'Windmill' question

slowburner

This 'windmill' has absolutely no purpose other than to provide amusement and some nice light patterns.
Every point of friction and rotation has a bearing or two. The thing is, (and I know I am getting into engineering here) the contraption is not rotating as freely as I would like - that is to say, it needs a fairly strong wind to move the vanes. I made another one previously and it was a whizzer (it was made to keep magpies off a roof) - same bearing set up, roughly the same surface area but I think the vanes were at a different angle.
Any idea what the optimum angle is?
What would be the optimum number of vanes?
I ran out of cd's and had to re-use some broken ones from a previous project - hence the cut off arc - I presume this is contributing to the lack of performance.


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diarmuidr

have you tried taping over the holes in the CDs?

slowburner

Never thought of it
I think the gap between the vanes is too big as well - where I had to cut the pieces off.
Thanks.

diarmuidr

no problemo, its just a guess really :P

Capt'n Midnight

really odd that the optimum angle for spinning is different to the one for starting !

at small angles of attack you get very fast spinnning but very hard to start



what are you using as the bearing ?
have you checked it's balanced ?

look up picture of old water pump windmills from the wild west to give you some ideas

slowburner

Capt'n Midnight wrote: »

really odd that the optimum angle for spinning is different to the one for starting !

at small angles of attack you get very fast spinnning but very hard to start



what are you using as the bearing ?
have you checked it's balanced ?

look up picture of old water pump windmills from the wild west to give you some ideas

Interesting. Starting is the problem alright - what do you think would be the compromise angle?
The main hub houses two bearings from skateboard wheels (I think).
It's fairly well balanced - although the main hub is wood and you can never be sure of wood's weight from place to place.
I made another one with 12 vanes - just to see would the increased surface area give added resistance and therefore greater ease of starting.
(experimenting is more fun than Googling) Thing won't run at all now.
I should try and get a better understanding of this angle of attack business.

KK4SAM

Me's think the furling tail is too small to maintain it into the wind.

slowburner

That was the Mark I version. The Mark II version had to be abandoned because it spun so fast that it threw cd's around like Ninja stars.
It is still doing its job on the roof of a house in Dublin - its job is to keep magpies away.
It's a fairly complex collection of variables for a dummy like me.
Angle of vanes off the vertical axis
Optimum angle for starting up versus running
Number of vanes (why do commercial windmills only have three?) Most extractor fans etc. have five

Capt'n Midnight

slowburner wrote: »

Number of vanes (why do commercial windmills only have three?) Most extractor fans etc. have five

the most efficient wing is long and thing, like a glider or an albatross , it means you have a greater swept area and the wing is thinner

for smaller fans the swept area may be determined by the size of the enclosure
for aircraft fuel storage is important, as is a place to stick the undercarriage
and shorter thicker blades/wings are far far stronger

shanemangan

OP you have given me a good idea for what to do with all my old CD's!

Im in the middle of making a fish tank out of an old monitor too, learning from this : http://www.instructables.com/id/Turn-Your-Old-CRT-Computer-Moniter-Into-A-Fish-Tan/

slowburner

CDs aren't as strong as you might think - just in case you intend to use them in a situation where they might break up and send bits into unwanted places :eek: