Research Update: The Blue Max
July/August 1985
By the Mother Earth News editors
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STAFF PHOTOS
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We put a little "backbone" between the blade skins and made a good thing even better.
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Those readers interested in small-scale alternative energy projects have no doubt seen the 350-watt homebuilt windplant we featured in issue 93. In an effort to develop that machine further, we've been experimenting with a few modifications . . . and can now report on an improvement we've made to the blades detailed in the article.
We initially went with double-skinned sailwing blades because they're lighter and easier to fabricate than carved wood or internally supported aluminum wings . . . and they still offer an acceptable degree of efficiency. But—as we discovered when faced with a spell of very breezy weather—wind velocities of 25 miles per hour or so (the range of maximum power output) produced an undesirable fabric flutter in the skins, partly as a result of the blade tips "ballooning" under the force of high RPMs, and in some measure because the blades were reacting to sudden gusts by adjusting their pitch for slowdown.
Our solution was merely to take the slack out of the blade covering, not by stretching it tighter, but by stuffing each hollow wing with a carefully cut section of expanded polystyrene insulation board. This not only gave each blade more integrity (while exacting only a 9-ounce weight penalty) but, more important, provided the airfoil with a definite shape, one calculated to improve aerodynamic lift.
The equipment used to make the modification was, for the most part, homemade. After we'd rescued a 3" X 36" X 72" piece of polystyrene beadboard, left over from one of our car body—building projects, and cut it into three 3" X 11-7/8" X 69-1/4" sections on the table saw, all we needed was a bench jig and a large hot-wire cutter to complete the job.
The jig was simply a 69-1/4" length of 2 X 8 with .040"-thick sheet-aluminum templates fastened to the ends . . . each drilled and marked, with stations set 3/4" apart on the shorter piece, and 1" apart on the longer one (see our diagram for details).
The hot-wire cutter, on the other hand, looks like a huge bucksaw but works like a cheese cutter . . . except for the fact that its stainlesssteel wire is actually a resistive element heated with a variable transformer. We made the cutter frame from one 73" and two 32" lengths of 1 X 2, two 1/4" X 2-1/2" carriage bolts, some baling wire, a 1/2" X 18" screen-door spring, two No. 8 X 3/4" eye screws, and a 15' length of 16gauge household electrical cord.
With the frame assembled, it takes about 8' of .040" stainless wire—stretched through slots cut into the ends of the wooden arms—to reach from one eye screw to its opposite mate. The cutting wire is wrapped to itself at the eyes, and one 16-gauge conductor is similarly connected to each wrapped joint to complete the circuit. Under no circumstances should the plug be inserted directly into a wall socket . . . the tool must be fed through a 5-amp transformer that delivers no more than 15% or 20% of standard (110 VAC) household voltage. (We used a variable model—a Powerstat 21—that's manufactured by the Superior Electrical Company, 97 Lee Ave., Bristol, CT 06010.)
