Donald Cooksey: NASA Wind Generation Project
(Page 3 of 9)
May/June 1976
By the Mother Earth News staff
COOKSEY: Well we do have other blade designs on the drawing board, and I believe that we'll eventually test them. We're going ahead right now with the particular rotor you see here primarily because it's relatively inexpensive and easy to fabricate. In short, it costs roughly two-thirds as much to build a two-bladed rotor as it costs to construct one with three blades. And the feathering mechanism is much less complicated, etc. It's also obvious that you can save a great deal of money on the construction of a plant this size if you can find a way to cut down on the number of components that make up the machine. Well, by using a downwind rotor, we don't need to add a tail to the WTG . . . and that, of course, cuts our initial costs considerably.
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We're not anti-three blades, by the way. A three-bladed rotor is probably the best possible choice for a windplant that sweeps a disc of 20 to 40 feet in diameter, which is exactly the area in which Jacobs worked. For a larger wind turbine, though, we think two blades are better.
PLOWBOY: I've been up to the top of the WTG tower, and I could feel it tremble as the wind up there blew against the rotor. I could see the blades bending a little too. Apparently there isn't much "give" in the rigid hub that holds the rotor. Is that a good idea? Shouldn't there be some flex up there to relieve that strain?
COOKSEY: We've taken pains to make sure that the frequencies of vibration set up by the blades as they spin are not the same as the tower's vibration frequency. If they were, the whole thing could shake itself apart in very short order. That problem, however—as long as the rotor automatically feathers and unfeathers itself so that it maintains a constant speed of 40 rpm when it's in operation-has been engineered out of the windplant.
The slight shaking you felt, on the other hand, was due to the difference in wind loads against the blades as they turn. There's a lot of variables working on that rotor as it spins, you know.
The tower, of course, creates a "wind shadow" at the bottom of the arc. This, plus the fact that the mass of air 40 or 50 feet above the ground's surface can be moving at only 10 or 15 miles per hour while the wind 150 or 160 feet above the ground is moving 50 or 60 miles per hour, can put a tremendous bending moment—out at the top and then in at the bottom—on each blade as it turns. Add in the fact that when you're sweeping a 125-foot circle you're going to get all sorts of little gusts and other variables between any one spot on the disc and any other.
All this does, indeed, put a certain amount of stress and strain on the hub . . . strain that we'd like to relieve. We'll probably be experimenting in the future with [1] blades of some other material or construction that will flex more as they spin and, thereby, absorb a great amount of these variable loads without transmitting them to the hub and/or [2] a teetering hub that will harmlessly dissipate the rotor's flex instead of absorbing it as the fixed hub now does.
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