NOTE: Although the idea of an overhead launcher is a good one, I am not happy with this design. It needs more work and I don't have the time now to work on it. I suggest this wind-up helicopter project instead http://www.sciencetoymaker.org/dragnflyHelicopter/index.html
The launcher is what gets the helicopter spinning. It accelerates the helicopter over your head using a shaft (wood dowel, not visible becaue it's inside the pipe) so if the helicopter flies apart it won't fly in your face. One launcher will serve several people. In the picture my daughter is pulling a string (barely visible) wound around the dowel. The helicopter blade rests on a platform at the top of the launcher that holds it with ratchets while being acceletated, then lets it go.
PVC pipe is usually sold in 10' lengths. The whole length only costs a couple of dollars (US). You can get it at building centers, plumbing supply stores and some hardware stores. The hardware stores usually charge more per foot, but they might let you buy only the 7 feet that you need for this project. Do not get CPVC pipe, and note that 1" pipe is not really 1" diameter either inside or outside. Go figure. The pipe provides a handle and a structure for getting the helicopter overhead where it is safer. If you can get them to cut the pipe 7' long feet to begin with, then you don't need the saw, listed below.
Dowels are usually sold in 3' lengths at hardware stores and building centers. Get a straight one. This dowel becomes a drive shaft inside the pipe to transmit motion from your arm to the helicopter over your head.
Any kind of drill will work. You will use the 1/8" and 1/4" drill bits, which are available in inexpensive packages.
Mason's line comes in garish florescent colors from the same store you get the dowels. It will be the string you pull to rotate the drive shaft. You need about 3 feet of it. The electrical tape is for wrapping around the knot in the twine so it doesn't come untied.
The cereal (or any food packaging) box provides the thin cardboard for a platform on top of the drive shaft that the helicopter will rest on during launch. And the two triangles sticking out of it hold the the helicopter while getting up to flying speed, then lets it go. You can use a milk carton, which is a little stronger and waterproof, but you have to thoroughly scuff the surface with coarse sandpaper where you want the hot glue to stick.
One nail keeps the helicopter centered until lift off. You'll also use these for the bending jig, so get at least a dozen. Finish nails are thin and have a small head.
There are actually two kinds of hot glue gun: hot melt and cool or low-temperture melt. Either kind will work. You are less likely to get burned using the cool-melt kind with the thinner glue sticks.
The saw is used to cut the pipe to 7 feet if you didn't have them cut it at the store where you got the pipe. You can cut PVC pipe with a hacksaw blade--you don't even need the frame.
The centering nail keeps the helicopter on the launcher until it has reached full spinning speed. Without it, the helicopter would be flung off sideways. Drill a 1/8" hole 1" deep in the center of one end of the dowel. A piece of tape on the drill bit can mark how deep to drill. Notice that in the illustration the hole is not quite in the middle. I have never been able to get the hole exactly centered, nor gotten it to go straight in. Drill a hole in the other end of the dowel. Choose the end that is best centered and that goes in straightest (if the drill didn't go in straight you can bend the nail straight after it's been glued in).
When you glue the nail into the hole with hot glue, you have to move fast. The narrow bit of glue that goes into the hole cools and hardens quickly. Make sure the glue gun has been plugged in at least 5 minutes so the glue comes out as hot as can be. Squeeze glue into the hole and right away push the sharp end in. Scrape away the extra glue. If the glue hardens before you get the nail all the way in, heat the nail over a candle. About 1 1/2" of the nail should stick out.
Click here and print out the pattern. Some browsers--especially Netscape--change the scale and the size of the printed pattern. If the printout says something like, "Scaled-60%" try another browser. Also, the printout has a scale check. It says 2" line to line or 5 cm line to line. Make sure it's accurate. Rough-cut out the circle with little triangles sticking out (save the other patterns, which will be used later). Cut open a cereal box for its cardboard. Make a couple of tape donuts and use them to stick the pattern onto the milk carton. Cut out the taped-together pattern and cardboard. Don't take off the pattern yet.
With a nail, punch a hole in the center. Push that nail through the circle with the pattern side facing up. You might have to expand the hole a little so it fits over the nail head.
Use hot glue to hold the bottom of the circle onto the dowel. The connection will undergo considerable stress, so use enough glue. But don't try to put it all on a once because it will run and drip. Put on a thin layer, let it cool, then apply another layer.
Bend both of the triangles 90 degrees straight up. Remove the paper pattern and tape. These triangular ramps will hook onto the helicopter and accelerate it. Then--when the circle is no longer accelerating and the helicopter is actually spinning faster--they will allow the helicopter to ramp off and up into the air.
In this step you will strengthen the ramps with hot glue (red in the illustration), actually building up the glue in the corner. As in step 2, apply thin layers of glue and allow some cooling time in between. As the side view illustration shows, pile up the glue in the corner to strengthen the ratchets.
Applying a layer of glue from the ratchets to the center of the disk will reinforce the remaining part of the disk that takes stress during take-off. Make it thin (about as thick as a coin) on top so it doesn't interfere with the helicopter, but it can be thicker and stronger on the bottom. A bit of glue on the outside of the triangle ratchets will make them stronger too. After you have launched some helicopters it will become obvious if you need more glue reinforcement because the launcher will slip instread of getting the copter to full speed. Use common sense to put the glue where it is needed.
The string attaches at the other end of the dowel. It helps to drill a 1/8" hole through the dowel about 1" or so from the end. After threading the end of the string through the hole, wrap it twice around the dowel before tying it back onto the long part of the string. In the illustration the string appears very short, but one end is about 3 feet long.
You can use the double half hitch shown. Here is a link to a page that shows you how to tie a double half hitch with an animated GIF: Double half hitch. Nylon string is very slippery, so even the best knots tend to slip off. However, even a bunch "granny knots" will hold if you wrap it with electrical tape.
If you haven't already cut the 1" PVC pipe to 7' long, do it now. It's easy even with a hacksaw blade with no frame. Place the dowel/ratchet/string assembly with the underside of the platform resting against the top of the pipe. Doing this shows you where to drill the hole in the pipe where the string will come out.
Drill a 1/4" hole into the pipe about 1" above where the string ties on so the circular platform that holds the helicopter won't rub on the pipe. Put the dowel into the top of the pipe. Fish out the string through the hole. A paper clip bent to have a hook at the end works well for grabbing the string. Once it is through, tying a small stick to the end of the string will make it easier to pull.
Note: I have not had a chance to try it yet, but it you put an old ball bearing and raceway set with a 3/8" inside diameter at the bottom of the dowel I think it would reduce friction. I've used the launchers for years without bearings and they've worked fine, but sometimes the string gets stuck. A bearing might help. I'll try it out soon, or you try it and let me know.
To wind the launcher spin the nail between and forefinger counter clockwise. Launching instructions are in the helicopter-making section.