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Forum topic by teejk posted 09-08-2014 09:59 PM 1026 views 0 times favorited 15 replies Add to Favorites Watch
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teejk

1215 posts in 2148 days


09-08-2014 09:59 PM

Topic tags/keywords: question

Hi. I dusted off a book I bought several years ago. Decided to try one of the projects. My question is finishing the blades. Right now they are simple flat pieces with the shorter edge angling out (I guess to catch the wind first and direct it to the longer blade to create the spin). I’m a retired finance guy and not an engineer but wondering whether shaping the blades in any way would create better power. It is supposed to carry some rings that hit a suspended pipe to be a wind-chime thing (enclosed) and it spins fine until I put any weight on the shaft. It’s all wood btw. Thanks in advance.


15 replies so far

View Wildwood's profile

Wildwood

1883 posts in 1598 days


#1 posted 09-08-2014 10:22 PM

See no problem with your wings.

Back when made whirligigs used mostly vinyl siding for wings and and used latex paint for wood wings. Both vinyl and wood wings worked well.

-- Bill

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crank49

3981 posts in 2435 days


#2 posted 09-09-2014 01:26 AM

If you make the leading edge of the blades pounded over and the trailing edge tapered to a thin line it would improve the flow.

Also, if the outer extreme of the blade was round instead of square it would make the fan quieter.

The large flat area of the mounting base is probably hurting your efficiency more than anything about the blade though.

-- Michael: Hillary has a long list of accomplishments, though most DAs would refer to them as felonies.

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jmartel

6570 posts in 1614 days


#3 posted 09-09-2014 01:45 AM

You have the pitch backwards. Shorter section pointed back towards the house.

As was said, the house is going to cause the most issues.

You also probably want the blades to be bigger. Those won’t catch much wind.

-- The quality of one's woodworking is directly related to the amount of flannel worn.

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teejk

1215 posts in 2148 days


#4 posted 09-09-2014 12:30 PM

jmartel-I think the pitch is correct. Not being an engineer I have resorted to experimentation with a large fan and the shorter edge catching the wind first makes sense to me. The design is flat blades and the “tails” are shaped to round to fit into bored holes in the sail block). The book didn’t get into details on the angle of the blades though but it seems there is some delicate balance between enough angle on the short edge to direct air to the longer edge but at the same time not hide so much of that longer edge with a steeper pitch. I’ll keep experimenting. Crank-I tried another set of blades (I’m getting pretty good at shaping them quickly now!) and eased the short edge. That seemed to actually hurt and not help, I guess because the goal is to catch as much wind on the short side and direct it to the long side. It might help to ease the long edge though but I only have 5/16” thickness to play with and it will be outside so I need to keep some material there. And not much I can do about the sail block. It has holes bored to accept the blades and not much material exists to round the face without getting close to the holes. I guess it might have ok to place the holes closer to the back of the block and leave more wood on the front that could then be shaped somewhat to reduce friction. I haven’t glued the parts yet and I’ll see if I can put even a slight relief to the outer edges of the face.

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ChrisK

1809 posts in 2545 days


#5 posted 09-09-2014 01:32 PM

teejk,

You got me thinking. I am an engineer with several credit hours of aero and fluid dynamics, though have never really used them. Crank is partially correct, Google windmill blade design and you will see that for real power generation and noise control the blades get very long and glider wing like. Maybe a little over the top for a whirligig, but maybe a very futuristic looking one. If you make one please post a pic.

Here is a little test with very simple blades: http://www.creative-science.org.uk/8blade.html

-- Chris K

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jmartel

6570 posts in 1614 days


#6 posted 09-09-2014 02:49 PM

Teejk, with all due respect, you are incorrect. The short side goes on the trailing edge. I have taken many aero/hydrodynamic classes as that is my field of work. I am an engineer in the marine industry, but fluids are fluids.

If you do not believe me, search for any image of windmills.

-- The quality of one's woodworking is directly related to the amount of flannel worn.

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teejk

1215 posts in 2148 days


#7 posted 09-09-2014 03:16 PM

Thanks jmartel. I think I tried it both ways but I may not have had the correct angle of pitch. Will be easy enough to change since I haven’t done the glue-up yet. It is one of those things that I guess I had never really thought about and my way seemed to make sense but then again I am usually right about half the time. That’s why I posted the question since there is usually somebody here that knows a lot more than I do. Short edge facing in…would it make any sense to ease that edge (something I read once about the science of flight)?

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Wildwood

1883 posts in 1598 days


#8 posted 09-09-2014 07:43 PM

I found just following a pattern to make my blades for a project easy. Think had only one project that required more than four blades. Never had any problem getting vinyl or wood blades balanced. Much easier to do with vinyl siding where only needed two blades.

Took little more time getting the propeller hub right. Hole for crankshaft has to be exactly in the middle and slots for blades equal angles. Used 45 degree angle for both 2 and 4 blade hubs. Once get everything marked out drilling and cutting pretty easy. I turned my hubs on a lathe after drilling and cutting. Sometimes turned hubs angled and sometimes just round.

Mechanical parts and supplies take more thinking. 1/16”- 3/32”- 1/8” brass rods ordered on line. Could not find 1/8” brass welding rod locally. 1/8” brass tubing makes great bushings.

-- Bill

View jmartel's profile

jmartel

6570 posts in 1614 days


#9 posted 09-09-2014 07:50 PM



Thanks jmartel. I think I tried it both ways but I may not have had the correct angle of pitch. Will be easy enough to change since I haven t done the glue-up yet. It is one of those things that I guess I had never really thought about and my way seemed to make sense but then again I am usually right about half the time. That s why I posted the question since there is usually somebody here that knows a lot more than I do. Short edge facing in…would it make any sense to ease that edge (something I read once about the science of flight)?

- teejk

For something this scale, it won’t matter much whether you make an airfoil shape or not. That being said, you want to roundover the leading edge, and create a knife edge on the trailing edge as a general rule of thumb.

-- The quality of one's woodworking is directly related to the amount of flannel worn.

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teejk

1215 posts in 2148 days


#10 posted 09-09-2014 08:21 PM

jmartel…Not a lot of time today but I did rotate the blades in. The book was very clear on the direction of the blades as per my pix. And it was clear that rotation would be clockwise. By turning them in, it turns counterclockwise! Not that it matters. If I can get enough power to rotate the internal “clangers” I really don’t care. But just FYI, I was fooling with a set of blades where I did relieve the edges as an experiment. Seems to spin better but maybe just in my head. I have to keep in mind that it is an outdoor thing and I have to plan on the effects of mother nature so not a lot of lee-way there. The book is old and apparently the design was copied from an old one, certainly before the Wright Brothers. I appreciate all the replies, especially for not asking why I am doing this now and not in January. I have been asking myself that very same question since it has occupied much more time than I thought it would (I get the periodic cravings for knowledge).

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jmartel

6570 posts in 1614 days


#11 posted 09-09-2014 08:29 PM

You can swap blades to the opposite side to get it to spin clockwise again if it would make a difference.

For that design, I think I’d probably increase the blade width for a larger Blade Area ratio. And use as lightweight of a wood as you can get away with structurally. That’s one area that shaping the blades will help with.

-- The quality of one's woodworking is directly related to the amount of flannel worn.

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teejk

1215 posts in 2148 days


#12 posted 09-09-2014 09:35 PM

thanks again jmartel. I’m using leftover poplar pieces from my window trim so it’s light enough I think. The plan (all based on grid-lines, no measurements) called for 2” wide at the widest part tapering down to about 1 1/4” before it starts to create the mounting shaft. Nasty weather coming so maybe tomorrow I’ll try to put some weight on it and see what happens. If it doesn’t work I’ll shape some wider blades (doesn’t take long). I’d kind of like to get this one to work because there are a few other very interesting ones in the book (winter work though). The book is going back on the shelf until I get the winter prep work done around here (it’s coming I think).

View Sylvain's profile

Sylvain

639 posts in 1963 days


#13 posted 09-10-2014 04:30 PM

As Crank49 said, the large flat area of the house behind will seriously impair the efficiency.
A round house (tower) would be much better.
If you google windmill to see pictures of real old windmills, you will see that the tower has a diameter much smaller than the wing span and that the axle is put very close to the top.
modern windmills have blades shaped like airplane screws and the mast is as thin as possible.

-- Sylvain, Brussels, Belgium, Europe - The more I learn, the more there is to learn

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mahdee

3551 posts in 1231 days


#14 posted 09-10-2014 05:47 PM

Here is the one I made for Baileigh contest… It spins counter clockwise.

-- earthartandfoods.com

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teejk

1215 posts in 2148 days


#15 posted 09-12-2014 08:33 PM

Thanks to all…as noted this is a replica of an old piece (authors thought Penn Dutch). Given the design it doesn’t seem to matter much whether the blades face in or out but I’m thinking about making a set wider at the tops. And then I think I know have a better understanding of things (thanks to all) and I can envision which edges need to be shaped to prevent friction. The book is going back on the shelf until winter where I may make the guy hand-cranking an old model T. The other interesting one involves drumsticks (with trip levers) hitting a leather drum skin.

But with that out of the way and Christmas coming…my grandmother had an piece with a helicopter type blade powered by heat from candles below. Was octangle shaped I think but it was a long time ago. The walls were 2 story and had windows that displayed figures that rotated as the rotor turned. Anybody built one of those?

And with that out of the way, thinking about marble toys too! Winter gets pretty long here!

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