I've got a few questions about the windmill episode:

1. When the team with the horizontal windmill realized that their fan turned in the wrong direction, couldn't they have simply place the windmill on the field so that it faced away from the wind? I don't think their blades incorporated any kind of airfoil (which would have made the fan more efficient in one direction than the other).

2. I'm completely mystified as to how the vertical windmill works. Apparently the airfoil is perpendicular to the spokes. I would think that would create a purely axial lift component, which wouldn't turn the windmill at all. In other words, the lift would pull outwards on the spoke, not turn the mill around. It would make more sense to me if the airfoils were colinear with the spokes, similar to the way those windspeed anemometers (with the hemispherical cups) work. What's the principal here?

3. I'm sure this relate to the answer to question 2: why does the vertical windmill have to be pull-started to function?

-- Eric Kollenberg (ekollenb@us.ibm.com), December 21, 2000

Answers

I'm no engineer, but if I understood Cathy's explanation, the airfoils DO create lift that pulls them away from the center, but the fact that they are firmly anchored to a rotating axis converts that force to rotary motion. This 'battle of forces' is the reason the windmill needs a pull start; it has to be 'coaxed' into converting that outward force into rotary motion by starting it manually and maintaining a higher speed.

Certainly this is a very similar principle to the one we saw in the land yacht episode; the sail doesn't catch the wind so much as use it to form an airfoil, which generates (sideways) lift, which the friction of the wheels converts to forward rather than sideward motion.

My question is why not replace the windmill's airfoils with flat, broad blades that catch the wind instead of slicing through it? The result would be better low wind performance and no pull start, yet maintain the ability to catch wind from any direction.

-- Derek Jensen (djensen@kconline.com), December 21, 2000.

Yes, that's what she talked about, but she pretty much glossed over the technicalities. There's got to be more to it than what you said, because a lift that is directly outward will "fight" with the inward pull of the spokes (Newton's 3rd Law), but that will just create stress in the spokes; it won't make the whole thing turn. For that you have to have a force component that is perpendicular to the spoke.

With the land yacht, what happens is that the wind blows at an angle that is partly against the direction of the yacht, and partly sideways to it. This creates a lift that is perpendicular to the direction of the wind, i.e. partly in the direction of the yacht and partly sideways to it. Since the yacht can only move in the direction the yacht is pointed, only the forward component of the lift has any effect, and the yacht rolls forward.

If the airfoils on the windmill were at an angle to the spokes so that there was a forward component to the lift, the same principle could be used. In fact, it seems to me that the ideal angle would be 90 degrees (as I mentioned in my initial post). That's what they do with windmeters I mentioned (which incidentally don't need a push start).

So I still don't have an explanation I can understand.

As for the idea of ditching the airfoil entirely, the problem with that is that the force exerted by the wind on one blade would be balanced by the force on the other blade. No net force means no power.

-- Eric Kollenberg (ekollenb@us.ibm.com), December 21, 2000.

For more information check out this website. http://www.awea.org/faq/vawt.html

-- Mike H (mharms.nospam@cemstone.com), December 22, 2000.

Yeah, I didn't consider the action of the wind on the opposite blade. It would be like immersing a water wheel completely under the surface of the river instead of half out of it.

You would have to build a shield to shade the opposing blade from the wind, or at least shape the blades like cones, so the one 'blade' presents a sharp end to the cut through the air while the opposing one presents a cup to catch the wind.

-- Derek Jensen (jensende@us.ibm.com), December 29, 2000.