Wind Turbine Mast Design

I'll let someone with a better grasp of statics and dynamics fill in the gorey details, but I suspect you are actually creating a problem by breaking the response to load in the middle. You are actually mixing techniques, either of which has good purpose, but not sure I am comfortable with them combined this way.

The gantry you describe initially, if the wires then return to the base of the pole ads great rigidity. Cables from the top to the ground (guys or stays) greatly add resistance.

Breaking the techniques leads me to suspect you are concentrating loads at the center rather than distributing them.

G.A ! FROM ME, EDIGNAN HE IS CREATING A MUCH BIGGER PROBLEM !!!

He doesn't want the wires horizontal because they would contact the blades of his turbine.

I do not propose this method, I make reference to a similar device where vertical guys are used to add rigidity. Yes, they do drive the compressive force up, but pipe is extremely strong in compression as opposed to bending moments.

Note the stays on the crane arms.

But to then break this at the widest point and try to carry the load wide to the ground is just going to concentrate the load at the center - to my humble brain.

Hi Miik,

For cost and use of materials, the lattice is hard to beat, which is why it was the standard for farm windmills. However, its small staying base means the the foundation must be very sturdy (and designed so the the entire foundation is not levered out of the ground).

As you say, using diamond stays will help keep the mast in column. The arrangement you proposed at first does not do well in this respect: If you draw this up, (without the diamond stay) and look at it from the side, you can imagine one stay in tension and the other going slack. The one in tension tends toward straightness, pushing the spreader to leeward, and the slack stay permits this movement. In sailboats, this problem is solved in two ways. In the very simplest, least sophisticated boats, the mast section is made larger, stiffer, and heavier. In the next step up, "lower" stays are used, which go from deck level to the point on the mast near where the spreaders are mounted. This arrangement could be made to work well in your case.

If the base is one meter wide, (and the mast 10 meters high) then the leverage pulling the anchors upward will be 20:1. The compressive load in the mast will be the same, so you would want to make sure that the underground structure ties together the mast base and the stay anchor points very rigidly -- with this small a staying base, relying the ground itself would probably not be wise. (If you've sailed, you might have been surprised at how slack the leeward shrouds (stays) become. A small part of this is due to wire rope stretch; a larger part is due to the entire boat structure flexing, particularly when the mast is deck stepped, rather stepped on the keel, and/or when the keel is not sufficiently triangulated with the "chainplates" (the points at which the stays are anchored).

The stayed mast shown on the page linked above shows an interesting staying arrangement in which the "spreader" works in tension, permitting the stays to spread out after clearing the blade tips. Seems like a reasonable approach.

If you go to a large boat yard with a wide range of boats, you will see staying arrangements ranging from very simple (just three wires, straight to the masthead, with no intermediate support to prevent the mast from bending -- and therefore a fairly robust mast section) to some quite complicated systems with spreaders at several heights, and various arrangements for inducing mast bend (which can be used to control sail shape). In the three wire setup, you may be surprised to see that the staying point, when looked at from the side, seems alarmingly close to the mast, perhaps only half a meter back from the mast base. This is required to allow the boom to swing out reasonably far with the wind aft. The saving grace here is that the very large forces in sailing are from side to side, rather than fore and aft: thus you see loads of capsizes to the side but few "endos" or "pitch-poles".)

If you do go to a boat yard, bear in mind that the distribution of forces is much different than for a windmill mast. The center of effort of the mainsail is well below typical spreader level, and the sail track transfers a lot of side load to the mast. On many simple boats, the mainsheet (the ropes connecting the boom to the deck) serves as another staying point. A spinnaker, on the other hand, applies much of its load near the masthead. Backstays (when present) often do not meet the mast directly across from the forestay: the forestay is perhaps 7/8 as high, so that tensioning the backstay can (deliberately) bend the mast back at the masthead and forward down lower. So don't take things too literally... but any time spent at a boatyard is time well spent. Time spent sailing is even better spent: the god's do not subtract, from a man's allotted time, days spent sailing.

Take a walk down to your local marina, and see how a sailboat mast is rigged. Notice how the shrouds (guy wires) are attached, near the top (not at the top), the top to base ratio of the spreaders (not carved in stone) and the type of fittings used. You might even find enough of a damaged spar to use for your project. I would reccomend aluminum for the mast, or a wooden box section from 1"x4" spruce, stainless for the rigging, tangs, and fittings. If you can't sgrounge a mast section, try Al electrial pipe. the fittings should (must) be SS fastened with screws or bolts using anti corrosion thread lock or epoxy. Weldments are prone to failure due to vibration. A hinged base or tabernacle with a gin pole will facilitate raising and lowering for service. I've got one of these systems holding a small wind generator, and another holding 203 sq. feet of sail (not counting the spinnaqker), on a 2 1/2" X 3" mast section, 27' tall. The whole magilla weighs about 31#. You guys that like to do the math on wind speeds and pressure can appreciate the forces. NONE of which should be transmited laterally to the mast.

Have fun with your project!

Carl

There are books on rigging, the larger glossy marine catalogs have diagrams, but the best bet is find a sailor with rigging experience.

Sorry guys, that was me, #6. I just spent the day doing maintanece on 3 laptops and forgot to sign in again. FYI the signature is from 'Aliens'. Love that Sigorney!

not enough info

the wind load of a fan is approx 60% of it's area

you have to know the max load you may face

do a wind survey, and check local NOAA archives

do you have a feathering system?

there are commercial towers available, at least check out their designs

there is a lot of liability involved in a wind machine.

your maximum tip speed should be about 7X wind speed

You could incline the post and so be able to have vertical guys at the front where you have the rotating disc. This is assuming that the dominant wind direction is used for the fixed orientation of the mast.

The spreader in the middle has 3 effects

1. Breaks up the unsupported span, reducing the risk of failure by compressive buckling.

2. Penalty is a concentrated force at mid span, which can cause failure by local crushing of the pipe wall (applicable to thin wall pipes).

3. Distributes the load into the guy wires, at the expense of increased compressive load, but removing the bending load on the pipe. As pipes are usually strong in compression and fail by buckling or bending, this enables it to take a far higher load than it otherwise could.

Dropping the lines vertically after the spreader has the practical benefit of enabling machinery to get in around the base for mowing etc without running in to angled wires.

Disadvantage is that the pull on the wires is vertically up (a straight uplift), requiring heavier anchorage foundations, than needed for an angled pull (you don't get as much help from the dirt around the anchor)

The effective guy wires you wind up with is the same as taking a line from the base of the wire to the top of the mast.

This angle often winds up disappointingly steep, giving high loads in your guy wires.

Basically, the wider you can get the base, the more effective your guy wires and the lighter the required wires for the same loads.

Of course you don't need to use a disc at the half way point. As suggested earlier, the spreaders used on yacht masts are fairly simple "posts" held against the mast. These posts are loaded axially in compression.

The object of the entire system is to eliminate bending and make all forces operate axially, either compression (mast and spreaders) or tension (guy wires).

A slender mast can be designed to reach quite high this way.

I saw a radio mast which started out with 2"" pipe, into which was nested the next size down ending at 1/2"etc at 240'. Guy wires at suitable intervals enabled this to remain operational in high gale force winds. (They chickened out at NW Cape and designed the system to be laid flat when a cyclone was coming, but in theory they could have made it strong enough to remain in use even in 140mph winds).

I would say a better anchor point would be 2.5 metres from the top to prevent flexation.

Especially if you have moving mass at the top.

It seems that the motive for vertical guys is lack of space to spread them out, the same motive sailors have. However, if there is insufficient space for guy wires, is there sufficient space for blade clearance and possible accidents? Quite apart from building codes, prudence would suggest placing the tower so that a possible structural failure of the tower or the turbine does not result in damage to valuable nearby objects. Perhaps an unbraced truss or tapered (fabricated) tube would be a better solution. Try to avoid exposed trusses (or discs or spreader bars) which will attract birds to perch on them. Neighbors are likely to object to chopped birds being flung about by the turbine. You also want to avoid a structure which will audibly "moan" in the wind, vibrating wires or shedding vortices, unless, of course, there are no people nearby to hear.

What a lot of aggravation for a Windmill!! High cost too if you need to buy the parts......

I am hoping that I can show you a few ideas to help simplify your work, reduce costs and to make far less noise downwind. (Have you ever heard a HAWT from downwind? The higher it is, the farther the noise reaches. Here where I live, they can only be erected when more than 3 kms away from living accommodation!!Due to the speed of the propeller tips, it can get very uncomfortable for neighbours, even if you can ignore it!!)

You appear to be building a HAWT, so think about a VAWT, simpler to build and easier to brace the tower (a tower is not always required for a VAWT and you can build a second one (or third or fourth...) close to without problems.....

They are not quite as efficient maybe, but chasing efficiency for that last %, when the energy itself is free is often a waste of time and material. Build 2 cheap ones for the price of one expensive one and have more output eventually.....

To sum up VAWTs need not be so high, reducing costs and simplifying designs.....

Here are some links for you to peruse...I hope they give you food for thought....have a great time building, no matter what your design eventually is.....

http://www.youtube.com/watch?v=98aCW1aTGu8&feature=related

http://www.youtube.com/watch?v=Vyvf7iyi-wM&feature=related

http://www.youtube.com/watch?v=9UPe6A_UVPc

I have done some design on a cable supported column that was re-enforced with with wire rope cables that were anchored close to the base of the column. I found that unless there was significant preload in the cables, the column would fail in bending. The cables simply had to much stretch to prevent the column. The column failed before the cables were loaded enought to stop the deflection of the column. You should do a deflection anaylsis on the cables and column to ensure the cables will take any of the load from column.

Consider the cross sectional area of the cable with respect to the cross section area of the column nver mind the fact that the modulus of elesticity of the cable is approx. 13.5e6, less thatn half that of solid cross-section.

Aloha milk999 from MAui, the land of wind turbine power.

We have several systems here and I think the one I am about to mention for your research is the best. It is perfectly suited for small areas and generates enough power to get you what you need. They are contained so disregard problems with blades hitting people. They fit in a 6 foot square enclosure. This photo shows how we installed a gang of 6 at the Maui Ocean Aquarium last year. They work great and the noise isn't all that bad. You would need to have some footings welded to mount them, but that would be no problem. California wind turbine maker AeroVironment Inc. made them for us. They cost about $50,000 for all six, not including shipping from California. Tell them how you found out! Stevan Holt from Island Door Supply in Maui. Hope this helps....Be sure to read newspaper article link after photo.

Read this link to a newspaper article that talks all about them...

http://www.mauinews.com/page/content.detail/id/508800.html