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Stability Science: How Tails Help a Kite Fly

An aerodynamic activity from Science Buddies



George Retseck

Key concepts
Aerodynamics
Wind
Flight
Forces

Introduction
Have you ever tried to build your own kite? Kites have been a source of entertainment through the centuries for kids worldwide. And they've been used for scientific experiments, too—Benjamin Franklin flew one to investigate lightning. (Something you shouldn't do!) In this activity you will have a chance to build your very own kite—a simple sled kite—and use it to investigate how tails help kites fly. After trying out this activity you may add tails to other kites to help them soar to greater heights!

Background
Did you know that, besides Ben Franklin, one of the most famous kite flyers of all time was a 10-year-old boy? His name was Homan Walsh, and without him the Niagara Falls Suspension Bridge may not have been built in 1847. Before construction could begin, someone needed to get a line across the Niagara River Gorge. Homan successfully flew a kite from one side of the gorge to the other over the treacherous rapids below and, after securing his kite string, heavier line was fed across until a steel cable could connect both shores and bridge construction could begin. Homan was rewarded with a $10 cash prize, which was a lot of money in 1847!

How does a kite fly? As someone runs with a kite, the wind going head-on into the kite causes a lift force on it. This force is perpendicular to the wind, pushing the kite up. At the same time, the force of drag pulls the kite back, in the direction that the wind is going.

There are many different kite designs. Some are very old, like traditional Chinese and Japanese kites. Some designs are very new, like the dynamic stunt kites used in sport-kite flying competitions. Their modern materials and designs make them ultra-maneuverable.

Materials
•     Printer
•     Paper
•     Scissors
•     Crayons and markers (optional)
•    Tape
•     Two drinking straws
•     Hole punch
•     Kite string
•     Measuring tape or ruler
•     Paper clip
•     Plastic grocery bag or other thin plastic bag
•     An open, clear area outside

Preparation
•     Download the Sled Kite Template (pdf) and print it out on a sheet of eight-and-a-half-inch by 11-inch paper.
•     Carefully cut out the sled kite. You can decorate it using crayons, markers or other media.
•     Trim the length of the two drinking straws so they will fit in the area marked for the straws. Tape the straws in place.
•     Place three pieces of tape in the marked areas covering the black circles to reinforce the holes for the kite string. Using a hole punch, carefully punch the two holes marked by the black circles.
•     Cut two pieces of kite string to make each 45 centimeters (cm) long. Tie a string through each hole. Tie them tight, but not so much that you tear the paper. Tie the opposite end of both strings together to one end of a paper clip.
•     Cut a one-meter-long piece of kite string. Tie one end of this string to the other end of the paper clip. Your sled kite is ready to fly!
•     To make some tails to test on your kite, take a plastic grocery bag or other thin plastic bag and lay it completely flat. If the bag has handles, cut with scissors straight across to remove the handles. Then keep cutting the bag in this way to create thin rings (which will look like strips when flat)—each should be about three-cm wide. Cut up the whole bag into these rings. Discard the handles. You can see this in an online visual guide for this step.)

Procedure
•     Try to fly your kite without a tail in an open, clear area outside. First walk with it, then try running with it. What does the kite do when you walk and then when you run? How well does it fly?
•     Using the plastic grocery bag rings you made, tape a tail to the bottom of your kite that is 10 cm long. (This will probably be less than one grocery bag ring.) The tail should be centered. Try to fly your kite outside again, first walking and then running with it. How does the kite fly compared with when it had no tail? Why do you think this is?
•     You'll now test a tail that is 100 cm long. To make this tail, loop two rings together and gently pull them tight (you can see this step—which involves overlapping and twisting the loops once—in the visual guide). Attach more rings to the tail this way to make it longer. Attach the longer tail to the 10-cm tail on the kite to give the kite that is 100 cm long. Try to fly your kite outside again, first walking and then running with it. How well does the kite fly with the 100-cm tail compared with the 10-cm one? How does it compare with flying a kite without a tail? Why do you think this is?
•     You'll now test a tail that is 500 cm long. Use the same looping process to expand your 100-cm tail another 400 cm in length. Try to fly your kite outside again, first walking and then running with it. How well does the kite fly with the 500-cm tail compared with the other tails or no tail? What differences do you notice? Why do you think this is?
•     Out of the tails you tested, which tail do you think helped the kite fly the best?  Why do you think this is?
•     Extra: Do this activity again, but instead of comparing different tail lengths, try adding additional tails and comparing the results. Add them symmetrically to the back end of the kite. How well does the kite fly with no tail, one tail, two tails?
•     Extra: In this activity you flew a kite using kite string one meter long. What happens when you fly the kite using different lengths of kite string, such as one meter, three meters or five meters?
•     Extra: There are a lot of other kite designs. Use the Internet, books or magazines at a library to find plans for different kinds of kites. Then test your new kite designs as you did in this activity. How does the tail length affect how well other kite designs work? What other variables affect how well kite designs work?


Observations and results
When you tried to fly the kite without a tail, did it spin and roll around a lot, and not really fly at all? Did flying the kite with a 100-cm tail work the best?

Trying to fly a kite without a tail may result in the kite spinning and rolling a lot because the kite is unstable. Adding a tail to a kite helps make it fly more stably by adding some needed weight and drag to its lower end. Adding a small tail, such as the 10-cm-long tail, may help the kite fly a little more stably, but it will still spin and roll around a lot. Adding a longer tail, such as the 100-cm-long tail, should help the kite fly well, allowing it to get relatively high without rolling much. A very long tail, such as the 500-cm-long tail, will help keep the kite from rolling, but the tail may be so heavy that it will prevent the kite from flying very high. Some recommend adding a tail that's around three to eight times the kite's length—but the best way to determine the right tail length for your particular kite is to try it out!

More to explore
Using a Kite as an Experimental Platform from Scientific American
Forces on a Kite from the National Aeronautics and Space Administration (NASA)
Kites from Aviation for Kids
How to Attach Kite Tails from Kites in the Sky
Making Kite Tails from My Best Kite
Let's Go Fly a Kite! from Science Buddies

This activity brought to you in partnership with Science Buddies
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