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Old 01-11-11, 08:54 PM   #29
nibs
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Sorry I do not get on the interweb much in the winter, as I have to go to a hotspot. We used to call a blade with no "twist" a drag machine, as the wind hits it and turns the blade, the classic example is the American Agricultural water pump, with its blades that almost fill the circle, it will turn relatively slowly but quite relentlessly. If you are building a constant pitch blade, the more you fill the circle with muliple blades with a low aspect ratio the better.
If you want a blade that flies you need washout or twist.
My 4' blade was carved from a single 2x4 first drill a center hole to mount it, then about 2 inches out cut on the leading edge draw a diagonal line from the upwind side to within 3/8's of an inch of the downwind side. This will be the leading edge, grind or cut the wood off using the trailing edge (other edge of the 2X4) as a guide leave about 1/8" of wood at the trailing edge. Round the leading edge by eye so that the whole looks like an airplane wing, use a protractor every inch to keep left side = right side. this will give you a pretty good jumping off point to experiment from, keep it balanced by sliding it on to a shiny metal screwdriver shank. When I get home in the spring, I will try and post a drawing of the blades I used to produce.
Yesterday I saw a turbine blade made from what looked like 4" white plastic water pipe, it was cut in such a way that it had washout, and was turning in a modest breeze. It looked kind of easy once you cut the tube lengthwise in the right shape, each blade was bolted to the hub with two 1/4"(+/-) bolts, once you got the design right, you could easily make a three bladed prop.
cheers and sorry for being gone for the winter.
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