Propeller design

[ZRTMWA]

I was thinking about the Egg Drop thread and the Math Homework help thread so I figured I would start a tech ed help thread. I don't really need help, its not like I'm going to fail if I can't think of a design but I always like to hear the opinions of LPF members.

That said, in tech ed we are designing propellers out of small tin rectangles (1.75" by 5") and hooking them up to a generator which is hooked up to a volt meter. The propeller goes in a wind tunnel and we must record how many volts the propeller generates. With a quick run through today mine was doing 3.57 V. I curved the blades a little to catch the breeze. This is basically what my propeller looked like except the edges weren't curved and the blades were twisted:

First we punch a small hole in the center of the rectangle, then draw our design with sharpie and then cut it out with tin snips and test it. Any ideas for designs? I think it should only have two long blades. But maybe it could have 2 long and 2 short blades. Or 4+ short blades (but these blades would have to be less than .875" long and thats not even including the hole. As always, thanks.

 

[Things]

What kind of generator is it driving? More blades = slower, but it takes less wind to turn them. If your driving a basic motor as a generator, you want the blades to be able to move through the air as fast as possible, with as little drag as possible.

 

[weetsie]

i would start with a 45 degree pitch with a decent aerofoil

 

[ZRTMWA]

Yeah pitch and surface area are key with these. The props are just driving a basic motor. Another kid in the class today just rounded off the edges of the tin, cut 2 small spaces in the middle to form two seperate props and his went up to 8.5 V. The record is 9 V I think. I can get any aerofoil because it's just a sheet of tin. The 45 degrees is a great idea though. I got my prop from yesterday up to 5.7 V just by twisting it a little bit more.

 

[ZRTMWA]

Now we are using 5" by 5" peices of tin, any more ideas? Generally people punch a hole out in the middle of the tin and cut it into a 5" circle, then divide it into 8 equall sections, cut them into blades and give them pitch. I tried a 2 blader, a 4 blader and a 32 blader. All of them got less than 6V. Any more ideas?

 

[schmag]

say I am not very good with props but I am awesome at google. see if any of this is good for ya.
Windmill Efficiency.
I do know that if you are just turning a regular motor you want rpms of your prop more so than torque.
have you tried more gradually sloping the blades themselves vs an all out twist of the blades
here is another link to a word doc I found.
http://www.copernicusproject.ucr.edu/ssi/Science Fair/5/Optimizing Windmill Blade Efficiency.doc
it looks to have some good info.
I googled
windmill efficiency

good luck sir.

 

[ZRTMWA]

Thanks but I need speed . Torque doen't matter because the setup is just a rubber band is attached to the shaft and a tiny gear which is on a tiny motor. The motor is hooked up straight to a multimeter. The faster the motor spins the higher the voltage.

 

[ZRTMWA]

I made another fail blade today. It was made out of the 5" x 5" tin and had 64 blades. It only got 3.3V. Yesterday I made a deathblade. It cut me at least two times. Sorry to keep bringing this back but any more ideas.

 

[bugattidude]

if you have any R/C plane blades you could test them and see the how many Vs ur gettin.. then maybe try to copy the design...

 

[weetsie]

Yeah pitch and surface area are key with these. The props are just driving a basic motor. Another kid in the class today just rounded off the edges of the tin, cut 2 small spaces in the middle to form two seperate props and his went up to 8.5 V. The record is 9 V I think. I can get any aerofoil because it's just a sheet of tin. The 45 degrees is a great idea though. I got my prop from yesterday up to 5.7 V just by twisting it a little bit more.

you can still make an aerofoil section just by curling it along the prop. i would start with something like the 2nd one up from the bottom in this img:

it will give more torque and less drag which will increase RPM.

EDIT: for blades stick with 2 or 3, more blades = more torque but as torque is not a factor apart from increasing speed by reducing the effect the weight of the prop has on the speed.

 

[ZRTMWA]

Oh cool, I'll try that design tommorow. Our teacher tried to show us something like this in school today but failed at explaining it. Thanks.