16 Footer fails to furl

[Hilltopgrange]

Hi Guys we have had some strong winds lately with 50-70mph gusts. I normally shut my turbines down before a storm hits rather than risk damage. This morning at 4.30am the alarm on my weather station alerted me to high winds, I got up from my nice snug bed and trudged down to the workshop in the lashing rain and shut the turbines down.

Well to the point, this afternoon it calmed down a bit so I started up the 16 footer within 20 minutes there was a sudden heavy gust of 53mph and the current jumped to 96 amps almost 5kw way to much, I ran to the window and was able to see the blades where pointing straight into the wind with the tail completely folded to the furled position. The blades stayed into the wind for about 2 minutes until the gust passed and then as the wind dropped it yawed out of the wind. I was then able to shut it down and go and change my trousers, That 2 minutes felt more like a lifetime!

so I think it is safe to say that it failed to furl, I think it has survived but only just.

I have previously watched it many times furl 90 deg to the wind at about 25-30 mph, I have a streamer mounted behind the blades to show wind direction relative to the blades. Normally when it furls power peaks at about 60A 3kw for a few seconds and then drops off to 45-50A 2.5kw so I am positive that it did furl, not so sure now though.

The machine is 16 feet dia, 48v with an offset of 11.5 inches (6%) the full build can be see here

Part one http://www.fieldlines.com/story/2009/5/23/135436/243

Part two http://www.fieldlines.com/story/2009/6/21/22551/6284

Part three http://www.fieldlines.com/story/2009/7/20/11923/2353

I am putting this down to the mysterious "prop seeking" phenomenon we hear so much about and is probably the cause of many burnouts.  

I went to a lot of trouble back in September to reduce the weight of the tail with aluminium and fabric sails, more detail here

http://www.fieldlines.com/story/2009/9/13/131418/233

So where do I go from here I hear you ask? And to be honest I don't know. I am now thinking the traditional furling tail design is not as reliable as many seem to think. What other options are there?

Ideally I would like to build a variable pitch hub but that's not  going to be easy without a lot of machine work with machines I don't have! I have been searching past stories about variable pitch control but not a lot of working examples to go by plus a lot of the past posts are missing their pics or I get that darned upgrading message.

Other Ideas that come to mind are tilt up furling but is it going to be any better another thought was to rebuild completely but as a down wind machine with coning blades similar to Proven, not an easy option either.

This machine was built around a Land rover stub axle and hub

The stub axle originally has a half shaft running though it that fits into splines on the front plate see pic above.

 I was thinking that I could fit a shortened half shaft through the alternator and out the back through a bearing to drive via gears a hydraulic pump (closed loop) max speed could then  be controlled with a flow valve triggered from a speed sensor on the shaft or pressure switch in the line. Might work or might not I need to do a lot more research that's for sure.

As always I am open to any and all suggestions or ideas

Sorry for the long post and I hope you can follow my ramblings.

Russell

[RP]

"the blades where pointing straight into the wind with the tail completely folded to the furled position."

If the blades were pointing straight into the wind then why was the tail pointing sideways to the wind?  I have to question whether it was actually furled.  Could it be that the streamer behind the blades was being affected by the disturbance of the blades?

[bj]

Russell--first off, glad it survived.  Even if some tightening etc is required, you still have a turbine to work with.

Your post, and many others on the same subject, have me convinced that the

Dan's ten footer I am building won't fly without some kind of mechanical

braking.  What form is undecided at this point.

Anyway , thanks, as I am no longer undecided.

[bob golding]

i think this is what happened to me last year. i wasn't so lucky and had a major failure with the stator burning out and the expanded stator ripping the magnets off. i went for a stronger design with heavier wire and well encapsulated magnets. i still have the same furling system and haven't had any problems. i cant offer a cure other than a smaller prop, and lots of luck. as i said in another thread i did have a phase go when i couldn't get the turbine down so it was over speeding in a 80 mph wind,but seems to have survived ok. if the magnets are well secured the occasional overspeeding incident should be survivable. i think it is better to let it overspeed rather than try to stop it by shorting the stator. it will slow down sometime.

bob

[Hilltopgrange]

If it had of actually furled as it is ment to the output would have dropped off. Normally the tail stays in line with the wind direction and the blades yaw out of the wind.

Russell

[Hilltopgrange]

Hi bj unfortunately I don't think a manual brake would have helped as it didn't run away as such, it was still under load it just failed to furl. The normal shut down method worked fine once the gust died down. A manual brake is handy though to make sure it stays shut down.

Thanks for your comment

Russell

[Hilltopgrange]

Hi Bob Ive had a few runaways in the past lol not something I want to repeat anytime

Soon  and something you will never forget! It was pure fluke that I was on hand to witness it at least I now know what it didn't do! I totally agree about not shorting it out at 5kw the shock load would cause it to explode and I don't think my switch could handle it either. I think its back to the drawing board for a better solution, needless to say I wont be leaving it unattended untill I know it is sorted.

Russell

[Volvo farmer]

I think it is better to let it overspeed rather than try to stop it by shorting the stator. it will slow down sometime.

I would agree with that. I'm pretty sure I burned up my stator by shorting in in high winds. It was running fast and furious but no rubbing noises till after trying to short it out.

[Ungrounded Lightning Rod]

Not necessarily.

If the tail was folded up the rotor was (largely) turned away from the wind.  (If it were still pointing straight into the wind your tail vane would still be flying straight back, no matter what your telltail streamer said.  So you were "furled" (for some value of furled).  The furling system was doing SOME of what you wanted.

But being furled does NOT necessarily mean that the prop isn't spinning and isn't pulling power - even a LOT of power - from the wind.

I suggest that the furled tail stop is not in the right position to have the rotor flying at a no-power angle.

And I suggest the following procedure.  Perform this on a very calm but not windless day - when you can brake the mill down to a gentle crawl by shorting it but the wind is enough to yaw the mill - perhaps on a low test stand so you can tweak it easily, but with a structure that mimics the top of your tower:

1. Tie the tail into the fully-furled position.

   
   

2. Let the rotor fly at roughly right angles to the wind.

   
   

3. Unload the genny and see whether it spins forward or backward.

   
   

4. Short the genny and adjust the tail stop - more folded if the rotor was going forward, more unfolded if it was going backward.

   
   

5. Loop to 3).  You're done when you can manually yaw it in both directions and the wind sets it spinning, then when you let go it yaws back and stops on its own.

   
   

6. Remount it, hook it up, untie the tail, and put it back into service.  Then see how that does in the next big storm.

   
   
   

I figure this should adjust the tail stop so the mill will be nearly stopped when fully furled in virtually any wind.  (High winds might peel some vortices off the tower that will spin it a bit, but not a lot.)

Ideally it should furl to something powered but safely under the max sustained power output in any wind below that which will just knock down the tower.  But if you go for that you want to approach it from the too-slow rather than the too-fast side.

[Ungrounded Lightning Rod]

The main problems from unloaded overspeeding are having something pulled off or broken by centrifugal "force" (quickly leading to imbalance and shake-apart or other catastrophie), being suddenly yawed enough that the gyro forces bends the blades or tilts the rotor until a blade hits the tower or the rotor hits the stator (ditto), or having the tail blown into the blades.

[bzrqmy]

This is what I observe with my 10 footer.  The blades stay pointed into the wind, but the tail does fold, sometimes fully, I have watched over 1800 watts pump out of it.  My next plan of action is to reduce the tail pivot angle.  I would like to see this thing furl way earlier.  Has anyone figured out a cushioned short circuit yet?  It seems like we could devise a circuit that counts RPM, when the threshold is hit, a series of decreasing resistnace loads engages until finally short circuit.

The root of the problem may be that the blades are no longer running in stall and the gyroscopic forces are allowing the tail to fold, but these forces keep the turbine pointed into the wind.  I am just speculating and have no evidence that this is really happening.

Would changing the direction of rotation make any difference?

[Dave B]

 If your machine acted as you explained then I would be very careful about the ultimate "change your pants" action also. This being starting up while completely shorted then picking up enough speed to fly right through stall and finally overspeeed. If your machine started flying with it shorted it sounds as if your tail could fold right up and the machine would just continue to accelerate. I am curious, looking at the front of your blades from up wind which direction do they turn and what side of the yaw bearing is your rotor ? (offset)

Dave B.  

[Hilltopgrange]

Hi ULR  If I had not witnessed this myself I would have been inclined to agree with you and RP. When I say straight into the wind it may have been yawed a few degrees bur defiantly no more than 10 degrees its not that easy to judge accurately from the ground and I certainly wasn't going to climb up there lol. From where I was watching I can see all 3 turbines and the weather station anemometer as well as the streamers the 10ft and 12ft where both fully furled and about 90deg to the wind according to the streamers but the 16ft refused to turn until the wind start to drop it then yawed out of the wind then the tail dropped and it yawed back into the wind again. Strange I know but that's what I saw.

I don't think I will be making any adjustment to it  just yet, I will keep a close watch on it and the weather while I decide my next move. To be honest I cant see me sticking with this system it is just to unreliable and problematic I think servo yaw may be a better option and just do away with offset and swinging tails

 All together. I am in a high wind area and gales are pretty much a weekly occurrence so I need to be sure these machines will protect themselves.

Thanks for the input.

Russell

[Hilltopgrange]

Hi Dave, When I do short it out I always wait for a lull rather than risk the trousers!

It has survived over 90mph winds while shorted even just shorting 2 phases will hold it. The offset is to the left and rotation is clockwise. It furls perfectly in winds upto 35mph. It was a very sudden and very violent gust perhaps it just happened to quick for the furling to work, probably never know the right way of it, the more I think about it the less I like this system.

Russell

[Dave B]

Russell,

 Very strange indeed. I looked back at your photos and previous discussions and saw your rotation / offset relationship after asking. I hope you get things worked out that you can rely on for your location. It certainly sounds as if you have more wind than most. Keep us posted with any new ideas you might try.  

Dave B

[Hilltopgrange]

I would think that reducing the tail angle would have a similar effect to reducing tail weight. I tried different directions of rotation and offset sides on earlier turbines and could never see any difference but it might help and is probably worth a try.

Thanks for the input!

Russell

[Volvo farmer]

Ahh. Perhaps I am drawing the wrong conclusions. Mine was fully loaded. It was just scaring me so I decided to shut it down.  Maybe I will never know the true reason that I burned up a stator, but I will continue to read here. Maybe I will gain more understanding someday.

[Flux]

This is a bit worrying, I don't have any experience of these larger machines and I am not sure that ideas that have worked on smaller ones can be scaled up indefinitely.

There have been obvious cases of machines not furling but in this case you have had it furling under normal conditions correctly. I have never met a case where a machine that normally furls properly has got away, but in very high winds a 16 ft machine has the potential to develop enormous power and holding it down to 3kW is not a trivial issue.

I presume that it got caught out during a gust and the prop seeking force took over. Once in overspeed the seeking force would be very large.

I tend to agree with you that furling leaves a lot to be desired on these large machines but Bergey get away with it but they use different blades and the type of blade does seem to have a lot of influence on this seeking force.

From all previous experience on smaller machines I would have thought your 6% offset was adequate but all I can suggest is that you increase it. If the offset is large enough it can't stay upwind held by the seeking force but what is safe is an unknown.

You must be close indeed so perhaps 8% would make absolutely certain. These large offsets cause other problems steering it directly into the wind under normal conditions but that is a minor issue compared with this one.

If you want to try blade pitch then the easiest one to try is the Jacobs scheme using the cf force on the blades to slide them out against a spring. The mechanism is easier to make than the other schemes and your blades are a similar size to the original Jacobs. I always had doubts about fixing the blades to the pitch mechanism but Jacobs don't seem to have done anything very special so perhaps this is not the big issue that I feared. Even so it is a significant engineering task and will need some decent welding and machining.

For your high wind site I would suggest trying more offset on your present machine at least for the time being or mount a small vane to the alternator bracket that is at right angles to the prop axis, the thrust on this will do exactly the same thing as the offset does and you can play with the size, it won't need to be very large as you are almost there now. This was the originals form of this furling scheme before people realised that the vane could be made redundant by offsetting the alternator.

I know how you feel , this wind seeking thing is absolutely frightening on a 8ft machine, I can only imagine what it is like on a 16footer.

Flux

[HaroldCR]

 Building the governored Hub is not THAT big of a deal.

  I used a piece of 7" round X 5" thick,solid steel drop from a machine shop. Had it center drilled for 1¼" and the back side I turned down to 2" so it was 2½" long.  Had the machine shop set up and drill 3-1" holes at 120° on the edge of the 7" surface. Makes a 3 spoked wheel. I turned a taper in that center hole that was fitted to a chair leg that was tapered. I hammered 12" lengths of well frozen solid bar into the 3 drilled holes in the hub. Then I pinned them with roll pins, to be sure they didn't fly out.

  I'm NOT a machinist. I just do whatever it takes to get where I need to go.

  I made a "Spider" from a piece of mechanical tubing that I trued up on my small lathe, so it would slip nicely over the 2" part of the hub. This will be for the "Governing" legs. I drilled and tapped ¾" solid bar for installing 90° swivels on the ends, that I bought from a place in Pa. that built Wind Machines of this style. I welded the 3 pieces to the ring that slips over the 2" section. The Spider legs were 2" longer than the width of the hub. MAKE SURE you match the legs to exactly parallel to the 3 legs on the hub.

  There is your completed Hub.

  Center drilling the blades was achieved by using a jig I built from wood, and fastened down on my work bench. I took a length of 1" thinwall conduit, and turned it down on the outside, on my lathe, for clearance of sawdust. I first drilled a 1-1/8 hole in the end of each blade about 2" deep. using a spade bit I cut down to 1-1/8" wide. Then, I cut a slot in the conduit, to fit the spade bit into, and brazed it fast, being sure to NOT get any brass on the outside of the tube. It should be just a tiny bit shy of the width of the spade bit. Then I proceeded to drill the holes in each blade to 14" depth.

  If Y'all want more info, I will be doing this again, next trip I make to Florida, where my Lathe is located. Might be in the next month or so. I'm also going to use 3/8" Heim Joints I just received. Those 90° swivels broke when a Tornado passed and the machine threw the blades into the brush.

  Meanwhile, look for Jacobs Wind Generators on Google. There should be photos, somewhere. Look for a rig with springs mounted on the face of the blades facing the wind. That's where I got the idea, from a book in the Library.

[dlenox]

Russell,

Sorry for your issues, this all sounds way to familiar with my 17'er.

Dan Lenox

[ChrisOlson]

I think that furling is not as reliable on larger rotors as it is on smaller ones.  Furling, in order to work properly, assumes that the force on the rotor always acts at its center and that the offset of the mainshaft from the yaw bearing can overcome gyroscopic forces generated by the spinning rotor.

The wind is also not a predictable thing.  Many aircraft have crashed because of wind shear and microbursts suddenly overwhelming the airfoil's ability to create enough lift to keep the aircraft airborne.  When you start talking windspeeds above 50 mph, wind shear is a very real phenomenon.  And the amount of power generated by a 16 foot rotor at 50+, facing into the wind when a powerful gust hits, is no trivial thing.

The gyroscopic forces on a large rotor are huge when it approaches overspeed.  I could envision situations where a powerful and sustained side gust hits the turbine, furling the tail, and the main force of the side gust is applied to the yaw shaft side of the rotor instead of at its center, totally overwhelming the torque effect of the mainshaft offset on the yaw shaft.  Add in the gyro force, and the blade circle's resistance to turning because of the gyro effect, and you could get exactly what Russell described.

Personally, I think that any machine larger than 12 feet or so should be incorporating some other method than furling to improve high wind survival odds.

--

Chris

[TomW]

This is a very interesting thread.

What I have gleaned from it is that the furling system on these turbines simply does not scale up very well 1:1.

I suspect it is that cube factor of wind power that does it.

The forces involved are not linear and any "fudge factors" that just work at 10 feet will be out of range at 17, etc.

Does that make sense?

I think of it as the "reason" in a nutshell.

Curious how my 12 footer will fair in the furling game?

Thanks to everyone who posted on the subject.

Tom

[ghurd]

Could it be something more simple?

"the blades where pointing straight into the wind with the tail completely folded to the furled position."

Meaning the tail was not only being subjected to 50MPH winds straight into the surface area, but also holding up its own weight?

What are the chances it was just the forces came together at the right time and angles,

causing the hinge simply bind itself up?

Possibly there was enough twisting force to make the lower part of the outside hinge bind to the pin?

Like the other side from where the paint is off in this pic,

http://www.otherpower.com/images/scimages/7813/7.JPG

Just a thought.

G-

[TomW]

[Hilltopgrange]

Hi Flux, I think ultimately I will be moving away from the swinging tail arrangement altogether and will probably try a servo motor yaw system but that will require some major mods and a lot of research.

In the meantime I very much like your suggestion of an extra vane to assist the offset it would be a simple and easy mod that I could try in situ with out to much effort, the more I think about it the better I like it!  

Thanks for your time!

Russell

[Hilltopgrange]

Hi Harold, Thanks for the info, I do have some machining facilities ie a very old lathe but it is very old possibly Victorian as it was originally line driven, it is about as accurate as an angle grinder lol. My son works in a small factory that makes drums (Adante),the type you hit with sticks not the oil containers lol they have all the latest cnc mills etc so machining shouldn't be a problem. I just need to get a good design sorted.

Thanks again for your time

Russell

[Hilltopgrange]

Hi Chris I totally agree with you the forces involved at this size are not to be underestimated . I think flux as usual has suggested a simple fix and I will be trying it out asap,  or as soon as the wind drops to a workable level.

Thanks for your input.

Russell

[bob golding]

been there done that. had the blades hit the tower with 9 inches of clearance, which was clearly not enough. same with the tail. the clearances you need to be really safe are a lot more than you would think. i have also had the tail lift off as well. i now have the tail secured with a long bolt.havent yet lost a blade to gyroscopic forces. i put this down to the fact that the blades are going so fast there are overcoming the turning moment,but i have no intention of getting anywhere near a overspeeding turbine to test that theory.

i think as regards servo control a friend of mine has a 10 kw turbine that uses it, it recalculates the position every 15 minutes, that seems to work ok, but it is more to go wrong, and wouldnt help if it did overspeed. my solution would be to fit smaller blades in the windy season to protect the stator,and use the bigger blades when the wind is not so  high. i am in a coastal location with high winds in the winter and seem to have found a compromise with  a 10 foot design with 9 foot blades. i might not be squeezing the maximum power out ,but it does seem to be holding together ok so far.

bob

[Hilltopgrange]

Hi Dan it just goes to show even if are sure it is going to furl that given the right conditions Murphy is just waiting to hit you a slap in the face. I was convinced this machine was well sorted as I had watched it furl many times. The extra vane as flux has suggested  may be a good option to try on yours, I will post an update when  get it fitted.

Thanks for the comment.

Russell

[Hilltopgrange]

Hi Tom it was very interesting when it happened though I would rather not repeat it if at all possible, I think it was the longest 2 minutes of my life to date. I have had a couple of runaways in the past but they where 6-10 footers and scary enough , however 16ft is a whole new ball game and has to be experienced to be fully appreciated . In this case it never lost load so it didn't run away as such but I very nearly did.

I don't have logging software as of yet but it would have been interesting to just see what rpm it was spinning at. I didn't even think to take a frequency  reading I just stood mouth agape, the temptation is to throw the shorting switch but I think that would have been even more entertaining to watch.

The big lesson learnt is to never completely trust your furling.

Thanks for your time

Russell

[Hilltopgrange]

Hi Ghurd I think the silver in the pic was silver paint from the tail, I had just brushed on a last coat while it was on the stand. I cleaned off all the paint red and silver on the pin and covered it in grease just before it went up in the air. But I must admit the thought that it had stuck had crossed my mind. I will be able to check it when I get a chance to get up the tower. I have a grease nipple on the top of the hinge as well as the yaw bushing so I will give them a few pumps of fresh grease as well.

Thanks for the input.

Russell

[Hilltopgrange]

Thanks to all who have taken the time to reply and offer help and advice it just goes to prove what a vital resource this site is.

The good news is the gusty conditions died down overnight and I was able to start it up again this morning, all is well and it is still producing power it was a close call.

Thanks again

Russell

[SamoaPower]

Russell,

Considering all the reports of burned stators, blade strikes, flying bits and pieces, many furling adjustment sessions and soiled trousers, I certainly agree that there are better ways to approach the problem. Instead of reiterating my views, please see:

http://www.fieldlines.com/story/2006/6/26/12214/2424

Bill