Research Update: MOTHER's Blue Max Homemade Windplant

MOTHER's Blue Max homemade windplant update shows how we put a little "backbone" between the blade skins and made a good thing even better.


| July/August 1985



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Dennis checks the rigidity of the finished Blue Max blade.


PHOTO: MOTHER EARTH NEWS STAFF

MOTHER's Blue Max homemade windplant update shows how we put a little "backbone" between the blade skins and made a good thing even better. 

Those readers interested in small-scale alternative energy projects have no doubt seen the 350-watt homebuilt windplant we featured in MOTHER EARTH NEWS issue 93. In an effort to develop that machine further, we've been experimenting with a few modifications to the homemade windplant . . . 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 inch by 36 inch by 72 inch piece of polystyrene beadboard, left over from one of our car body—building projects, and cut it into three 3 inch by 11-7/8 inch by 69-1/4 inch 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 inch length of 2 by 8 with .040 inch-thick sheet-aluminum templates fastened to the ends . . . each drilled and marked, with stations set 3/4 inch apart on the shorter piece, and 1 inch apart on the longer one (see our diagram for details).





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