warm ground. Those thermals in turn create wind as cool air rushes in to replace the warm air that rises upward. And as everyone knows, wind lifts kites.

How does it do that?

Short of putting two economists together in a room, asking how a kite (or an airplane wing, which is just a fancy kite) generates lift is probably the most surefire way I know to start an argument. Theories abound. Some are simple, some are complex, some are flat-out wrong, most have a little bit of truth to them but are mostly wrong, and one or another is probably correct. Nobody agrees which is which. Let's have a look at some of those theories.

 

The Bernoulli Principle

The most common explanation involves the Bernoulli Principle. You may have heard of that. It states that a fluid in motion exerts less pressure on a nearby object the faster it moves. So if air flows faster over the top of a kite (or a wing) than over the bottom, then it would generate less pressure on top, sucking the kite upward.

That's right: According to the most common theory of lift, kites aren't pushed upward by the wind; they're sucked upward by vacuum. There are several proposed mechanisms for how this might work. One of them says that the curvature of the surface forces the air to move faster across the top than it does across the bottom in order for the two air masses to reach the trailing edge of the kite at the same time.

This is one of the theories that's just plain wrong. Experiments with puffs of smoke and high-speed cameras show that the air does move faster across the top of the kite, but it moves so much faster that it actually reaches the trailing edge before the air from below the kite. There may be suction involved, but it's not because the air has to speed up across the top to reach the back of the kite at the same time as the air that went under the kite.

A closer look at what's happening below the kite tells the true story: The smoke puffs approaching the underside of the kite pile up against a cushion of high-pressure air long before they ever reach the surface of the kite. They're slowing down; the air on top isn't speeding up. (The air on top is actually slowing down, too, just not as much as the air on the bottom.) And the two air streams don't come together again the way this theory says they do. Adjacent smoke particles never match up again. The ones that took the lower path remain behind the ones that took the upper path forever after.

So scratch that theory.

 

Research!

What happens when air piles up against something? Let's find out. For this experiment you need a car, a driver (not you, because you're going to be busy doing scientific research), and a stretch of open highway.

Have your driver take the car out on the open road and get it up to cruising speed. Obey the speed limit; this doesn't have to be really fast. [45-55 miles per hour is perfect.]

Roll down the window. Now stick your hand out the window, palm facing forward. Feel that push? If you're moving very fast at all, it's going to be strong, so be careful you don't whack your arm on the window frame.

What you're feeling is air piling up against your hand. It can't get out of the way fast enough, so more and more of it stacks up until it makes a little invisible cone of high-pressure air in front of your hand. That cone diverts the rest of the air around itself, and around your hand.

Okay, that shows how moving air can push on something. But how does it generate lift?

Tilt the top of your hand forward a little bit. Wow! Something just shoved it upward. (The Bernoulli people would have you believe that something just sucked it upward.) What's happening is that the air that was piling up in front of your hand now has an escape route. It can dip down below your hand. But there's aready air there, so the air that's trying to get around your hand starts piling up against the air below it. When air piles up, the pressure rises, and pressure, well, presses on things. Like your hand. Only now the pressure is below your hand as well as in front of your hand, so it pushes your hand upward as well as backward.

Note that you could also say because the air piling up in front of the hand increases the air pressure, that means the back of the hand has lower pressure (compared to the front), which means it's sucked backward and upward by the low pressure, just like the Bernoulli Principle argument says it will be.

See why we argue about this?

Play Scientifically experiment with the angle of your hand. You'll soon find the optimum tilt for generating the most lift. That happens when the maximum amount of air piles up underneath your hand. Level out your hand too much and the air doesn't pile up anymore (except a little bit in front of your thumb). Put too much palm against the wind and the air all piles up in front of your hand again rather than underneath it.

Okay, now you can go home. As your driver is slowing down to turn around, notice how the optimum angle of your hand changes with wind speed. We'll use that discovery later.

 

A Balance of Forces

Kites work just like your hand did, with a few extra tricks thrown in for fun. The angle of the kite against the wind provides lift. The backward push from the passage of the wind is called drag, and that's countered by the string you tug on. (If you let go of the string, the kite doesn't fly away; it loses its lift and flutters to the ground, although it may be blown quite a ways downwind while it's falling.) Because the kite is above you, the tug of the string also counteracts some of the lift, as does the kite's weight. When all these forces balance out, the kite stays put.

Determining where that balance point is in the sky is what makes kites so much fun to fly.

The angle of attack determines how much lift the kite will get out of a steady wind. Like your hand out the car window, there's an optimum angle that will provide the most lift for a given size and shape of sail and a given wind speed. You can adjust that angle by changing where the string (called the "line" by serious kite fliers) attaches to the bridle (the rigging between the line and the kite).

If the wind gusts harder for a moment, or if you tug harder on the string, then the lift increases. The drag increases, too, but that's directed straight backward, so it doesn't matter (until the line breaks). The kite's weight doesn't change. The downward tug on the line becomes stronger, but if the kite is angled correctly, the lift increases faster than the tug does, so the kite rises.

Wind is seldom steady, especially close to the ground, so a kite experiences little puffs of turbulence all the time. Sometimes a puff of wind will hit one side of the kite more than the other, and that will generate more lift on that side, which will make the kite veer around in an arc away from the side with the extra lift. If the kite doesn't have some way to recover, it will go into a spiral and eventually crash to the ground.

There are several ways to stabilize a kite against the death spiral. The most common, at least in the old days, was the tail. The tail doesn't provide any lift; it just provides drag and weight. But all that drag and weight is at the bottom of the kite, so it tugs the kite back upright after a gust tilts it sideways. Kites with tails can still spiral if the lift becomes too uneven, but tails at least help.

The other, much more effective, solution is called dihedral. If you angle the sides of the kite back a little bit, making the kite's cross-section a shallow V, then neither side of the kite operates at maximum efficiency. That sounds bad, but it's worth the expense because when a gust of wind hits one side of the kite, it not only generates a pulse of lift on that side, it pushes that side backward, making it even less efficient than before, while the other side simultaneously tilts forward until it faces the wind more directly, becoming more efficient. It lifts that side of the kite back into place.

 

Really Flying a Kite

Okay, so you can put a kite up into the air and hold onto the line, and a well-designed kite will hang out up there, weaving around as gusts of wind push it this way and that. You can tie the string to your toe and take a nap, and if the wind stays steady the kite will still be there when you wake up (although your toe may not be in the greatest of shape). But some people can really fly these things, making them swoop around where they want them to go. How do they do that?

Several ways. One involves multiple control lines, but it can be done with a single line if you're really, really good. What you do is keep your eye on the kite, maybe even pull the line sideways, and the moment the kite moves in the direction you want it to, you give it a tug to increase its overall lift and increase the motion in the direction it's turning. When the tail or the dihedral starts to counter that motion, you let off the tension and let the kite drift, still aimed in the direction you want it to go. It will eventually veer the other way, but you will have gained some ground, and you can repeat that gain again and again. Some fliers reduce the dihedral so they'll have even more control.

 

Multi-Line Kites

There's a much easier—and way more exciting!—way to control a kite: Add more lines to it.

Imagine stringing two bridles side by side and attaching a line to each bridle. One line pulls on the left side of the kite and the other line pulls on the right. When both lines pull evenly, the kite flies like a normal single-line kite, but if you tug on one line, that side gains lift and the kite veers toward the other side. But the dihedral can't push it back upright because it's not strong enough to overcome the lopsided pull of the line. The kite keeps spiraling toward that side...until you pull on the other line. Instantly, the kite swerves in the other direction.

That's control. You can steer the kite all over the sky, make it do loops on command, figure-eights, spirals, and even fly it out to the side, clear out to the "edge of the wind" where it will hover just a few feet off the ground just as well as a single-line kite hovers overhead.

These kites seldom have tails, because tails would just diminish the degree of control, but some people add enormously long tails that describe beautiful swirls in the sky, tracing out the kite's path as the pilot steers it around.

The only thing a two-line kite can't do is fly backward. For that you need four lines.

Imagine that not only could you change the amount of lift from side to side, but you could change the overall angle of attack, altering the degree of lift across the entire kite. That's what four lines gives you. And OMG, four-line kites are the coolest things in the world. You can take them off and land them. You can fly them sideways—backwards. You can fly them upside down.

Four-line kites, often called "stunt kites," are addictive. People can (and do) spend thousands of dollars on perfectly tuned kites that give them such precise control that they can fly side-by-side with dozens of other kites in beautifully choreographed aerial ballets. There are kite competitions, kite conventions, and kite festivals where people show off their skills to the amazement of everyone, even their peers.

But any kite, from a simple single-line diamond to a four-line stunt sail, flies because of the same principles.

Why do kites fly? Because they're so much fun!
 

__________________________________

Jerry Oltion has been a science nut since he was old enough to spell "curious." He has written science fiction almost as long, and has done astronomy somewhat less. He writes a regular column on amateur telescope making for Sky & Telescope magazine, and spends many, many nights a year out under the stars.
 

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