How does a kite stay up in the air?

By balancing four forces: lift (wind hitting the tilted kite face deflects downward, pushing the kite up by Newton's third law); weight (gravity pulling the kite down); drag (air pushing the kite downwind); and tension in the string (which anchors the kite so the wind keeps flowing across it). Take any of these away—drop the wind, drop the string, lay the kite flat—and the kite falls.

A kite is a tethered wing. Wind hits it at an angle (called the angle of attack), and the kite deflects that air downward. By Newton's third law, the air pushes back upward on the kite, producing lift. The string holds the kite stationary against the wind so the airflow keeps coming, which is the equivalent of an airplane's engine producing thrust.

What forces act on a kite?

Four: (1) lift, perpendicular to the wind direction, generated by the kite's angle of attack; (2) drag, parallel to the wind, pulling the kite downwind; (3) weight, the gravitational pull on the kite's mass; and (4) string tension, which has both an upwind horizontal component (countering drag) and a downward vertical component (adding to weight). When all four balance, the kite hovers.

Why is it hard to fly a kite on a non-windy day?

Without wind there is no relative airflow, so the kite can't generate lift. Running with the kite creates 'apparent wind' as the kite moves through still air, which can be enough to get it airborne briefly. But once you stop, the kite stalls and falls. A windy day means you can stand still and let the wind do the work.

How do kite-fighting strings cut other kites?

In kite-fighting competitions (popular in India, Pakistan, Afghanistan, and Brazil), the string—called 'manjha'—is coated with a paste of finely ground glass or quartz mixed with rice or adhesive. When two such strings cross, the abrasive coating cuts through the opponent's string under tension. Because manjha is also dangerous to birds, vehicles, and people on the ground, several Indian states have banned glass-coated and metal-strand kite strings.

The Science Of Flying A Kite

Table of Contents (click to expand)

Forces Involved In Kite-flying
Release And Launch
Climb And Cruise

A kite flies because four forces stay balanced: lift (the wind deflecting off the tilted kite face, like a wing at an angle of attack), weight (gravity pulling the kite down), drag (air resistance pushing the kite downwind), and the tension in the string (which holds the kite in place so the wind keeps flowing over it). Unlike an airplane, a kite has no thrust of its own—the string is what stops it from being blown away.

If you’ve ever flown a kite, you would probably vouch for the fact that it’s not easy the first time. In fact, even after you’ve got yourself acquainted with flying kites, it takes a great deal of skill to keep a kite airborne in changing wind patterns and perform maneuvers like rapid climbs, quick dives and cutting others’ kites.

In this article, we will introduce you to the science of flying a kite, in the hope that the knowledge of various physical forces involved in kite-flying may give you a head start in acquiring this unique skill.

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Science Of Kites: How Do Kites Fly?

Forces Involved In Kite-flying

Anything that flies in the air for more than a few seconds is wrangling a set of physical forces. For an airplane those four forces are lift, drag, weight, and thrust; for a kite the same physics applies, but with one key swap—a kite has no thrust of its own. Instead, the role normally played by an engine is played by the tension in the string, which holds the kite stationary against the wind so air keeps flowing across its face. Balance lift against weight plus the downward component of string tension, and balance drag against the upwind pull of the string, and the kite hovers.

The lift itself comes from the kite being tilted at an angle of attack to the oncoming wind. The kite face deflects incoming air downward, and by Newton’s third law the air pushes the kite up (with a slight downwind component). This is also why a kite stalls if it lies flat or if the wind drops—no angle of attack means no deflection means no lift.

Many of us (except physics nerds) may not have realized that, just like rockets and airplanes, there are different phases of flying a kite too. These different stages are release, launch, climb and cruise, although they aren’t as clearly defined as in the flight of airplanes or rockets.

Release And Launch

To get the kite airborne, first of all, you need to provide it with enough lift to counteract its own weight. Although there are numerous factors (e.g.,the shape of the kite, its motion in the air, quality of air, etc.) that affect the amount of lift produced in a kite, the principal contributor is the velocity of air that goes by the kite. The design of kites helps too; if you notice, they are made of a very light (but rigid) material and have minimal accessories (in the form of thin, wooden spokes) attached to them. Their shape is also streamlined (with a pointed leading corner) to help them split the oncoming air.

structure of a kite — The aerodynamic shape of a kite helps it to fly

On a windy day, after telling your friend to hold the kite in the direction of the wind, you will give the kite some initial velocity before releasing it. Now, all you need to do is give the kite a little tug to lift it in the air. On non-windy days, though, your friend may have to move backwards a little to get the kite flying. Running with the kite also helps to create an ‘apparent wind’ that pushes the kite up. However, once released, you (and your pal) don’t necessarily need to run to impart velocity to the kite, as the velocity of wind usually increases with increasing altitude, which in turn provides enough lift to keep the kite rising in the air.

Climb And Cruise

After the kite has been successfully launched, it will continue to climb into the sky with the help of well-timed, skilled tugs of the string before cruising at a general altitude. For a kite to cruise, all the forces and torque acting on it should be balanced. If by any means (like, if you hand the control of the string to an inexperienced kite-flier, such as your dog/cat) that balance is disturbed, the kite will start moving, swooshing or dipping until the forces are re-balanced. For example, when you pull the control string, the velocity of the kite increases slightly, which boosts the lift, consequently making the kite climb. Similarly, when you let go of the string a bit, the kite loses altitude due to the increased weight of the string and dips until it’s pulled back up by tugging at the string.

Kite fight

In some countries (like India), people play a game of ‘downing’ others’ kites by cutting their control strings with theirs. To do that, a person first brings his own kite in proximity to someone else’s so that both of their control strings touch, and then rapidly pull at his own control string to cut the other’s string, thereby ‘downing’ their kite. Note that the string used in such kite-contests is not ordinary kite string, but is instead made of highly abrasive materials and is therefore not safe for use by children.

Flying a kite can be really fun, I can tell you that. However, I won’t give you any false hope by saying that the skill of flying kites is an easy one to master. That being said, some knowledge of the physics involved in flying a kite will definitely give you an edge while learning how to master this pleasant pastime.

References (click to expand)