NOTIFICATION:
This homemade propeller really works--powered by a rubber band--and it is fun to make a model airplane completely from scratch. However, it is a challenging project that I do not ask my middle school students to attempt. When they build planes, they use commercial 6" nose hook propellers from Kelvin.com, part number # 850654.
It will take a lot of time, trial and error to get it to work.
Below is only a stopgap description until I can make an instructional video detailing the steps.
What you need
a two-liter soda (Coke/Pepsi) bottle
It should be un-crushed and smooth. Try to get one with the label still on. Rinse it out before using.
small paper clips
Use the all-metal ones, not the plastic multi-colored ones. The ones I use are about 3 3/4" (95mm) long when un-bent so they are
straight.
needle nose pliers, scissors, tape, razor or sharp-pointed knife
You could use regular pliers in a pinch.
a propeller to copy
This isn't absolutely necessary but if you have a commercial 6" (150mm) or so long propeller made for rubber bands and it works well, why re-invent the wheel? It helped me sort out the angles the first time.
PROCESS:
Step 1
cut out the blade halves
If you look at a factory-made propeller you will see that the angle twists as it goes from the outside to the center (this is to compensate for the outer part of the propeller going faster than the part of the propeller nearer the center of rotation). Theoretically, you can attain this complex twist by cutting the blades out of the soda bottle at a slight slant--5 degrees or less. In practice, I put very little twist in my propellers, but I'd like to hear how other people do it.
The picture shows a paper pattern about 7/8" (22mm) wide. Later on I will shape it more. It is tipped slightly from vertical (represented by a red line) to the left for twist. You need two of these to make one propeller.
Step 2
make the paper clip shaft, etc
This is going to be two pieces of paper clip straightened, then hooked together to form a "T" shape. Bend a small loop into the middle of one straightened paper clip, and a small hook into the end of the other. Each wire must grab onto the other, or else the propeller will be flimsy. Crimp and put a drop of glue at the intersection to hold everything steady. I use hot glue because I lack patience, but any glue should work.
Step 3
attach the blades to the shaft
The easiest way to attach the two plastic propeller halves to the shaft it to make a loop in the two paper clips (the top of the "T"). Then just tape them on, as shown in the picture.In the picture the actual shaft (the bottom of the "T") is hidden by glue and various bearings (made from a cut-up ballpoint pen ink tube, see step 4, below).
The other way to attach the plastic blades to the paper clip shaft is to punch or melt a small hole near the end (5 mm or so) of the plastic. Then poke the end of the paper clip through, bend it around and crimp hard as shown.
Whichever method you attach the paper clip "T" to the plastic blades, the concave part of the blades should face the plane (or to put it another way, the bottom of the "T"). If you forgot and put them the wrong way, you should be able to twist the paper clip wire until it faces the right way.
By the same method of twisting the wire, set the blades to their proper angle. This is difficult to convey with words and two-dimentional images. Here is where a commercial propeller to copy comes in handy.
Step 4
attach to plane
The top picture shows how an old ball point pe
n is cut open and the thin ink tube used to make various bearings. The main bearing is a short piece of the ink tube with paper clip wire wrapped around. I glued the wire to the tube with hot glue. The ends of the wire are glued onto the front of the body of the plane. Make sure the tube is lined up so the propeller shaft that will go through it will point forward, not up, down or to one side.
The lower picture shows the assembly. I thread on a couple more very short tubes so they will be between the the propeller and the main bearing. I haven't really tested whether these additional bearings reduce friction or not. Next, I put the shaft through the main bearing. Only then do I bend a hook into the end of the shaft; otherwise it will not fit through.
Step 5
odds and ends
The commercial propellers we use for the squirrel plane are 6" (153mm) long. I make the homemade propellers between 6" and 7".
Sometimes I trim the leading edge of the propeller a bit so it tapers slightly toward the tip.
Be aware that the soda bottle propeller is lighter weight than commercial propellers. You might have to weight the front of the plane or readjust it
