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Table of Contents
- Introduction
- Parts list:
- Layout Drawings
- Description:
- Turntable Assembly
- Wire Sizes
- Alternators
- Drawing for Alternator Adaptor Bracket
- Drawing, Wireing for Chrysler, Ford and GM alternators
- Drawing, Bottom View
- Drawing, Full Scale Layout (drill centers) Part C2
- Makeing 3 Blade Impeller
- Balancing the Blade
- Drawing, Blade Layout, Foil template
- Pulley Alignment
- Adjustments, First Run Tune-up
- Tower, Tilt Mast and Tabernacle
- Adjustments, Attitude
- Windspeed-Power Chart
- Lead Acid Battery Care
- Troubleshooting
- Site Selection, Gathering Windspeed Data
- Force Beaufort Scale
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design source USDA
1939 6v. Wind Plant
Introduction
The plans can be printed from your computer. The page numbers on the part list are off because I've been updating sporaically since 1980. They also have drag-n-drop printable templates that can be pasted to the material for locating drill centers.
Part numbers listed in a description, such as E-13 or T-5, are referenced in the Parts list. Which, with the smaller version of the Layout Drawings, can be opened and printed separately. You'll need this list in hand when you read the drawings. The part numbering code is: E = Electrical, F = Frame, C = Feathering Carriage, T = Turntable, B = Blade, and = Drawing.
Sometimes my source links go out of date because suppliers change their catalogs and don't tell me. If you reach a dead link, please google it, that's what I do. And tell me if you find a good one.
I'm unfamiliar with standard metric sizes for lumber, pipe, and fasteners. Please round the decimals up or down to fit the occasion.
As with any internet poll, these plans rate from excellent to incomplete. Descriptions are generally self evident with the actual part at hand. If you find something is obscure (or faulty links), please let me know. a
Just to be clear: simplicity and repairability are features, not bugs. They're designed that way on purpose.
Bill Cornelius
Turntable Assembly Procedure
1. Solder the wires (E-l and E-2) to the inside of the two sections of copper tube (T-8 and T-9), which are the commutator rings. The wires may connect to either tube, but remember which is which, so the mast wires don't get connected to the wrong terminal. To solder them without melting the wire insulation, strip about 3/4" (1.9 cm) of insulation off the end of the wire and form the wire to the right shape so it'll fit flush against the inside of the tube. The insulation should extend inside the tube a way but not touch it. "Tin" (melt some solder on) the ends of the wire and also a spot on the inside of the tube. Then just lay the bare tinned wire inside the tube and apply enough heat to the outside of the tube to melt the solder and it's done. Be sure the wire insulation extends inside the tube so the circuits don't short against each other. I use a propane torch, but it can be done on a stove.
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2. Prepare the copper tube commutator rings to fit with the plastic pipe couplers used for insulators (T-10) by bevelling the edges with a file. Clean the outside of the commutators and the inside of the insulators with sandpaper and see that they fit smoothly. The short commutator tube goes on the bottom end and the wires all come out the top end. See right, also Exploded View D-1, in the lower left. Prepare the end of the one inch pipe (T-3) with a rat-tail file. Remove any burr and smooth down the seam that sometimes sticks up as a ridge on the inside of the pipe. The actual dimensions of these pipes vary from l" to 1-1/8" (2.54 - 2.86 cm) usually closer to 1-1/16" (2.7 cm) . If you should have to buy another insulator to replace this one, they come in various outside dimensions depending on the mood of the manufacturer, so measure them before you buy and, if you don't find it at first, look around. They cost about 25 cents (in 1980). You'll only need half of a coupling for this insulator (cut across the diameter), and as the coupling is tapered slightly toward the ends, start the small end. Coat the inside of the pipe and the outside of the insulator with a formica adhesive (any brand) and drive it in with a few blows of a sledge hammer and a block of wood to prevent breaking the edges. The preparation with the file will let them fit easily. Let the glue dry for a few hours. Meanwhile assemble the other two copper commutator tubes the same way. Take care though that they don't touch each other in the middle of the insulator (that's what insulators are for), Measure and mark them and put a piece of cardboard or garden hose washer in between them if necessary. The insulators that come with the kits have a ridge inside in the middle, but be careful anyway. While all this glue is drying, you can read a book or hoe your garden or do dishes or something, but don't drink no beer because you'll end up with a lawn ornament instead of a wind generator.3. After the glue is set, glue these two subassemblies together, using the same technique. To get the right gap for the commutator brush to fit between the ends of the insulators, clip one over the copper tube before you press the tube into the insulator. Stop driveing when you have just a little more than 1/16" (0.16 cm) space on either side of the brush. Now, while the glue is wet, quickly eyeball the commutator tubes so that they're centered on the same axis as the pipe, if they're a little bit off it won't hurt. In fact, if they're quite a bit off it won't hurt, so long as the insulators don't rub against the frame. Its easy to bend them straight before the glue sets.
4. The commutator brushes (E-6) transfer electricity from the windmill to the battery without twisting the wires on the tower when the wind changes direction, they are made from .015 gauge (0.318 mm) automotive brass shim. They contact parts (T8) and (T9). Here's a drawing to get them the right shape:
Dragonfly Wind Generator
Sunday, May 1, 2022
DIY Plans

5. This tube guides the feathering cable through the wooden frame and anchors a vinyl tube inside the turntable. The vinyl tube should be pushed over the inside end of this 2-1/2 inch piece of 3/8" copper tube (T-5) that seals the hole under the pulley (with a dab of calk). Install it just before bolting the pipe flange to the frame because the copper may accidentally get bent if it sticks out while you're working on other parts. The plastic vinyl tube (T-6) should be inserted from below. It's there to contain any water coming in through the hole for the manual feathering cable. It also prevents the bare feathering cable from shorting any circuits of the commutator assembly. (See drawing for the fancy details). The end is split with tin snips, formed in a copper tube flaring tool, and hammered flat on top. If you don't have a flaring tool, it's not critical, just make several cuts with tin snips and fold the tabs out like a star, but see that two of them go under the cheeks of the awning pulley where it touches to hold the tube solid. The hole should be open enough to allow the feathering cable to pass through without chafing the copper.
6. If you have to replace the feathering pulley (F-8), use one without an aluminum roller, the feathering cable saws them in half.
7. Here's a drawing of the outer turntable sleeve (T-4): Thread this hole* for ground lug screw.
*Thread to
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Part T4
This rectangular hole may be cut by drilling a series of 3/16 (0.476 cm) holes on 1/4 inch (0.635 cm) centers. Knock out the connecting metal with a cold chisel and file off the sharp edges. The cross cut is easiest with a hack saw. Remove the dotted area for systems with two field wires.
The round hole on the right end is to attach the ground (negative) wire with a threaded lug. Like this: --->
Another way is to slip the bare end of the wire under the straps that clamp the pipe to the masthead, which looks crude but works fine.
Alternators
Automotive alternators are amp rated from about 30 to 75 amps. I recommend those from about 35 to 45 amps. The bigger alternators need too much wind before their output exceeds what is required to charge their own field, so they won't produce at all in lower winds. The smaller alternators will turn on in lower winds, but they have less output. If the wind never gets over 10 MPH (4.47 m/sec), there's not much power there anyhow but a motorcycle alternator might work. Below 7 MPH (3.13 m/sec), it's not worth the effort.
There are two basic types of alternator mounts. The one shown on the assembly drawings is simpler. It's used by Chrysler, Prestolite and Datsun (in 1980). The others will need an extra bracket to adapt it to the Dragonfly frame, like the one shown here.
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on Delco-Remy
37 Amp Carriage
Bolt

Template Figure
one Figure
two Figure
three All Holes C-2 C-3 C-3 C-9
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Ford alternators need to be trimmed about one inch to fit this adaptor. The size of the bolt and swivel pin for mounting different kinds of alternators varies between 3/8, 7/16, and 1/2 inch (0.95, 1.1, and 1.27 cm).
Always use the voltage regulator that goes with the alternator you get. Ford and Datsun have a very good voltage regulator with some extra wires to turn other things on and off, and the ignition wire will also tolerate a panel light to act as a malfunction indicator. The best place to mount the voltage regulator is up on the machine, because: (1) it can dissipate its heat there without effect. (2) The mercury switch must be between the V. reg. and the battery. (3) Temporary disconnections in the heavy wire circuit while charging will fry the regulator, and the big ground wire from the alternator passing through the turntable is a possible point of faulty connection (through corrosion or vibration). If the regulator is grounded between the turntable brush and the alternator, it eliminates that particular hazard.
The Ford type won't turn on until the wind is blowing 15+ mph (6.7+ m/sec). Once it's on though, it will stay on, self-energized, until the wind speed drops below 7 mph. I wouldn't recommend them if your sites average windspeed is below 16 (6.7+ m/sec) or doesn't have frequent gusts above that . An important modification would be to cut drainage holes in the back of some alternator cases. When feathered, some types collect water which may freeze and split the case.
A solid state battery isolator is handy because it will allow you to heat water and charge batteries at the same time and it will protect the battery from shorts in the wind generator circuits, or, if your alternator isn't self exciting, you may want a separate battery for the wind generator so there will always be enough charge for the field even if your kid leaves the wood shed lights on all weekend. Be sure you get a heavy one (Sears or auto parts stores).
Get a drive pulley appropriate to the size alternator for RPM's required. A three-blade, 9 foot (2.743 m) diameter blade can use 12 inch (30.48 cm) diameter for Fords and Delcos, and about 10 inch (25.4 cm) for Chrysler, but there are so many hundreds of varieties of alternators, all I can recommend is to try a few. If the amp meter needle shows too little output in high winds, try a larger pulley. If the needle jumps and the blade slows down a lot when the field comes on in low winds, try a smaller pulley. The blade should slow down some when the field comes on, but if it goes from a hummingbird to a crow, the blade looses efficiency.
If you need to determine the blade rpms to select a better pulley, place your ear against the mast or tower while watching the second hand of a clock. You'll notice rhythmic vibrations that correspond to one blade rotation (though perhaps mixed with a series of lesser vibrations). Count the number of rotations in 5 or 10 seconds, (like drum beats per measure) and multiply that by 12 (for a 5 sec. interval), or 6 (for a 10 sec. interval) to get RPMs. Big pulleys with large alternators produce lower RPMs.
Bottom View 
Wire routing
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After 10 yrs of feedback, the most common falure of this machine is from vibration fatigue of the field and switch wire terminals (2 reports). So the updated plan is to shrink wrap the wires and secure them to the frame to damp any vibration.
Note Whenever the Dragonfly is feathered, the field is on. If you plan to leave it that way for a few days, turn off the field below / or put on an additional mercury switch up there to do it for you.
Wireing for Chrysler, Ford and GM
These alternators are 1975 to 1985 models, more recient ones usually have built in voltage regulators and fewer connections to worry about. The wire that goes to the cars ignition is the field control, so that one takes the on-off and mercury switch.
Mercury
Switch Mercury Switch Mercury Switch 5 Amp fuse Amp Meter ON - OFF Toggle Switch Tower commutator brushes field (Ignition) wire field (Ignition) wire Double Throw Momentary Switch Solid State Battery Isolator Voltage Regulator Voltmeter Field Battery House Battery
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Wire Sizes

Total Length of Wire in Feet from Source of Current to Most Distant Fixture on Circuit.
Heavy wire costs more but it might save money in the long run by reducing the size of the battery bank which will wear out before the wires do. Various wire gauge conversions. *************************************
Layout (drill centers) for part C2
Making A Three Blade Impeller
A three blade impeller is more efficient than a four blade, because it has one less trailing edge, so less drag. (Two blades are better but they'll always vibrate at a wind speed that is determined by their over-all weight and how it's distributed). A graphite 2 blade, with a lead hub would work for our speeds. One blade with a counter balance is best, but I object to the high tech. N American, milled wood is sold by unmilled dimensions, for example 2x4 lumber is planed smooth while green to 1.6 x 3.6 inches and then shrinks to around 1.5 x 3.5 inches as it dries. The wood for our blades should be dry and near to these dry dimensions, so metric conversion of 5x10 cm is probably closer to 3.81x 8.89 cm dried. I designed the blade to be cut from that standard size lumber, but don't know if it's the same everywhere (the world is big). Select three straight-grain 2x4's x 4 -1/2 feet (5x10 cm x 1.37m) long of fir, spruce, redwood or some local wood with smooth texture. Place these boards flat on a table and mark as follows. From the left end (this is the tip), mark as shown below. Then measure up from the bottom (the back or downwind side) and mark a point at each of these intervals along the edge nearest you as shown in Fig. 3. Blade Drawing
Tip
End Tip Tip Lay out Leading Edge Lay out Trailing Edge Leading Edge Trailing Edge Root Root Root
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Connect all these measured points with a line. This is the leading edge. Then measure up along the back edge and mark and connect those points.. Fig. 2 shows how the twist taper is supposed to look. When the wood above this line is removed, it will reveal a dandy twist taper with about a 14 to 1 slope at the tip, and about a 6 to 1 slope at the root. The twist is made that way because the tip of the blade is traveling faster than other points nearer the hub. Design-wise, it is more efficient to have an equal wind pressure along the whole length of the blade, so the faster moving points are cut with a shallower pitch and the slower points with a steeper pitch. To remove that wood, use a handsaw to cut through the top side down as far as the marked lines on each edge, but not through them.. Make a series of parallel cuts about 3 inches apart, and with a chisel remove the wood between the cuts. Take care not to split into the wood below the cuts, (watch the grain) . Then with a Shureform or plane, smooth these surfaces FLAT from edge to edge. Flip the board over and on the back side, round off the foil shape with a hand plane. Fig. 2 can be traced onto a piece of cardboard or thin metal to be used as a template.
Root
Tip
Tip
Foil Template
Leading Edge
Trailing Edge
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The trailing edge should be about 1/16th inch (0.15 cm) thick. One-half the thickness of all layout lines should be visible when you're finished. If you take off too much or nick it with the chisel, the holes can be filled with epoxy body filler or wood dough and sanded smooth.
MAKING KIT BLADES: All you have to do is plane a bit off the upwind side to the line of the leading edge, and about 1/16th inch (0.15 cm) thick for the trailing edge, or as thin as you dare. Keep the upwind face flat from edge to edge. Don't get fancy and invent a special airfoil on the spur of the moment, remember NASA uses a computer and birds have taken 50 million years. The layout lines should all be visible when you're through. Trim the root ends so they fit together at the center on the hub. The flange bolts should pass through their ends so there'll be support to tighten the carriage bolts (B-11) without warping the hub plate. After the pitch and foil are sanded and painted, protect the leading edge with a strip of stainless steel tape (these used to be available from Napa Auto Supply, but now I can only find them through JC Whitney, whom I would not recommend under any other circumstances, if they offer you something of "equal quality", you should decline.) held on with nonferrous tacks or staples about 5/16th (0.79 cm) inch long. Thin copper flashing works well also. Glue the flashing with formica adhesive. The metal tape is to keep dust and weather from raising the wood grain, which would spoil the airfoil. Aluminum tape works but will need replacement occasionally because it corrodes. There's also a window sealing product made of thin copper foil backed with tarred paper. It works if you can peel of the paper and wipe off the tar with solvent. Needs to be cut into 2 inch wide strips. Measure the tapes all the same length (about 3 feet, or 1 meter) long and about 13 inches (33 cm) from the hub. Stick them to the flat side first and fold them over the leading edge and onto the back side by slicking them lengthwise, folding about 1/8th inch (0.3 cm) at a time with a piece of smooth wood. To prevent wrinkles, slick from the middle towards the ends. With a gun, put 5/16 (0.8 cm) staples about 1 inch (2.6 cm) apart along the edges to hold the edges down and prevent wrinkles when the blades flex. Staples are usually unnecessary with aluminum tape. The next step is mounting the blades on the hub: Lay out a 9 ft. (2.743 m) diameter circle on the floor with chalk. Still using the same radius (4-1/2 feet / 1.3715 meters), mark off six equal radius length points around the perimeter. lt should come out even. If it didn't, you muffed it, so lay it out again. Pythagoreas or Aristotle claimed it was geometric proof of the order of the universe that the arc of a circle will subdivide into six equal parts using the radius as one unit of measure, (but lately it seems to have become a matter of opinion). When it comes out close to even, erase every other point and there should be three equally-spaced points around the circumference. Lay the blade tip centerline on those points, up-wind (flat) side down, foil (curved) side up. The hub is made of a triangular piece of 3/4th inch (1.9 cm) plywood (B-8) and a triangular piece of 18 or 20-gauge galvanized tin (B-9). They're both cut to the same pattern.
Drill a 1- 1/8th inch (2.86 cm) diameter hole in the center of the tin. A sharp cold chisel on a hard surface does an acceptable job but ragged edges should be filed.
Slide this piece of tin under the blades at the hub and line up the hole on the center of the 9 ft. (2.743 m) circle. Place part (B-8) plywood triangle on the top (downwind) of the blade root ends, and line it up with the tin on the bottom. Hold it in place with your knee and drill five holes (3/16 inch or 0.476 cm. dia.) in each root end through all sandwich pieces (mind the floor!) and bolt them solid with 3-1/2 inch #10 (8.98 x M8x1.25) RH (Round Head) machine screws. Stainless are best, if unavailable use more or heavier bolts, 1/4 inch (8.98 x M11x1.5) maybe. Measure to be sure they are equally spread, then line up a pulley flange on the upwind side shaft hole and drill 5/16 (0.79 cm) + holes through the tin and all for the 3 flange bolts (B-11). Drill a one or 1-1/8th inch (2.857 cm) hole for the shaft to go through. Run wood sealer or penetrating epoxy into all these holes until they are well soaked, otherwise the bolts may work loose with changes in temperature and humidity.
The balancing Gadget is made from Next, whip up some kind of pulley mount, depending on what kind of pulley you use. I'm sold on aluminum "V" belts because they are cheap and handy almost everywhere. Some people like cog belts or chain drive, I think they're too much hassle, noisy, heavy, and expensive. I've used iron pulleys but they rust and the corrosion chews up the belt. Wood might be good if the surface is hardened with Polyethylene Glycol, or some of those chemicals used for turning thin wooden bowls. Don't use epoxy because it makes the belt crack. If the wood shrinks across the grain, the pulley will get out of round and make the blade vibrate from the uneven belt tension. Another advantage to the aluminum pulleys is that the spokes are usually identical between sizes from a given manufacturer, so they can be interchanged in a second to try out the best ratio for your alternator, if you cut your pulley mount to fit the spokes. A spacer block (B-2) should be placed between the rear hub board and the pulley mount (B-3) so there will be some clearance between the blades and the alternator. Drill oversize holes for the bearing mounting bolts (B-11), through the spacer block/mount (B-2 and 3) to allow for alignment adjustment I recommend the Google products search, because they list several sources to compare prices, buyer reviews, and shipping cost. Their picture shows the pulley you want. Do a primary center alignment of the pulley on the bearing when all the holes are drilled but before attaching the 3 blades to the hub: Assemble the hub as shown above, maybe using short 2x4 scraps in place of the blades, but don't bother to secure them. Temporarily put a piece of pipe through the bearing shaft holes and tap the pipe with a hammer while restraining the pulley with your free hand. Measure from the edge of the pulley to the pipe or bearing. When it's centered, secure it in place with some drywall screws. The pulley mount and spacer block (B-2) should be screwed to the rear hub board and NOT GLUED to it, so that it can be tapped into center alignment once the blade and belt are mounted. If the pulley is off center slightly it will cause uneven tension on the fan belt and the blade will appear to be vibrating even though it's well-balanced. To center it, loosen the bearing and flange bolts then tap the pulley with the heel of your hand until the belt tension is equal throughout a complete revolution. If the shaft (C-1) was off center in the hub boards (B-8 and 9), tapping will force the shaft against the inside of the hole (that goes through the hub boards) before it reaches alignment. If that happened, it would stop moving even if you pound on it, so you'd have to 1) re-align the shaft to allow clearance, and possibly drill bigger holes in the spacer block, 2) re-align the pulley block, 3) rebalance. Don't hit the pulley with a hammer, they're zinc or pot metal and deform easily. Then tighten down the flange bolts and the pulley mount screws, but not enough to deform the front hub plate triangle. I've not had a problem here, but f you feel the bolts (B-11) need to be set more than 3 ft lbs (15 Kg/M2), use a small backup plate to spread the pressure. Moving the bearing too far in this adjustment may push the blade out of balance.Seal it all and paint with floor enamel or topside paint. A primer coat wouldn't hurt either, you can put a latex topcoat over an oil base primer or sealer because the primer is thin, but latex on oil paint will crack. Generally, the Belt Sizes are; For 8 inch (20.3 cm) pulley, 3/8 X 35 inch (0.95 x 88.9 cm) outside diameter "V" belt; for 10 inch (25.4 cm), 3/8 X 41 inch (0.95 x 104.14 cm) OD diameter "V" belt; for 12 inch (30.48 cm), 3/8 X 48 inch (0.95 x 121.9 cm) OD "V" belt. But set it up and measure to be sure. BALANCING THE BLADE The blade must be assembled with bearing, flanges, pulley, bolts, paint, and the leading edges covered before balancing. If it's put on afterwards, it'll affect the balance. Suspend the blade from the balancing gadget shown in Fig. 4.
If the dull tube cutter method seems too esoteric, cut it through at that location and solder a copper pipe connecter to another short piece of 3/4 tube and just push them together so they can be disconnected, the idea is to align the other bearing in a straight line. Don't remove the burr on that cut. If the tube is inserted too far, it will not be sensitive enough, not far enough and it will never balance. 2.5 inches should be about right. You'll have to remove the upwind bearing to remove the 2 piece inside copper tube before the blade is mounted on the axle.
First Run Tune Up ADJUSTING FOR THE FIRST RUN When sliding the blade onto the shaft, loosen the carriage bolts (part B-6) temporarily so that both bearings will line up more nearly along the same axis. The shaft is a piece of galvanized water pipe and there may be small fluctuations in diameter from batch to batch so if the lock cams on the bearings slide all the way around and won't grip against the pipe, a small piece of shim can be fabricated from a strip of tin. Don't use aluminum shim because it's malability makes it wear loose and the bearings will wear out faster. Roll the shim and insert it between the bearing assembly and the shaft. A better fitting shaft can be made from a stock 7/8 inch (2.22 cm) diameter cold roll, round iron bar. I don't use that because it corrodes, is heavy, cost more and is less available. Both cams should be rotated the opposite direction the blade turns and tapped lightly to wedge them snug. Then tighten the set screw and give the cams and flanges two coats of rust inhibitor paint. Make a hub cap out of a light weight tin, stainless, or aluminum salad bowl. Measure the bowls' diameter and scribe a circle on the hub plate. Secure it with 3 or more evenly spaced sheetmetal screws through the bowls rim. SET THE DRIVE BELT loose, about 3/4 inch (2 cm) play, with plenty of belt grip, (available at auto supply, or Heating and Air Conditioning stores). It seems common sense to set it tight, but with the belt stiff and new, it will cause so much friction that the machine will need about 20 -30 mph (9 to 13 m/sec) wind to work, and it won't generate according to the graph. Also the bearings might be stiff, so leave it up for a day or two before allowing yourself to be claimed by suspicions. A slipping drive belt causes the ammeter needle to stop consistantly at a given amperage even though the wind gusts are much higher. Snug the belt up a little, apply some belt dressing, and check the Troubleshooting chart. If the ammeter needle jumps from zero to 5 or 10 amps: the angle of the mercury switch is too steep. Lower the machine or climb up there and loosen the nut that holds the mercury switch clamp, and tip the switch into a slightly more horizontal position. If you have a handy fence post, you can mount the machine temporarily on that (without the blade). Tap the screw eye (C-15) forward or back, or loosen the 2 U-bolts holding the alternator pin (C-7), and adjust it so there's about 1/8 inch (0.3 cm) of free space before the spring (C-12) engages to restrain feathering. Rock the feathering carriage back and forth and adjust the switch to turn of and on within that space (you'll need a volt-ohm meter or troubleshooting light). When it's properly adjusted, the needle will climb from zero without a sudden jump start. If the switch is adjusted past its optimum, the needle will show a dip to the discharge side of zero before it starts to charge. It should never show a discharge.
Tilt Mast and Tabernacle not to scale ![]()
Attitude Adjustments The best way to insure your wind machines' continued to service is: Don't think energy dependence or independence is anything absolute or it will crunch you. To set doom and eternity riding on the back of technology is foolish. Why trade dependance on a technological monster for dependance on a technological elf? What I really mean is: laugh and stay loose, diversify, invest, plant early, have kids, you already knew it.
Power Chart
LEAD ACID BATTERY CARE
If you use a 12-volt car battery, don't let the voltage get below about 10 volts. Mount a volt meter right next to your light switch and test it whenever it feels appropriate. Used car batteries are sometimes available from wrecking yards for about 10% of the new price guaranteed to work, but not for as long as new ones. Deep Cycle batteries are a better choice, but they are more expensive. They can be run down to 3 or 4 volts without too much harm. Used ones are available from golf cart maintenance companies and paraplegic supplies (electric wheel chairs), possibly also from electric fork lift maintenance companies. They are usually returned when they are eight months old and exchanged. The used ones may be good for several years and cost very little, but they may also fail quickly in exotic ways. Get the opinion of the dealer. If they contain lead/zinc they will last longer but their charge will be slowly tapering off. If they contain lead/antimony or lead/calcium, they can take repeated quick-charges, (if you expect frequent hurricanes), but fail suddenly. The choice is: bang or whimper. A better battery is nickle-iron, they last 50 years but are big and expensive. A large new set of Deep Cycle, lead-acid could cost more than $1,000 at 1980 prices, God knows what they'll cost by the time you read this, but I don't advise you to throw your money around like that. A little care will keep you from having to replace them as soon as the warranty runs out. To make them last:
Or here's the Definitive
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