BUILDING A HELICOPTER

Note: Before you build the actual helicopter you will need to build a simple bending jig (a board with some nails pounded into it) to twist the tongue depressors.

What you need:

2 1/2" long finish nails

Finish nails are thin and have a small head. They are the same as the one used to build the launcher. Here they are used to make the tongue depressor bending jig.

Small wooden board or short "2 by 4"

This will be the base that the nails for the bending jig go into. This could be just about any scrap of wood or plywood with a flat side that is is at least 3 1/2" by 3 1/2" by 3/4" thick.

Pliers or sturdy needle-nose pliers

These are for bending the nails over on the bending jig.

6" long wooden tongue depressors

These provide the frame of the helicopter. I prefer the wider "senior" tongue depressors because they seem a little sturdier, but I've used the "junior" size too. Look in the yellow pages under "medical supplies" and call around. They usually come in boxes of 1000 but they're cheap and useful for lots of things. You can also try craft stores.

Drill and 1/8" drill bit

If you don't have a drill, you could substitute a nail heated in a fire, held with pliers, to just burn a hole through.

Hot water

It's amazing how you can bend wood and have it retain its new shape by dunking it in hot water first. You will be twisting the tongue depressors so the airfoils have a slant, called an "angle of attack." I like to use water that is close to boiling--using a stove, hot plate or microwave oven--and pull out the tongue depressors with pliers, but you could use hot water out of a tap and soak it a little longer.

Cardboard

Using cardboard to make an airfoil shape makes the tongue depressors go dozens of times higher. I use old breakfast cereal boxes (or poster board of comparable thickness) or milk carton. Cereal box cardboard is easier to bend into shape but it has to be kept dry. It will wilt and weaken even after landing in dew covered grass. Milk carton can shake off water but it's a bit harder to bend and you will need to sand (scuff) the surface where it glues to the tongue depressor. It would probably be best to start with a cereal box.

Scissors, ruler

Electrical tape

This will hold various parts of the airfoil together. Although black is the most common, get red if you can because it shows up if the helicopter lands in tall grass.

Hot glue gun and glue sticks

I prefer the cool-melt kind with the thin glue sticks because you are less likely to burn yourself.

Non-hardening modeling clay.

The weights in the tips of the helicopter use momentum to keep it spinning. Paradoxically, adding weight to the center would just weigh it down, but weight at the tips makes the helicopter go dozens of times higher.

When the weights are spun fast, they are subjected to centrifugal force, (although that is not technically correct) and people underestimate just how strong it pulls on the weights. If not properly contained in the tips of the helicopter, the weights fly out. I use non-hardening modeling clay available from craft stores, mail order or perhaps you could get a little from a sympathetic art teacher.

I have to confess that I did not always use non-hardening clay and I am sure other people will rediscover using coins as weights. They provide a denser mass than clay, they are easy to put in, don't deform and they don't have to be weighed to get the right amount. However, if they are flung out they are really dangerous.

This is a true story. Back in the early days of making helicopters with classes of 9th graders the launchers did not spin the helicopter up above our heads, but rather right in front of our faces. So we used full face protection in the form of a clear face shield. One day, however, I was absent for part of the day, conducting science activities at a nearby elementary school. When I returned to the jr. high school, I saw drops of blood leading to my room. A student stopped me to ask if Eric was all right. I almost fainted.

I learned that although I had left instructions for the substitute teacher to only to show a video tape about flight, my 9th grade students had somehow talked him into letting them go outside and launch helicopters. Furthermore they assured the substitute teacher that no face or eye protection was needed. A coin in a helicopter launched by a student named Eric was flung out. It hit him in the thin skin right next to the corner of his eye. He required 3 stitches. Although the scar is virtually invisible and it was an unforgettable learning experience for everyone, I was so horrified by the incident that for years I didn't do the activity with students. It was only after developing non-hardening clay as a weight that I started it again.

Although we now accelerate the helicopters over our heads, flung out coins would still be a danger to other people nearby, not to mention a threat to windows. I think using coins as weights is a bad idea. Stick with modeling clay.

Triple beam balance (or other way of measuring weight to nearest gram.

This is for measuring 4 or 5 grams of clay for each tip of the helicopter. Science teachers always have at least one. They are fun to use. If you can't find one I'll show you how to make a homemade balance in step 7.

Step 1

Make the bending jig.

Click here and print out the page. 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.The dots are a pattern for where to put the nails. Cut out and tape the pattern onto the wood. Save the rest of the patterns for making the helicopter. In the illustrations on this page I've only nailed in 3 nails on each side for clarity. For a class projects, however, I have long lines of nails so as to bend many tongue depressors at a time. After you pound in the first pair of nails on the end, each additional pair will bend one more tongue depressor.

Pound in however many nails. I find it easier to pound them in perpendicular, then bend them over later. The nails should go about 3/4" into the wood.

Using pliers, bend the nails over about 30 degrees from vertical (60 degrees from the horizontal wood). Take care that whichever way you are holding the jig, the nearer row of nails always slant to the right and the far row always slants to the left.

Step 2

Mark, drill and bend the tongue depressors.

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. Center the tongue depressor on the mark finder. Use it to make a mark (a blue "+" in the illustration) exactly in the middle of the tongue depressor.This is where you'll drill a small hole to keep the helicopter centered during launch. Also draw two marks 1 1/2" away from the middle. Take these lines around to the other side of the tongue depressor. Finally, write "TOP" to help keep correct orientation as the airfoils are glued on.

With a 1/8" bit, drill a hole right in the middle of the tongue depressor.

The tongue depressors need to soak in hot water so they stay twisted. If you can heat the water to almost boiling, the tongue depressors only have to stay in a minute or two. If you are using hot water from a tap you should let the tongue depressors soak for 15 minutes or so.

Even if you have to pull the tongue depressors out of nearly boiling water with pliers, you will find you can handle them with bare hands almost as soon as you shake the hot water off. This is because the wood is thin with a large surface area. Get the tongue depressors into the bending jig before they cool down completely.

The wood will have to dry in the jig. If you cannot wait overnight, put the whole fixture in an oven at about 180 degrees F. (____ degrees Celsius). They will dry very quickly.

Step 3

Cut out, form and tape the airfoils.

The following instructions will show how to make a symmetrical airfoil. It works almost as well as nonsymmetrical shapes and it is easier to make and glue on right. I'll have some pointers at the bottom of this page for making nonsymmetrical airfoils.

Click here and print out. 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 two airfoil patterns. Hook a piece of any kind of tape back onto itself like a snake biting its tail so the outside is sticky. Use several of these "tape donuts" to stick the patterns onto the print side of the cardboard, oriented so the top/bottom sides point toward the top and the bottom of the box. It is well known that wood has a "grain." It is less well known that cardboard also attains a grain when the fibers are poured onto the screen. It will will be easier to fold and bend if you tape it on that way. Cut on the solid outlines.

Use a sharp corner of a table or a straightedge to fold exactly on the dashed line.

Curving the cardboard is difficult to describe but you will know as soon as you get it. It is not the previous step of creasing on the dashed line, but rather making the whole piece of cardboard curve. In the illustration the right hand firmly grabs one of the "up/down" ends and pulls it down and to the right around a sharp corner. The cardboard should bend as it goes around the corner. The left hand pushes down hard on the cardboard, making it resist the pulling. The operation is somewhat similar to the way people curl ribbons. Start the sharp corner at the middle where the dashed line is and pull to the edge. Flip it around 180 degrees and curve the other half.

Take off the pattern and the tape holding the pattern. When you hold the two edges together as shown, you can see how the curves make it form something close to a symmetrical airfoil.

Cut off some electrical tape (red in this illustration) 3" or a little longer. Stick half the tape to one edge. Press the edges together and fold the other half over the other edge. Trim the ends if tape sticks out on the ends. This taped edge will be called the "trailing edge" from now on because it will be the back of the airfoil. The other edge is called the "leading edge."

Step 5

Glue the airfoils onto the tongue depressor

Gluing the airfoils is the most challenging part of making the copters. The hot glue can burn you and it hardens quickly, giving you limited working time. The airfoils have to go on a certain way which can be confusing when you are working hurriedly. I suggest you practice the maneuver first without actually putting on the glue. It's also a good idea to get a second person to help the first time you do it.

It is quite common for people to botch up the job the first time they put glue on airfoils. They orient them wrong or don't make the glue connection strong enough so they fly off the first time they spin. Be patient. You'll learn from the mistakes and be ready the second time around.

Start applying glue to the bottom side of the tongue depressor so you end up on the top (shown in the illustration). From one of the outer marks apply about a 1/8" layer of hot glue while using the nose to spread the globs (this smoothing can help prevent dripping). Quickly turn the tongue depressor over and apply glue the same way to the other side. Squeeze the front and back of the airfoil so it opens up wide. That way it fits over the glue. With the leading edge facing as shown, slide it on the gluey end of the tongue depressor until it reaches the mark.

Center the airfoil on the tongue depressor. Pinch the airfoil onto the glue with enough pressure that it forms the shape of a symmetrical airfoil. If you cut off the tongue depressor where it goes into the airfoil (don't), the illustration shows what you should see. The leading edge (right side of the illustration) might not have the smooth curve shown, but you will fix that in the step ________.

Glue on the other airfoil. Try to use about the same amount of glue so the helicopter is balanced. Notice that the taped trailing edge (red in the illustration) is on opposite side of the tongue depressor. This can be confusing to people the first time they glue on airfoils. It is similar to the way two people facing each other and shaking right hands actually have their right hands on opposite sides.

Step 6

Balance the helicopter

Put something thin like the wire from a paper clip in the hole in the middle of the tongue depressor. If one end drops down it means the helicopter is unbalanced. Put some hot glue on the lighter end--where the tongue depressor goes into the airfoil--to balance it. An out of balance helicopter will vibrate, which will reduce efficiency. Think of what an unbalanced load of laundry does during the spin dry cycle.

Step 7

Tape in the weight

You will need to measure either 4 or 5 grams of non-hardening clay for each wing tip. A 4 gram weight in each wing tip is a little easier to fit in and it is a little easier to accelerate. But 5 gram weights have the potential to go higher and stay up longer if you can provide enough power to accelerate them. A triple beam balance is the usual way to go, but if you can't get hold of one click here.

Flatten one of the 4 or 5 grams of clay into a rectangle about 1 1/2" by 3/4", which will leave it about 1/8" thick. Wipe or wash the clay residue off your hands so you don't get it on the outside of the airfoil. If a lot of clay residue got outside, the tape holding the clay weight in might not stick as well.

Push the flattened clay into one end of the helicopter. It should go into the cardboard sleeve until it hits the tongue depressor so the clay just peeks out. Don't worry too much about how you pack it in because the first time you spin the helicopter fast the clay will pack itself evenly into the tips.

Cut off a 2" piece of electrical tape. Stick half of it in the middle of one side of the airfoil. Pinch the airfoil so there is only about a 1/8" gap between the two cardboard edges. Pulling the tape tightly, fold it over and stick it to the other side. The end should be quite thin. When the helicopter rotates fast and the clay is flung to the ends it will stretch the tape and bulge out the end a little.

Cut two more 2" pieces of tape and apply them to either side of the first piece, pinching the end of the airfoil and pulling the tape tight as you stick it on. It is important to completely cover the ends. If you even have a pinhole the clay will leak out when the helicopter is spinning fast. Repeat the taping to the other end of the helicopter.

The helicopter will fly well at this point and you can skip the next step if you are in a hurry to fly your helicopter. However, if you can wait couple of minutes you can increase the performance a little more by rounding the leading edge. An airfoil actually performs better with a rounded leading edge than with a sharp leading edge. When folding the cardboard airfoil in step ____it is hard to round it, but applying several layers of tape in certain way does give it some roundness. The illustration at right is somewhat exagerated, but taping does seem to effectively round the edge.

Cut off two pieces of tape 3" long or a little longer. Slice one piece lengthwise into two strips--one of them 1/3 the original width and the other 2/3 the original width. Apply the thinnest (1/3 piece) right on the leading edge

Apply the thinnest strip to the leading edge, centering it right on the fold. Next apply the wider strip, again centering it on the fold so it completely covers the first strip. Finally, cut off another piece of tape a little longer than 3" but don't slice it. Apply it over the first two strips, centered the same way. Look at this illustration of a symmetrical airfoil. See how close you can shape the cardboard to that shape.

Step

Launch the helicopter

The spinning helicopter can fly apart if the airfoils were not glued on right. There is no danger to the person launching and perhaps a few people very close because it is spinning well above head level. However, a flying airfoil could endanger people standing a little farther a way. So if there's a crowd of spectators, they should be 20 feet away.

I find it easiest to wind up the launcher without the helicopter on it. Pull the platform a little bit out so out of the pipe before you start winding so it doesn't rub on the pipe. As you look down on the launcher platform you should wind it counter-clockwise as indicated by the green circular arrow. It is common for people to wind up the launcher the wrong way with the result that helicopter just sort of drops off. The string should be under a tiny bit of tension as it winds onto the shaft (the dowel). You can do this by wrapping the string once around the pipe, giving it a little resistance. Or you can pull on the string gently as it goes in.

When you put the helicopter on the launcher, remember to put the side with "top" written on it facing up. Assuming you are right handed, grab the pipe with your left hand and keep your arm straight at the elbow. Pull the string hard, but start out smoothly. If the copter gets stuck on the head of the nail you might have to enlarge the hole a bit.

Where to go from here

Although this helicopter flies tens of times better than other pull-string copters I think the full potential is a long way off. I hope you contact me and let me know how your work goes. I have used a symetrical airfoil in the instructions because it is easier and you might still get the thing to fly even if you get it on wrong. However, non-symetrical airfoils work even a little better. Step 3 shows one.

I'm going to experiment with