Windmill kids challenge

[taylorp035]

The local high schools have a thing called the "Tech Challenge" every year.  There are several events where the kids have to do some type challenge, from tug-of-war vehicles, CO2 cars, mousetrap power cars, rube goldberg machines, and a spontaneous challenge (how high can you support a golf ball with drinking straws or make a hot air balloon).

This year's challenge is going to be a windmill challenge, where the kids have to make something that spins a hobby motor the fastest.  I believe the restriction will be on the size of the devise and no store bought plastic propellers.  From what I hear, the wind will come from a large fan and it is legal to concentrate the air with a cone or something of the like.  I have been recruited to give pointers and help with the designs.

My thoughts:
I don't know the legal dimensions yet, but probably 12"-18" square area
Competition date is next fall
two blades would probably be the best
cnc made blades would ideal for less wobble
a straight axle
Maybe some outside bearings if they don't want the blades attached directly to the motor.
Ceramic bearings would be nice, but the budget is probably <$10.
There will now be ~4 cnc machines to work with (two routers, a mill, and my birthday present (more on this later) ).
With a cnc machine, the blades could be one piece for perfect balance and thickness.
I wonder if one blade would even be an option? Autodesk Inventor would make it easy to find the center of gravity to drill the hole in the right place.

What should happen is that several blades should be made and compared, so design changes could be made.


My birthday present this year was a 4- axis cnc mill.  It has the normal 3 axis and a rotary table.  12" x 6" x 5.5"  This could be nice for some awesome mini blades.  I have been so busy with school work that I haven't touched it in 2 weeks    We assembled it, but it runs really slow, like a factor of 100.

[DanG]

Maybe CNC making a frame to stretch ultra thin clear cellophane over top of - think narrow tipped dragonfly or the back wings of some Australian grasshopper or mimic a high tsr airfoil profile, stiff frame to avoid flutter but no extra mass to waste potential thrust...

[ghurd]

One blade will not work.
Search the board for "maple seed".  There are quite a few related hits.  Mr. 'rotornuts' put a LOT of work into a single blade, and had a lot of problems.

Bummer about fan generated wind.
The turbulence wreaks havock on good blades.
Something that works well in natural wind may not even turn in front of a fan.
Might be good to suggest some kind of 'turbulence reduction assembly' in front of the fan.
Myth Busters did it with drinking straws.
Jerry and windstuffnow Ed posted some ideas on here on FL too.

I think the fan is deal killer to the best types of small blades.
Crappy blades may work a lot better in front of a regular fan.
G-

[taylorp035]

Hmm... so maybe I should consult with the overlords of the competition and make some smooth air flow.

I have yet to see the "wind source" yet or the motor, so everything for now is speculation.


I worked on the CNC machine some more today, and it still isn't running the way it should be.  The axis move, but they are unresponsive when it comes to the feed rate control.

[ghurd]

"maybe I should consult with the overlords of the competition and make some smooth air flow"
Yes!

Thinking out loud-

I never tried a large ducted blower wheel, like from a furnace.  Seems like the 'wind' out of it might be less turbulent?
Do not underestimate them.  My average household duct output is 16MPH, which is not a lot less than a 20" box fan on high speed.

Might find 1000 toilet paper tubes, in 2 layers, front over the back in an "Olympics" logo fashion, would work fine and be within budget?

BTW- "I have been recruited to give pointers and help with the designs" means someone noticed your efforts and knowledge on all these projects.
Congrats!
G-

[wooferhound]

Build Your Own Inexpensive Wind Tunnel
http://www.apogeerockets.com/education/downloads/Newsletter252.pdf

On page 13 - Building a Wind Tunnel
http://www2.estesrockets.com/pdf/2845_Classic_Collection_TR-TN.pdf

Advice For Building a windtunnel
http://www.physicsforums.com/showthread.php?t=191978

Homemade Wind Tunnel - WindStuffNow
http://www.fieldlines.com/board/index.php/topic,137885.html

Jerry's wind Tunnel
http://fieldlines.com/board/index.php/topic,140755.0.html
http://fieldlines.com/board/index.php/topic,138403.0.html
http://fieldlines.com/board/index.php/topic,138410.0.html

[taylorp035]

I'm pretty sure the school has two wind tunnels already built, so I bet they could use one of them.

[dnix71]

Spin the fastest? Is it under load or no load? What are you going to use to measure rpm? Harbor Freight has non-contact rpm meters that use a peel and stick reflective dot.

http://www.harborfreight.com/digital-photo-sensor-tachometer-66632.html  $40 and in stock.

If you used a cone concentrator that might reduce the losses from turbulence some.

[Madscientist267]

A high school with a wind tunnel? Two?! Hmm... maybe there is hope for the next generation...

Steve

[taylorp035]

The rpm will be measured with a voltmeter, hence basically no load. 

As for the wind tunnels, one is homemade (I have never seen it run), and the other is good for about a 12" x 12" area.  It's main use is for CO2 cars, because it has a bunch of scales that can measure the drag force.  It also can measure the difference in down force on the the front and rear wheels.  I'm not sure what the top speed is, but probably over 40 mph.  It would probably work well for testing the windmills.

What would be more fun would be the wind tunnel at Penn State Behrend.  I think it is good for 120 mph and a 24" x 24" area.  I wish it was a tad larger so we could fit our supermileage car in it, but I guess the CFD softftare will have to suffice.  They do have an Air X windmill ... 400W or so.  I'm not sure what they are doing with it, but it's at the exhaust side of the tunnel.  One of these days 

[DamonHD]

Steve: those tunnels are not for the science classes, they're for the politics sessions!  %-P

Rgds

Damon

[Madscientist267]

I knew it was too good to be true... haha

In all seriousness, the fact that one of them is 'student-brew' is even better. We need to see more of this.

Uh oh, I feel one coming on........

My favorite subject in school was science, and it seems like as time has gone on, the generic tendency is to remove the hands on portion of education.

Got to play with all kinds of goodies even as early as the 7th grade, when others were having to wait until sometime resembling more of a high school setting, as did any kid that was able to illustrate that they could handle what they were working with.

Seems these days, the problem has become that there are too many people afraid of being sued when someone gets hurt doing something. Waaaaah... buncha babies. If they taught respect for the subject as well as teaching the subject itself, this wouldn't be a problem.

We all know for example, that 'around here', there's plenty of ways to get hurt. It's a fact of life. Flying blades, battery acid, sparks, magic smoke/fire, explosions, high speed interaction with gravity and hard surfaces, the list goes on and on.

Also known, is that the trick is to be educated as to what these dangers are, to respect their potential, and be responsible enough to avoid them as much as humanly possible. Granted, this still does not prevent all accidents, but I'm fairly convinced that there's a solid relationship between respect for danger and lack of accidental injury. Me thinks.

Every time I ask my oldest son (15 yr) if he's encountered something in school, and he looks at me as if I just morphed into a creature with three heads (either disbelief or a blank stare), a little piece of me dies inside.

We've got to do something. We're producing idiots in mass quantities. I blame the educational systems; at least as a place to start.

We, gentlemen (and the occasional lady), are a dying breed.  

mv /steve/rant /dev/null

(might as well, my singly voiced concern will have about the same effect - but at least I feel that the crowd on here can get what I'm pointing at, bare minimum)

Sorry, didn't mean to jack the thread or anything... the concept just @!$$es me off.

Steve

[taylorp035]

I agree.  The tech classes anymore are usually for the kids who are kicked out of the other classes like the music programs and art classes.

My school started this "home-improvement" class that teaches the basics on how to do just about everything around the house.  It has been a huge hit and the kids really like it.  It's kinda sad that very few of the kids like to design and build stuff anymore.  Even in college, there are ~5000 students, but only about 10 really like to build things (supermileage club is the only place where students can let their imagination run free).

It's amazing to see senior M.E.'s who don't know what a 1/4-20 is... the system has failed in that way.  I think they need a class where we do hands on stuff where we can learn things about various systems like motors, batteries, machining, and the like.  I believe the M.E. program only has 1.5 credits of shop class.

[SparWeb]

Might find 1000 toilet paper tubes, in 2 layers, front over the back in an "Olympics" logo fashion, would work fine and be within budget?
G-

Exactly what it looked like on the wind tunnels at my old college.  They called it a "diffuser", of course, because you can't spend that much money on a machine with "paper tubes" in it!


Is there any chance of getting the motor "load" onto a little dyno to check how it loads the prop?  Knowing its power curve and how much torque it needs to just start turning over would be valuable for finding just the right shape and pitch of blades, to basically have the highest TSR possible.

Gut feeling is that there should be as little blade chord as possible, tapering to fine tips.

Good luck.  Sounds like fun!

[taylorp035]

From what I understand, there will be no load  -   they are just measuring the voltage. 

I did get my cnc machine running, so maybe something will happen over spring break.

[SparWeb]

There is "always" a load, even if it's only bearing friction.

5000 RPM  X  1 inch-ounce  = 4 watts

The power available in 15mph wind with a 12-inch diameter rotor is also only 4 Watts.

[taylorp035]

It would be nice if I could quantify all of the numbers, but I can't. 

[gotwind2]

If kids have any sense they will find my contribution, I made it just for this purpose 
Fairly easily Googled, Let them find it..

http://www.gotwind.org/diy/12-inch-mini-turbine.htm

[taylorp035]

^ very nice.  Some cnc blades would push that thing up to 7500 rpm with my calculations.  You could hit 12v in ~15-18 mph winds.

Maybe they should change the rules to how many volts anyone could generate in the same wind.  It wouldn't be very fair though if people used different generators.

[Madscientist267]

I made it just for this purpose

Well alrighty then. Doesn't get much simpler than that!

Gotta agree, it's quite a tight little design. How long does it take to build one from parts to pole?

Steve

[taylorp035]

I got some more info yesterday on how this thing is going to work.  The motor is going to be one of those small 1" x 1" diameter hobby motors.  They have almost no cogging, so start up will not be an issue.  It seems like everyone will be limited to a 3' x 3' square for the cross sectional area.  The wind source will be a box fan, most likely with 2-3 speeds.

My initial thoughts were to make  very small 2-bladed windmill, with a diameter of ~6" on the cnc machine.  But the possibility of having a bigger blade and gearing up the motor is also possible.

[SparWeb]

Low low low solidity, then...
The full swept area probably not necessary either...
... just agreeing with you then.

[ghurd]

Low low low solidity, then...

I should keep my mouth shut since I am NOT a blade guy... BUT...

Low solidity implies high TSR?
My experience is that will not work much in front of a non-stable very-turbulent air flow, like in front of a box fan.

My RichH 12" CNC blades will hardly turn themselves in front of a box fan on high.  My hand held digital wind gauge said something like 12MPH?
In real outdoor wind, they are screaming along in 6MPH wind, and maybe 800RPM loaded at 8MPH.

However, a 16" dia vent fan blade, with squareish or pie plate like blades, actually gets going OK in from of the fan.
(OK means moving much faster than the CNC blades, it is not implying they are OK for a real wind turbine, because they are not.)

Sorry.   
I have a feeling the whole operation will be a flop if something is not done about the box fan air flow.
G-

[taylorp035]

I agree with the idea that the air is very turbulent in front of the box fan and that it will probably punish the blades with an airfoil on them.  I will see what I can do to change the set up.

[wooferhound]

A lot will depend on the contestants choice of the direction of blade spin. depending on a clockwise or anticlockwise spin, one blade would have an advantage over the other as it interacts with the air spinning out of the front of the box fan. Why aren't you using one of the 2 wind tunnels that you have available?

[taylorp035]

Why aren't you using one of the 2 wind tunnels that you have available?

They aren't that big.  They are big enough for CO2 cars and model rockets, but that's it.  Maybe a cross section of 12"x12".  Also, the competition is held about 5 miles from where the wind tunnels are located. 

[zap]

Have all the kids start saving all their used Pringles cans... cut the bottom off, set 'em on the floor and tie 'em in a bundle big enough to cover the fan... straight airflow?

[taylorp035]

Have all the kids start saving all their used Pringles cans... cut the bottom off, set 'em on the floor and tie 'em in a bundle big enough to cover the fan... straight airflow?

Wish I would of thought of that earlier.  The Pringle's music tour came through the school just a few weeks ago.  There were literally dozens of cans of pringles that could have been had for free.

[zap]

The Pringle's music tour came through the school just a few weeks ago.  There were literally dozens of cans of pringles that could have been had for free.

DOH!!!

[Bruce S]

Because of the fact that the fan is going to have a lot of turbulent air , maybe it would be better to build the blades as a downwind unit instead?

Just a thought,
Bruce S