Before I go into how I make propellers, here are some pictures Prabhuraj.C.U from the state of Kerala in India sent me. He reports that it flies. For details you can contact him directly: prabhurajpudussery at gmail.com (replace the at with @).
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UNDER CONSTRUCTION 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.
It should be un-crushed and smooth. Try to get one with the label still on. Rinse it out before using.
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.
You could use regular pliers in a pinch.
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.
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.
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.
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.
The top picture shows how an old ball point pen 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.
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.