Shouldn't there be bigger (or more) windmill blades?

Iamme

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Everytime I see pictures of those wind farm windmills, that have these expensive towering rigs that have those 3 skinny blades on them...I say to myself, "What's wrong with this picture? Why don't they utilize more of the tower and put on bigger blades, or more OF them?" It just doesn't look right.

I already realize that you have to have open area between the blades to allow the air to pass through...otherwise the rotation would simply cavitate, and not produce useful 'work'. But aren't these rigs OVERLY minimalistic? Weight I'm sure factors in. But, if THAT is the reason, then surely there has to be a better way. I.E., use the same tower (for economy) but put on two overlapping blade systems through two seperate bearing systems.

You know how you can just tell if something doesn't look right? I can tell that strings tied to the feet of an elephant, are not going to stop him. I could have told you that the "Big Blue" monster crane that toppled in a twisted heap during the new Miller Park stadium constructionh during a windstorm in Milwaukee was an accident to happen. I can tell you that just by looking...that 2 X 4's stretched across a 16 foot expanse as floor joists, just is not going to cut it. Same with these windmill blades. Look how monstrous looking the Dutch windmill blades look.

Comments please?
 
As someone who has attempted the fine art of sailing in the past, and not done too well at it, I can tell you that one of the most important considerations of harnessing wind power is not what to do when there isn't much mind, but how to handle a storm. It is amazing how quickly you want to lose power. A sailing boat can just drop it's sail if it wants to, a wind generator has to stay out there in all kinds of weather.

Also note that propellor planes have small blades as well.
 
I don't know much about the underlying physics, but I heard that basically the fewer blades you've got, the stronger the wind the rotor can a) effectivley use and b) withstand. There's actually a one-bladed version (with a kind of counterweight knob on the other end) for really high wind speeds. The farmyard variety can take just any mild breeze and start pumping the well, but croaks in a coast strength wind. Or so I seem to remember. Roll on the physicists ;)

cheers
floyt
 
Don't remember the physics but it has the same restrictions as the serface area of a sail (AUP kinda aluded to that ) you can only make it so large with diminishing returns until the curve becomes negitive i.e. the force imparted by the wind can get no larger and the weight of the sail (prop ) becomes the major factor.
 
Iamme said:
Everytime I see pictures of those wind farm windmills, that have these expensive towering rigs that have those 3 skinny blades on them...I say to myself, "What's wrong with this picture? Why don't they utilize more of the tower and put on bigger blades, or more OF them?" It just doesn't look right.
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TLAR design only gets you so far in aerodynamics. You can be sure that the design of commercial wind generators has been carefully optimized for the expected conditions.


I already realize that you have to have open area between the blades to allow the air to pass through...otherwise the rotation would simply cavitate, and not produce useful 'work'.
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<pedantic>Actually caviation is a phenomenon which is only relavant to propellers working in liquids. Since the air is already a gas, it can't suddenly turn into a gas if you take the pressure too low.</pedantic>


But aren't these rigs OVERLY minimalistic? Weight I'm sure factors in. But, if THAT is the reason, then surely there has to be a better way. I.E., use the same tower (for economy) but put on two overlapping blade systems through two seperate bearing systems.
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My wild-a**ed guess is that the bending load on the tower is one of the limiting factors. I don't think that would be helped by going to a multi-disk configuration. You'd still have about the same reaction force for a given delivered power. Maybe a little more, since there may be additional interference drag between the two disks.

Comments please?
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Windmills are cool. They're building another row of them in the I-10 Palm Springs farm I ride my bike next to.

--Terry.
 
If I recall correctly, fewer blades => higher rpms at a given wind velocity, and higher rpms generate electricity more efficiently. Of course, my memory is not what it used to be.
 
The trust generated by an airfoil is not directly proportional to its area, but the friction is. Thus a long, narrow foil will generate more lift per area unit than a short, broad one. A long broad one will generate more than the narrow, but at the cost of increased friction, and thus unproductive structural load.

Note that glider planes have long, slim wings, modern sailboats have tall, narrow rigs.

Hans
 
I think its interesting to refer to the development of props during WWII when they probably reached their highest state of refinement.

In the late '30s props had two thick blades.
By 1940 props now had three thinner blades, and later four and then five or six blades, always getting thinner. This pretty much backs up what Hans says above.

Probably the ultimate evolution was to have two sets of contra-rotating six bladed props, which cancel out the torque.
 
Actually, the ultimate props were placed on Zeppelins, where space and ground clearance was not an issue. 4-6 meters diameter, slim two-blade monsters. Or look at helicopter rotors.

Hans
 
Jon_in_london said:
In the late '30s props had two thick blades.
By 1940 props now had three thinner blades, and later four and then five or six blades, always getting thinner.
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Kind of like razor blades.
 
On planes a two blade prop is always more efficient than multi-blade types. Single blade props are used on racing model planes, as they are more efficient still.

The reason for 3, 4, 5 and so on bladed props on planes is that an optimum designed two blade prop, for the same engine and flight conditions, would catch on the ground, and/or demand longer more impractical landing gear.

Early spitfires had two blade props. As the engine was developed, and grew more powerful, the only way to absorb the power was to fit more blades; but the designers would have prefered to increase the diameter of the prop while retaining just two blades, if they could.

The least efficient part of any wing, sail or prop blade is the tip. To gain efficiency, minimise the number of tips.
 
ceptimus said:

The least efficient part of any wing, sail or prop blade is the tip. To gain efficiency, minimise the number of tips.
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Hmm... I know that some have tried fitting 'feathers' to the end of wings to stop that vortex thingy forming. Wonder if you could do the same with a prop.
 
Jon_in_london said:


Hmm... I know that some have tried fitting 'feathers' to the end of wings to stop that vortex thingy forming. Wonder if you could do the same with a prop.
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The only reason to do winglets is if your span is limited for some reason - eg jetway spacing at the airport (airliners), competition rules (gliders). Otherwise it is structuraly and aerodynamically more efficient to just increase the span. Also, winglets must be optimized for a single C<sub>l</sub>. Winglets on a prop is a nice idea, but the structural design of props is hairy enough already that the complication might be pretty hard to deal with.

--Terry.
 
What's that glider in your avatar Terry?

I fly a Tragi 702 in F3F (no, that's not a cypher - it might actually mean something to Terry)
 
ceptimus said:
What's that glider in your avatar Terry?
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It's a Raptor Aerosports Feather XL. About the last generation of javelin-style HLGs before the discus takeover. Weighed a hair under 10oz with flaperon/rudder/elevator control.


I fly a Tragi 702 in F3F (no, that's not a cypher - it might actually mean something to Terry)
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Slope racing? Sounds cool, but I've never tried it. I think my reflexes are getting a little slow these days anyway...

--Terry.
 
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