See photo here. This is the “classic” windmill form, and if you magnify the image and examine the blades of the turbine, each appears to be comprised of a bunch of ribs and a spar, with precious little surface area to catch the wind. They bear a passing resemblance to modern grid fins, but those are densely packed with short-chord airfoils; the same cannot be said for those windmill blades, on which the ribs are widely spaced and do not appear to have much of an aerodynamic shape. They appear to have a root-to-tip twist in their general shape, same as a modern wind turbine, but there’s no solidity; it’s like stripping the barbs from a birds feathers so that all that remains of its wing are the bones and quills.
So what’s going on? Every picture of an old windmill I’ve ever seen shows the same thing, but I would have thought that those skeletons would be wrapped in cloth to make an actual wind-catching airfoil of some sort.
How do these things develop torque when the wind blows over them?
My understanding is that the blades were based on ship sails of the time, but I could be wrong on this. In any case, and airfoil shape is not required, it just makes modern windmill designs more efficient than the old style.
The wind is a pretty impressive force, you need surprising little surface area to harness it under some circumstances.
They’re called “sails”. Each of the little slats is actually an airfoil designed to catch the wind. They’re designed to adjust depending on wind speed.
Also, are the blades actually trying to catch the wind that’s coming from the perpendicular direction, rather than some other angle? Because modern wind turbines (being very thin) don’t look so efficient for that purpose either.
At least half the old-fashioned windmills I see in a quick Google search look like they have fabric spread over the framework of the blades, which would indeed catch the wind.
I suspect that many of the ones without fabric might be currently used just for decoration, where the aim is to look like a windmill but with as little actual force generated as possible.
I do see an image that looks like the blade is not just a very open framework, but actually rows of wooden slats (I’m presuming airfoil-ish shaped) set in the framework. This seems like it could produce reasonable force without fabric covering.
The framework is to support the sails. If you Google windmills for images you can see the sails on many of the windmills furled and tied to the framework in a sort of wrap-around fashion like here.
There’s a tradeoff between efficiency at higher wind speeds and at lower wind speeds.
Fewer thin blades are more efficient at high wind speeds, since there’s enough wind to propel them, and they slice through the air more effectively, reducing drag.
More, thicker, blades are more efficient at low wind speeds, since they catch more of the air. But at high wind speeds, they don’t move well, and can even get ripped up.
Modern windmills generally get put in places that have consistently high winds since they’re not cost effective elsewhere. Old windmills got put… well, wherever, since there often weren’t other good ways to get power at all. Plus, we have a better understanding of fluid mechanics now.
I think the key to this is simply that canvas or other fabric rots and gets untidy. So it is seldom now present on old windmills: all that is left is the supporting framework. The sails are gone.
Yes - when operating, those “classic” windmills had fabric sails stretched over that framework. Most of them are just for show today: sails would cost money, wear out in the sun & wind & rain, and make the structure more vulnerable - so they are omitted.
“Common sails consist of a lattice framework on which a sailcloth is spread. The miller can adjust the amount of cloth spread according to the amount of wind available and power needed.”
