Index (in reverse order so the latest stuff is at the top)Tower DesignBlade Carving Blade Design References Back to the Overview Tower Design, 4 June, 2006Having mounted the rotor on a short test stand, I have been confronted with the effects of turbulence on windmills. Observations so far: 1. They ain't kidding, it's gusty down here at ground level. 2. Gusts impede the start-up of a wind mill by suddenly stalling or unloading blades that haven't sped up yet. 3. Gusts cause high yawing stresses that seem to be a major load factor on towers. This link is a PDF of an analysis of the peak yaw load that was applied to this rotor during a brief period on the test stand. Yawing is the motion the windmill makes as it turns into the wind. When it's turning slowly, sudden yawing interrupts the wind passing over the blades. When the mill is turning quickly, it's like a gyroscope, and forcing it to turn left or right puts a load on the tower in a up/down direction. This "Yaw Moment" can be very large if a large windmill is hit by a gust of wind from a different direction. Blade Carving, 30 March, 2006To start, I grabbed any ol' piece of 2x6 from the woodshed. Not a single piece could I find without knots every 18 inches or so. I decided at that point that I would be "in school" and whatever I make is simply for the value of making it and if I want to produce something useful, I'll have to find better wood. With that behind me, I set about teaching myself how to shape and cut a blade with the taper, twist and airfoil shape that I wanted. Naturally, I made some mistakes my first time out. For anyone who might be planning to carve a blade like I did, I recommend the following general order of events: 1. Cut the taper. Leave 1/16-1/8" extra material to shave off. The cut edge is the Trailing Edge. 2. Cut the thickness. Leave 1/16" extra material to shave off. The cut face is the back of the blade. It will get the most curvature. 3. Carve the front face. This will define the twist. This face is pretty flat, depending on the airfoil you chose. 4. Carve the back face LAST. This defines both the airfoil shape and thickness at the same time. It requires the most care and if you save it for last there's no more fiddling around to do that would upset the balance. After each step, measure how much you've left at several places along the blade to make sure both blades are the same. I've attached some photos of my first blade to show what I did, and also illustrate problems to avoid. 
 You can see that I've already carved the front-face twist before cutting the thickness. Blade Design, 30 March, 2006As I came to the end of the alternator design, it became obvious that the math would point to specific sizes only if the limitations of the rotor were known. I read up on wind-mill propeller design, and what a surprise! I thought I knew a bunch about propellers, because airplanes use them - "How different could they be?". It turns out that the power density of a wind turbine is about 100 times less than an aircraft. Aircraft props, designed to pull an airplane against its drag load, with tips travelling just under the speed of sound, are vastly over-designed for a wind-mill. Worse, the blade's airfoil profile is backwards, and you can't fix it by just bolting it on the other way. Aircraft propeller design charts typically don't dare to extrapolate their data into the range where wind turbine blades would operate. To put it in aeronautical terms, the finest-pitched climb prop is still too coarse for a windmill, by a factor of two. Voilą: a case where two nearly identical devices, operating in the same environment, but for different purposes, must be designed in different ways. Due to the relative infancy of wind-turbine technology (at least compared to airplanes) the data available for designing wind-mill props is much less rigorous than that used by aeronautical engineers for propulsion types. References:
Updated 30 March, 2006 Created 30 March, 2006, Steven Fahey |
