A question about propellor design

[JimSawyer]

My niece has a science project coming up that she needs me to carve her a propellor for. It's a car that will run on an elastic band with a propellor. What she's looking for is distance not speed. What my question is/are ; 1) how many blades should I go for? I'm leaning towards 3 but I'm not sure. I plan to make three different ones, with 2, 3, and 4 blades, and she and her father can experiment. Also, should the blades be wide or thin. And should the ends of the blades be more square or tapered.
If there are any other specifications I haven't thought of, please fill me in.
This project is due February 17th, but I'll need to have them ready for my brother and his daughter to test and install them at least a week ahead of time.
Thanks in advance.

[AlArchie]

Jim, here's a site that pretty well describes the prop building pocess. I have an article somewhere on it also but don't know if i CAN FIND IT...i'LL ;OOK IN THE MAG STACKS AND SEE WHAT i CAN COME UP WITH. If I find it, I'll post it....if I can't I won't. Now there's a brilliant statement, huh?
Al

[Curly Early]

Jim, I may kick myself for answering this question, but this is my take on the subject. Like some others on here that were modelers, I started young, building rubber powered airplanes. My memory serving me correctly, the props were square tipped, wide bladed and of aggressive pitch. You don't want the prop spinning at a great RPM because your rubber band will soon run down. Perhaps you know this about pitch, we bought propellers for gas engines by size, 10-6, 12-8, ect. 10 being diameter in inches, 6 being the distance the prop would, in theory, go through the air in one revolution, thus 6 inches. I googled rubber powered airplanes and found them on U-Tube in action. You'll notice the props turned very slow, were very large in proportion to the size of the airplane, but duration lasted quite some time , so use the largest diameter you can, aggressive pitch, square tip, thats what I would do. I've rambled enough but, we use to control, (interupt) the flight of our early R/C planes with rubber band driven escapements. By the way, we lubricated those rubber bands with something for more torque, was it cornstarch? Maybe someone else will remember. Earl

[mark yundt]

Glycerin and or silicone spray works well. And ,if you stretch the rubber band as you are winding it,,you can get much longer spin time for longer power.Go to hobby shops and you can get lengths of rubber to make good "motors".I don't know the weight of the car or its resistance to rolling...but for ultralight micro planes..they use a single blade design..Of course there isn't anything to these planes as they weigh less than a penny.

[Claude]

I'm going to go out on a limb here and suggest that she make the propellor blades out of a gallon plastic milk jug. By cutting a vertical piece out of the jug from near a corner, the blade can be made with a small curve to it, when looked at from the end (do two of these). The longer it is, the slower it will turn, and the longer you'll get thrust. When mounting the blades to a hub, the easiest way may be to just glue the two together, glue a couple more little disks of plastic on each side of the center hole to make a bit thicker and stronger hub. Insert your wire that the rubber band hooks to (unbent paper clip section?) through the hole in the center of the propellor, bend it over into a small "U" insert the short end into a second small hole in the hub, so the wire won't slip, and then glue in the wire to the holes using the same glue you used to stick the plastic together. The leading edge of the blade should be pointing almost straight out from the wire, and the trailing edge of the blade should be "parallel" to the hub wire. Now, having said that, don't use a 90 degree angle - that's way to much to be efficient. The curve you're looking for would be maybe 20-30 degrees - meaning press one edge of a blade to the table, and the other edge is lifted up only 10 or 20 degrees. If my explanation is about as clear as mud, let me know and I'll try to photograph a piece for you...

BTW, here's a video of making one of wood, also: Classic Propellor - Video

Whatever you decide to do, please post photos of the process for us.

claude

[AlArchie]

Efficient Propeller Design

GEEZE, I woke up dumb this morning and been dumb all day.......here's the link I thought I posted. Hope it helps.

Al

[JimSawyer]

This is great you guys! Thank you one and all. I'm going to make a few different blades with different configurations and let my brother and his daughter experiment.
Al, That was a great link. I had not even considered the chord measurement. That just means more blades with different cofigurations.
I'll take some pictures tommorow after I've made a few, and also of the vehicle itself when I can, (that's my brother's end of things).
Thanks again, this has been extreemly useful.