Furling Calculations

[ChrisOlson]

Most people used to start off with a spring on the furling tail and then realised that gravity is more reliable and also has a constant rate.  You seem to be going in the opposite direction so maybe you know something the rest of us have missed.

Thanks for the comments, Hugh.  I do appreciate your insight as you probably have more experience with these machines than most of us put together.

I agree that gravity is very reliable being that nobody has floated off the earth lately.  But I would like to offer something for consideration.  It seems that many people have furling problems.  I doubt that any two machines are identical, even built from the same set of plans.  The specs might say a tail hinge angle of 18°.  But when it gets welded on it ends up at 20° - not perceptible to the human eye but it changes furling results considerably.  The angle of the tail boom - whether it be perpendicular to the yaw shaft, or slanted up or down, changes the furling result.  The tower can flex in the wind and change the relationship of the tail hinge angle and tail moment to gravity's force, and thereby change the furling result.  Two machines might be identical but one has a longer wire run than the other, changing the blade loading, and thereby changing the furling result.

There's many things that just simply cannot be factored in.  And I think that everyone here who has built several machines, and especially large ones, will agree that arriving at a concrete amount of tail weight, and being able say definitely that it's going to work as intended is more of a black art than a science.

That's why I'm pursing this new idea.  I want something that's easily adjustable where I can use all the available data to arrive at where it SHOULD furl, crank it off by at least a 25% safety cushion, then adjust it until it's right, a little bit at a time.  I think a mechanical test should be devised for the shop - like actually pulling on the rotor axis with a spring scale to simulate the expected thrust and making certain the tail folds up well before that point is reached - especially on big machines.  Maybe somebody with more experience with this than me could develop a chart of some sort that says, as an example, for a 17' rotor you should expect minimum "x" amount of thrust and if you pull on the rotor axis with the scale at "x" amount the tail's fully flying position should be overcome in the shop.

I think everyone would agree that it's quite satisfying to watch your machine furl and work like it's supposed to - even if it furls early.  It makes your blood pressure go up when it doesn't.

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Chris

[ChrisOlson]

Here's another photo of the new tail setup, taken from the rear.  I repainted the turbine head after cutting and welding the new tail assembly and mounting post for the spring cylinder base on it.  After the paint dries overnight I'll put the genny and prop back on it and put it back on the tower tomorrow afternoon.





If I must say so myself, this spring loaded outfit works very smooth.  Before I cut the old angled tail hinge off I measured the pull force to furl it with a spring scale and it was 31 lbs but it didn't furl at that setting with the new blades.  It furled properly before I changed the blades.

I set the new cylinder so it takes 21 lbs of pull on the rotor shaft to fold it up.  If I crank the adjusting screw on the cylinder tight it takes 34 lbs of pull to fold it up.  I'm going to fly it at 21 lbs first and see what happens.  If these new blades are wind seekers, like I suspect they might be, it might take a really low amount of spring pressure to get it to furl.

Typical, whenever I want to try something new the wind doesn't blow for days.  But as soon as I find out how it works in the real world I'll report on the results.  If this works like I expect, all three of my turbines, and the new 21, is getting this same setup.

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Chris

[RP]

I can't see what you have for stops.  If it's the spring tube assembly, will it hold up to being slammed into full furl or straight on?

[scoraigwind]

On turbines where I am fussy about the furling being just right I set it up with a cam on the top hinge (something wrapping around a cam) and a ball joint at the bottom of the tail hinge.  In the beginning I always did it like that, but then I began to realise that a simpler hinge was more reliable and worked well enough on the whole.

The big 'nirvana' machines that we have built over the last ten years or so I went back to this idea and we use a chain to hang the top of the tail.  As it furls the chain wraps around a cam.  In one case the cam has adjusters that move it to the side or backwards but in the other one I simply add pieces of steel and screw them onto a wooden block.  If I could figure out how to add photos to comments I would show one.  I put a new video on my youtTube here http://www.youtube.com/watch?v=PMD-Jh01890 but it doesn't show the tail hinge at all well.  It does show a couple of smaller machines furling nicely.

Mostly my smaller turbines are very easy to lower using tilt-up towers and so adding or removing weight would be easy but in reality I rarely do that because the furling usually works out OK for me somehow.  I agree it is a black art and it can go wrong although often it is due to the blade running too fast or the offset being too small or both of these factors that lead to the machine going crazy.

One comment that I do have on your spring idea is: have you checked the moment of the tail right round to fully furled?  It looks to me as if it will increase as the spring gets tighter.  In the very old days when I used springs I set them up so that the radius of the spring reduced as the tension increased.  That way I was able to get the right tail moment in all windspeeds. It's not hard to get a tail set  so that it starts to furl at the right power but to get the right power in strong winds as well is more of a challenge.  Looks to me as if yours might not furl right round very easily.  That's why the nirvana turbines are set up for adjustment of both the side and the back angle of the tail hinge arrangement.

[Flux]

My early machines had a spring to hold the tail at right angles and a rope passed down the centre of the tower with a weight on it. A pulley converted the vertical pull to one in a direction to hold the tail into the wind.

Furling was adjusted by changing the weight. The snag was that it needed some form of slip rings as the rope went down where the cables normally go.

I later changed to a cam type scheme which I presume is very similar to what Hugh described ( I haven't watched the video yet). This is easily adjustable and is really the only scheme where you have full control of the power out at various wind speeds. I had failures with chains and ended up with steel cables. These are a weak point but the failure takes a few years and if you change it every 3 or 4 years it works fine.

These machines have used a sliding sleeve round the yaw pivot to work a mechanical shut down by turning the tail at right angles.

I have built several with the inclined hinge and again with the sliding sleeve shut down and these have performed well enough but I have made the hinge angle adjustable and my tails are very light in comparison.

On the last machine I built that is running with a mppt converter I went back to the old scheme of a rope down the middle but this time I have modified the pulley system for the rope to pull effectively at constant torque on the tail. In many ways this has been the nicest machine but it has properly constructed and protected slip rings and was quite complex to build.

The earlier rope controlled versions were run with dc connections and did not use true slip rings. The centre connection is an insulated tube and the weight of the machine is carried by it, this makes perfect contact always. The other connection was carried via the yaw tubes and doesn't always make perfect contact even with an alternator hanging offset on it. I have recently done 2 small machines this way and added a brush to the earth connection and this has worked fine. I wanted three ac cables down the tower for the mppt machine hence the need for proper slip rings.

The one thing I have never used is a spring control although I have used a spring assist to increase furling speed on one of the cam machines rather than increase tail weight and turn it into a battering ram. That works fine and is easily adjustable but the spring linkage is such that it does have a virtually constant force rather than constant rate as you propose.

Under many conditions the increasing spring force with furl will work ok as many machines drop power greatly when full furled, this will reduce the drop off. For a few machines that tend to level out at constant power you may find it doesn't reduce power enough in high winds. Much will depend on the spring geometry and pre load.

As far as I can see all schemes work as long as the offset is above a critical value and the thing actually furls in the intended wind speed. This will be way beyond the point where the tails starts moving away from the stop ( I think this is where most fail as they consider this furling).

Nice to see new ideas but in the end it has to be trial and error. If the offset is enough it will work but you will have to experiment to get the maximum power limited to a safe value. Any thing that makes adjustment easier is a good move but I have always had to increase furling speed rather than lower it hence my worry about too steep a hinge angle and very heavy tails.

The machine I built with too little offset didn't furl and I tried many clever and complicated tricks to fool it but it never furled even with an auxiliary thruster fin forcing the tail upwards against its weight. It turned the tail but the prop headed straight into the wind.

Flux

[ChrisOlson]

One comment that I do have on your spring idea is: have you checked the moment of the tail right round to fully furled?  It looks to me as if it will increase as the spring gets tighter.

It does stay very constant thru the entire swing.  If you'll notice from the photos, rather than using one long spring I used four shorter ones.  It seems to simulate the action of an angled hinge quite well.  This machine has a right offset (looking from the front) and at rest the tail is about 5° to the left.  The tail moves thru the first 10° of movement (5° on either side of center) fairly freely, but will return to the "rest" position if you let go of it.

I tried this with a bathroom scale this morning instead of a spring scale, by placing the bathroom scale on the prop hub and pushing on it with the tail held stationary by a rope.  At about 20° of total movement it takes 21 lbs to push the rotor around, and at the peak fully furled position it takes 23 lbs.  So it only varies by 2 lbs, which is less than the old angled hinge setup I had on it before.  When I tested that with a pull-type spring scale it moved very freely up to about the same point and took 31 lbs to move it further, but it peaked at about 35-36 lbs to move from 45° or so to fully furled.

One thing that must be noted about my turbine designs is that I use a lot of forward "lead" from the offset bracket.  As my machines start to furl the moment of the rotor increases because the lever is longer as it gets turned out of the wind.  For instance, on this machine the offset is 8.5".  Fully furled, with the offset bracket parallel with the wind force, the effective offset is 14".

I used constant rate springs in this thing that I bought at a local hardware store.  Even though they call them "constant rate" I found that they do vary a tiny bit from full length to full compression.  I found that using four of them stacked up is more constant than using one long spring of the same rating and wire diameter.

I'm very pleased with how it works in the shop.  It's very smooth in operation and takes a constant, steady push to fully furl it on the test stand.  The cylinder is long enough so I'm only using a portion of the full travel of the springs.  So if I add preload to the springs by cranking the adjusting screw it raises the amount of push required to 34 lbs with a peak of 35.  The more I preload the springs, the more constant the required push is thru the full swing.

The trick here in the design is to not use the full travel of your springs.  You only use about 2/3's of the total travel.  I have 12" of travel capability in the cylinder with only 8" of actual shaft travel.  I wrote down all the details of this a bit ago and my adjusting screw can add or subtract 2.9" of preload.  So at full adjustment the amount of possible spring travel in the cylinder is 9.1".  With 8" of shaft travel there's still 1.1" of reserve in the springs.

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Chris

[fabricator]

Do you think you will need to incorporate a shaft seal to keep the guts from filling with water and rusting everything up? Or freezing up?

[ChrisOlson]

I believe it will.  The tail still uses a conventional hinge with a slotted pipe over a pipe.  So there's built-in stops there.  The cylinder is made of a piece of 1" Schedule 40 pipe and the rod is 1/2" cold rolled steel.  The cylinder end is a screw on 1" pipe cap that I machined on the lathe so it looks nice.  The piston inside the cylinder is made from two nuts with a washer between and the outside nut is ground so the spring pilots on it.

The furling stop is where the rod end hits the end of the cylinder.  Fully extended the piston hits the end of the cylinder inside at the same time that the slotted hinge hits its stop.

My rough guess is that you could slam it back and forth steady for hours and it'll shear the mounting points off before you'd wreck anything in the cylinder.

I used a spray-on graphite coating for lubricant both inside the cylinder and on the shaft.  I drilled a tiny hole in the bottom of the cylinder to let any water that might get in it out, and I made the mounting post on the base long enough so the cylinder is slightly slanted downwards.  This way, when it rains water won't enter where the shaft slides in and out and it will only get any water in it from the shaft being wet if it compresses.  And most of that will be wiped off by the brass bushing I pressed into the end cap where the rod slides in and out.

It's one tough unit.  I experiment with a lot of things and some of my things don't work and some do.  But rarely does anything I build ever break, regardless of whether or not it works.

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Chris

[ChrisOlson]

Read above, my reply to RP.  She won't freeze up, stick or bind.  It'll outlast the machine.

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Chris

[ChrisOlson]

I should perhaps explain this offset vs effective offset thing.

When I design my turbines I consider the offset to be the distance from the center of the yaw shaft to the centerline of the rotor shaft.  But because I use a long "lead" on the blade hub I use a different calculation that I call "effective offset".  The effective offset is the hypotenuse of the equilateral triangle formed between the distance from the centerline of the yaw shaft to the centerline of the rotor shaft, and the distance from the blade hub plane to the point perpendicular to the yaw shaft on the offset bracket.

So on this 8 footer, for instance, where I'm trying this spring cylinder, I have 8.5" of actual offset.  The blade hub is 14.1" forward of the yaw shaft.  When the rotor is straight into the wind the offset is 8.5".  When the rotor gets pushed out of the wind to the point where the angle of the blades to the wind is about 45°, it becomes 8.5^2 + 14.1^2 = effective offset^2 or:

effective offset is now 16.5"

This greatly aids in furling.  Once one of my machines furl they tend to stay there and run at partial power even if the wind dies down a bit.  And they come back into the wind very gently with none of the typical "snapping around" that many machines can exhibit.

The downside to this design is that it takes a longer tail boom to control it properly.  The other downside is that if you want to always extract maximum power from high winds, and don't mind abusing your equipment, this design is not the way to go.  Once it goes into furl it will usually limit the power to about half of what you could get with a conventional machine that aggressively steers itself back into the wind.

I've spent a lot of time getting machines to furl properly in the first place, and spent a lot of time playing with this forward "lead" to make them stay furled once they get there.  This is the primary reason I went to my thru-shaft design as opposed to using trailer hubs and spindles.  All I have to do is change the length of the shaft to move the blade hub further forward.  After much trial and error I've settled on a design that roughly doubles the offset at that point where the rotor is running at around 45° to the wind and it has worked very well for me.

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Chris

[ChrisOlson]

The effective offset is the hypotenuse of the equilateral triangle formed

What a misprint......

Not an equilateral triangle - I meant to say right triangle.

It's warming up outside.  Need to get my 8'er back up on the tower with this new tail on it.........

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Chris

[Dave B]

 To help put things into perspective as far as furling the larger machines with the tilt tail.

 Just as a simple example for a 20' diameter machine : If you wanted to furl this machine (I'll simplify things even more and say (start to furl) at 22 MPH wind speed. A considered good turbine might have an output of say 6000 Watts. Do we realize that just 1 MPH wind speed higher and we are playing with about +1000 Watts more output ?

  This is about the TOTAL output we are able to play with for a 10' machine from 0 - 25MPH. We are talking about 1 MPH more and 1000 WATTS.

  Is it a surprise that there are issues with mechanically trying to tweak thousands of watts with a couple MPH wind change ?

  Dave B.

[ChrisOlson]

Dave, that sums it up better than any explanation I've seen yet.  And it's why I'm so concerned about developing a good furling control system for my big machine.

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Chris

[Dave B]

Thanks Chris,

  I applaud your efforts for doing stuff and then posting your photos and results besides.

  There are a lot of opinions and suggestions of how to do things from those who may not have the resources and or back ground for actual "hands on". There also seems to be a few out there who have done and tried it all with regards to furling but unfortunately we see very few if any photos of what worked or did not.

  So, keep on keepin on. It's obvious to the "doers" that 20' is not just a big 10'. I'm sure I'm not the only one who looks forward to your progress reports and photos. It's refreshing for sure from all the "you shouda, you coulda and especially the I woulda" done it this way talk only.

 Dave B.  

 

[TomW]

Dave B;

Well said! I think it explains quite well what I was trying to understand.

There is a reason the "Big Boys" steer theirs mechanically and use braking and/or pitching arrangements.

Wind Power does not scale linearly [swept area/speed] and so the machines' dimensions are not likely to.

I suspect there is some upper size limit to the common furling arrangement seen here. You guys may be on that frontier. Good Luck.

Thanks for the explanation.

Tom

[fabricator]

Dave B, you put into words what had been nagging me for some time, when you go from a 17' to a 20' you almost double the power output, but we want to still use the exact same simple furling mechanism with its linear action, how can we expect a linear furling mechanism to control power output that is not linear?

[ChrisOlson]

As usual, I put my test turbine back up on the tower yesterday and now the wind won't blow.  The forecast is for light winds < 10 mph for the foreseeable future.  She's up there turning, barely putting out 18-20 watts, but it's definitely not going to test my new furling control cylinder at that wind speed.

According to all the data I've reviewed, input from you guys with formulas and data, and using some in-shop testing along with applying some gut instinct, my test machine SHOULD start turning itself out of the wind at about 350 watts with this new tail setup.  It should start limiting power slightly above that and be fully furled, overcoming the 21 lbs of spring pressure I have the cylinder set at, and running in stall/limited power at around 400 watts.

It has proven it can easily put out 800 watts if it stays in the wind but that's a bit much for the generator.  I designed it for 750 watts continuous with peak to 1.1 kW for a short time, and spikes to 1.5 kW for a few seconds.  If it actually hits 1.5 kW it'll blow the fuses on the rectifiers.  Although I haven't seen it yet with these new blades, I have no doubt it will exceed 1 kW at wind speeds in the high 20's.  The generator is this machine is pretty stout for a 8 footer.  But it's only 12 volt (it runs my shop office computers, lights and coffee maker) so the rest of the system won't take high outputs because of the amps.

When I see it do what I expect, I'll crank the adjusting screw a few turns and see what it does then.  So I'm going at this from a different standpoint than usual - I don't really care if it furls at a specific wind speed.  My goal is to build a compact and simple system that has a maximum power adjusting screw.

To use an analogy, for any of you guys who have ever worked on diesel engines with Bosch injection pumps, this would be called the "smoke screw" (technically the fuel rate setting that limits the max rack travel) in the injection pump.  It has a rated power setting - turn the "smoke screw" and you get more power and smoke.  I hate to call it a "smoke screw" on a wind turbine, but I know if I turned it all the way in, based on my shop testing, I would get smoke - and a lot of power - for awhile.

Now I just have to wait for the wind.......

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Chris

[don1]

Hey Chris,

 I like your idea of a compression spring over a tension spring. pull type springs tend to break where as compression springs hold up well.

 Case in point. Valve springs in engines seldom ever fail. garage door springs another story. pull springs tend to wear through the hooks or break near the bend transitioning from coil to hook.

  What would happen if a spring failed? Would it destroy the machine? Or just furl?

[ChrisOlson]

If a spring fails it'll just make it furl sooner.

Your analysis of the spring situation is why I used compression springs.  A stretch spring would be hanging out in the open exposed to the weather, the hooks would eventually wear thru the brackets, and it would break.

Still haven't gotten enough wind to see what speed it furls at now.  It reached 16 mph today and that was enough to make the tail go from 5° on the anti-furl side (where the stop is set) to about 10° on the furled side.  But it still had the turbine pretty much steered right into the wind at 16 mph.

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Chris

[fabricator]

The suspense is killing me.

[bob golding]

if it works one could have a motorised  drive on the adjuster screw and adjust it on the fly. would be a lot simpler than  trying to vary the pitch of the blades. you certainly seem to have improved on the old idea of  having the spring on the outside. i am really hoping it works.

[SparWeb]

Hi Chris,

Boy I'm sorry I missed this thread until now.  Been away for a week at a little sports thingy in Vancouver...  

I don't think I can add to the advice and experience shared by Flux and Hugh so far, and rather late to boot, so I'll just harp on this comment here (taken a little out of context):

"If I can work out what I need for a starting point for spring pressure on the smaller one, hopefully I can scale it up to the big one.  The key here is knowing EXACTLY what the rotor thrust is, fully loaded at the point where it's supposed to furl."

I hope that the ensuing suggestions from Flux and Hugh dissuaded you from this point of view?  Watching my little machine from the ground, in the lee of a row of trees at times, the thing dances around all the time.  Any notion of an exact amount of force goes by the wayside when considering the magnitude of fluctuations.

I haven't been able to read through everything (just got back from the trip, and need sleep more than I need to catch up on my life).  I don't remember right now if you are talking about a 17' machine or something bigger, but there is a point IMHO that the furling tail should not be used and a more controlled furling system is "more economical" to employ.  I'm suggesting that a 20' diameter rotor flinging itself against a steel stop is going to bend something out of whack no matter how much 1/2" plate you weld to it.  

Did you receive my spreadsheet by e-mail okay?

I don't remember what formula I use for thrust in it.

[ChrisOlson]

Yes, thanks Steven, I got the interactive spreadsheet but haven't had time to play with it yet.

Of course, since I built a new furling control system the wind refuses to blow anymore.  The turbine has been running but it seems the weather is rock stable and we get no howlers to try the new system out.

I think I've got a good design here, and I'm trying it on a small enough machine so that if there is a problem the disaster isn't as big.  It's not a new idea - spring loaded furling tails have been used on the past.  I just made one with enclosed springs to be weather proof, and easily adjustable.

I'm still waiting for the wind......

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Chris

[ChrisOlson]

Finally got a few gusts of wind around 20 mph this morning and my furling spring cylinder works!  I was doing some paperwork in my shop office and happened to glance at the ammeter on the turbine and it was hovering right around 28 amps.  I looked at the anemometer meter and it said 20 mph.  It should put out more than that at 20 mph.

So I ran outside and looked - it was running partially furled!  I watched it for a bit and it works perfectly - so perfect and smooth it's like an amp regulator.  When I noticed the ammeter at 28 amps it wasn't doing the normal varying up and down - it was staying very steady at that rate.

I ran back into my office to grab the video camera to take a movie of it working so you guys could see it.  Don't you know, but the time I got back out to the tower the wind had died off and it hasn't done it again (yet).  This is the first time we've had a wind this strong in a couple weeks - most days my turbines have been just coasting on their towers, running just below cut-in.

I got the spring pressure in it set at 21 lbs, which is obviously a bit low.  But I'm going to leave it there for now so I can catch a movie of it furling the next time it happens.

When I get the data for the exact spring pressure I use to get it to 600-750 watts I'll post it in this thread so if somebody else wants to try building one of these things they got something to go on.  Right now I know I got 21 lbs of spring pressure, the tail is 84" from the hinge to the tip of the feather, it has 8.5" offset with 8' PowerMax blades - and it furls at 400 watts with that combination.

When I built it I figured out that it doesn't really matter where you attach the spring cylinder rod to the tail boom - as long as the spring cylinder applies its pressure to the same offset point (8.5" on this one) as the rotor axis is pushing on, it will work.  And neither does it really matter how long the tail boom is, or how heavy it is, because it swings in a plane perpendicular to the yaw shaft and doesn't "climb the hill" against gravity like it would on an angled tail hinge.

As I learn more I'll post it here.  Right now I'm pretty happy that it works!

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Chris