direction confusion

[dlenox]

All,

As I have been without my wind turbine over this winter - not such a bad thing considering the severity of this bad weather during the winter... I recently have been thinking about past comments pertaining to the blade rotation as it relates to the direction of offset of the yaw bearing.

I have reviewed the OtherPower.com Dan's wind turbines as in the 10', 17' and 20' machines and have some general observations.  Today I went back over the various websites to confirm the following:

Size    Rotation Direction    Yaw Offset (viewing from front)

10'           CW                 Right

17'           CW                 Left

20'          CCW                 Left

As far as the Dan's have reported all the machines appear to be furling properly and do not exibit the characteristic of a wind-seeking machine where by; once furled at say 25mph winds, that at a higher wind speed (say 36+mph) that the essentially unfurl themselves and produce vast amounts of power.

As I am thinking about getting my 17' turbine back into operation for this spring I remind myself about the guidelines between the amount of yaw offset as well as potentially the side that the tail bearing is located relative to the yaw bearing.

But in reviewing the Dan's wind turbines it doesn't appear to matter, in spite of the problems that others have had during the last year with the larger (17' +) wind turbines.

So I find myself confused, as I know from experience that my wind turbine 'appeared' to furl ok in a 28mph wind, but then became wind-seeking in a 36+mph winds.  Granted that I initially had only an 8.5" offset (Center to Center) and over the winter increased this to 12.5"

I hate rehashing this all over again, but for these wind turbines (10', 17' 20') have to wonder what the 'optimal settings are', including:

  1) yaw forward offset

  1) yaw side offset

  2) yaw tail offset as related to blade rotation

Not only am I interested in getting my 17' turbine back into service, I have been considering a different design from the 'typical' axial flux machines, and as such am very interested in making a couple of smaller 10' wind turbines up and running here, so any thoughts would be appreciated.

On how to beat/tame the essentially 'furl or burn' scenario.

Dan Lenox

[Flux]

This is how I see the issues but I haven't built anything as big as 17ft.

There is a minimum side offset and if you go less than this it will not furl. With my type of blades and methods of loading I have got away with 4% but it is obvious from reading here that this is not enough for some machines using Dan's style of blade under some types of loading condition.

What I think happened in your case was that the offset was too small and it never furled. The tail will go at an angle and it looks to be doing so but your power would be controlled by stall. Above a wind speed that the stall will control it it increases power again and possibly it would have furled in higher winds than the alternator can stand, but I suspect not, once you have it windseeking you loose control.

The cure for that is more offset.

The forward overhang seems to affect the drop in power when it does furl. If it does furl it will reduce power, I have never seen a case where a properly furling mill will continue to give increased power out with rising wind speed. The forward overhang also seems to have effects on the way it goes into furl although this may be site dependent.

Chris Olson seems to have come to the conclusion that the forward overhang can be similar to the side offset for a good compromise and I have some reasons to agree but not based on many cases.

The direction of rotation and the side the alternator is offset is not a clear cut issue but there is an advantage in using the opposite of the conventional design as it has been done in the past. When it does hard into furl it is better if the blade tip is thrown away from the tower rather than towards it. I have seen no difference in furling with either set up but others claim to have done. There may be some small effect that only shows on big machines so I would say go for the offset on the other side to normal with clockwise rotation or reverse blade rotation if you want. It certainly will help from the tower strike point of view and it cant hurt in any other way.  

I started by copying the Lucas Freelite and this was arranged that way. I have changed at some time and it works on my size machines just as well and a very large number of machines are working ok but I suggest going back to remove one possible issue.

CW rotation , offset to left or keep offset to right and rotate anticlockwise is probably good advice looking at the thing from upwind.

Flux

[poco dinero]

Hello Flux,

I have been reading this forum for over four years, and you have clearly established yourself as one of the preeminent authorities on the subject of furling in particular, and small wind turbines in general.  You have been a most valuable resource for all of us on this forum.  This is my first response to one of your posts.

From dlenox's post,

" . . . have to wonder what the 'optimal settings are', including:

  1)  yaw forward offset

  2)  yaw side offset

  3)  yaw tail offset as related to blade rotation."

By "yaw forward offset" I assume he means the same thing as what others, including Chris Olsen, have referred to as "lead", or the horizontal distance from the yaw axis to the plane of rotation of the rotor.

A review of the specifications of the various working designs out there indicates to me that there is no set of "optimal settings" for these parameters.  It also indicates these parameters are not all independent variables in the furling equation.  Once you choose one of them and make it the independent variable, the other parameters become dependent variables for which there may. or may not, be an optimal value.

Case in point.  I recently acquired a 5 kw wind turbine.  It has a 5 meter (16.4 feet) diameter rotor, and a yaw axis offset of only 3 1/4 inches.  This makes for a 0.198 ratio of yaw axis offset to rotor diameter.  However, it has a HUGE "lead" measurement:  32 inches from yaw axis to rotor plane of rotation.  And yet rotor overspeed is controlled only by yaw and dynamic braking supplied by its controller.

This particular machine also has a very interesting furling system, different from anything I have seen on this forum.  I described it for another reader (cdog) here:

          http://www.fieldlines.com/story/2010/3/5/102612/7971

The title of that post is "hydraulically operated wind turbine towers", but later in the post, in response to a request from cdog, I described the furling of this 5 kw wind turbine.

I would really appreciate it if you would review my description of how that furling system works, see if it makes sense to you, and also render your opinion as to what you yourself think of that furling system.  Have you ever seen anything like it?

poco

[Flux]

Poco.

Sorry I didn't see your description before.

Yes basically I agree with your idea of how it works. There have been schemes with a vertically hanging vane using this idea, I think the plans produced years ago by Jack Park were on these lines.

What you describe seems basically the same as this but it does seem to incorporate some other ideas of a force balance system using two fins at right angles and both capable of rotating and coupled together via bevel gears. At least it has the off centre pivoted vane with differing areas.

I think the main criticism of the hanging vane device was that it was subjected to a lot of nasty buffeting forces when hard against the wind. With this differential area rotating vane it probably stands a lot more chance of survival than a freely hinged vane flapping in the wind.

I strongly suspect that this is satisfactory for high wind protection but instinct

tells me that once it furls it just about shuts down until the wind drops ( bistable). I can't imagine it acting as a proportional control like the gravity hinged tail when it is working right. This to me seems to have some of the characteristics of the Bergey machines with small offset and large overhang. It may be rather critical to the type of blades used and substituting different types of blade may a recipe for disaster but as the whole thing should be designed as a unit this is not an issue.

The little Rutland Furlmatic machine is a bit like this, being part way between a hinged tail and a variable area flap but in that case the fin is hinged vertically and loaded against a spring.

This seems an interesting idea, I have had thoughts on these lines but never got round to trying it, I see no fundamental reason for it not to give adequate protection and if it all holds together it should be fine.

Flux

[ChrisOlson]

I guess I've never seen where it makes a huge difference as to what direction the blades rotate vs which side the offset is on.  I think the general consensus is that if the blades are clockwise rotators, viewed from the upwind side, then the offset should be to the left.  So I went against conventional wisdom and put clockwise rotators on right offset machines.  It doesn't appear to matter.

What does matter is the amount of offset and the forward offset (or "lead").

This is what I've gone to:





You have dimension A, B and C.

a^2 + b^2 = c^2

If the offset (dimension A) is 8" I like dimension C (the effective offset) to be 16", or thereabouts - roughly double.

So if you solve the equation:

b^2 = 256 - 64

B = 14" (or thereabouts).

I've gone to this setup on all my machines to eliminate this so-called "wind seeking" phenomenon.  It doesn't appear to change WHERE it furls at at all.  But once it furls it tends to stay there and not aggressively steer itself back into the wind and suddenly start putting out dangerous levels of power.  With the particular example in the drawing, with the blades furled at roughly 45° to the wind direction the effective offset, as I call it, is now 16" instead of 8".

You still have to set the machine up to furl at the right power level.  But going to this setup has eliminated the "wind seeking" problem I had on two of them - the third machine I built this way from the ground up and it has never exhibited "wind seeking".

--

Chris

[poco dinero]

I made a calculation error in my previous post that I would like to correct before some sharp-eyed bugger nails me.

As I stated, the rotor diameter of this 5kw wind turbine is 5 meters (16.4 ft).  And its side offset from the yaw axis is 3 1/4 inches (.271 feet).  The ratio of yaw axis offset to rotor diameter is correctly calculated as 0.0165 (1.65%), rather than 0.198 as previously stated.  That is about a third of the amount of offset used on the Otherpower machines.

Quoting from Flux's post:

"I strongly suspect that this is satisfactory for high wind performance but instinct tells me that once it furls it just about shuts down until the wind drops (bistable)."

The company's website is:

          http://www.exmork.com/5kw-wind-turbine.htm

and it contains a curve of power vs. wind speed for this machine.  The rated power of 5kw occurs at a wind speed of 10 meters/sec (22 mph), but as the wind speed increases the curve indicates that the power continues to increase up to where it reaches a maximum of 6000 watts at a wind speed of about 13 meters/sec (28.6 mph)  Then the power gradually and smoothly decreases until it is less than 4000 watts at 17 meters/sec (37.4 mph), which is where the graph stops.  Survival wind speed is listed as 50 meters/sec (110 mph).  No signs of bistable operation there.

Of course, brochures and website claims are one thing, real world performance is another.  This machine is presently resting in my workshop instead of at the top of a tower, but I hope to have it flying in a few months.  Too muddy here to pour cement right now.

And Dan, by now you must be thinking that I have stolen you're thread, but hopefully what I'm about to say will convince you that I haven't.  I just wanted to get Flux's comments first.  All of the foregoing discussion is related to your post.  I know that you are looking at alternate designs, such as bringing the alternator down the tower, etc.

There was a recent post by Hilltopgrange in which he described how his 16 footer apparently failed to furl during a storm.  It appears at:

          http://www.fieldlines.com/story/2010/1/21/16166/2676

In Hilltopgrange's post, he, I, and everybody else that submitted responses were extremely and totally confused about what was going on and how this could possibly happen.  Total confusion.  Then Hugh Piggott chimed in and cleared up the confusion.  

Hugh figured it out that Hilltopgrange's 16 footer was running downwind, with the fully furled tail slammed hard against its upper stop, and held there by the wind.  And it would have done that no matter how big the offset was, and no matter how light the tail.

From that day forward, I knew that I never wanted a large wind turbine with a tail boom that folds up against a hard stop.  On the 5 kw machine I was talking to Flux about, the tail boom is rigidly bolted to the alternator, not hinged.  That machine could never run down wind, a condition that would be extremely unstable for it.  It is my hope that that discussion, along with Hilltopgrange's post, will steer your research in a new direction.

If I were going to go into a big R&D (research and development) program, as you are, that is the direction I would go.  Fixed tail boom, do something with the tail feather to provide the yaw moment.

Just my two cents.

poco

[ChrisOlson]

That being said, I only drew the illustration that way for simplicity of explanation.  This is the way I actually build my turbine heads now:

--

Chris

[Ungrounded Lightning Rod]

I think the general consensus is that if the blades are clockwise rotators, viewed from the upwind side, then the offset should be to the left.

Easier way to think about it:  Blades should be going DOWN on the side toward the yaw bearing.

(Choice of rotation vs. offset is just about avoiding blades hitting the tower due to gyroscopic forces when furling abruptly, not at all about wind-seeking forces or other things affecting the timing or rate of the furling itself.  Furling produces more gyro forces than unfurling.  Gyro forces cause the motion to be "carried along with the rotation" by 90 degrees.  So having the blades go down on the yaw bearing side makes them push away from the tower during furling, toward it during the more gentle unfurling.)

[dlenox]

poco,

no problems - this is all good information - a nice open conversation.

btw: you mention the side offset of the exmork but what is the forward offset?  I'm curious to know.

Dan

[dlenox]

Chris,

ok I was with you until you said (as I call it, is now 16" instead of 8"), then you lost me...

what is it that you are saying here (B=14 and A=16, or B=16 and A=8, or something else)?

Dan Lenox

[dlenox]

Flux/Chris/poco,

For the time being I have eliminated the idea of using a drive shaft concept to get the rotational forces down near ground level.  Initially I had forgotten to consider centrifugal forces on the shaft - causing whip.  It would be nice to do, but I'm not crazy about the concept of having to support this drive shaft every 10-12 feet apart.

I am going to be building a couple of 10' turbines as proof of concept for what I am working on, as my parameters have changed and I am still working through a whole bunch of ideas.

Right now I'm not quite ready to put these ideas 'on the table' as I am not only trying to work out some physical parameters, but also trying to make sure that the concepts/ideas are financially feasible.

Right now I have a long laundry list of ideas but most of them are centered around:

  a) ease if maintenance

  b) better control

  c) eliminating or drastically changing the tail

The initial size for the prototypes will be limited to 10/12' diameter turbines as this size has a relatively low cost to performance ratio.  The 17' turbines definitely are a different animal, and to some extent they seem to create a different set of issues.

Dan Lenox

[Dave B]

 

  How many on this board have built and are currently flying a 17' diameter or larger axial design machine with the furling tail ?

  Of this number, how many are experiencing non-effective furling ?

  Of this number, how many have tried to correct the furling ?

  Of this number, how many have successfully corrected the furling and what corrective measures were taken to do so ?

  Just curious.  Dave B.

[Ungrounded Lightning Rod]

For the time being I have eliminated the idea of using a drive shaft concept to get the rotational forces down near ground level.  Initially I had forgotten to consider centrifugal forces on the shaft - causing whip.  It would be nice to do, but I'm not crazy about the concept of having to support this drive shaft every 10-12 feet apart.

Not to mention resonances.

When the RPM matches a bending vibrational mode of the shaft the bending from the vibration quickly increases essentially without limit.  This was the original issue that made turbines a pain - damping the resonance enough that you can spin them up past it before they self-destruct. (Some people thought they'd be impossible until a fix was found.)  It's also an issue for long drive shafts.  This is part of why they are large-diameter - to make them stiff and thus raise the frequency.  It's also why the transmission is up near the engine on most vehicles - to get the RPM down below the resonances at any vehicle speed.  (If it weren't for this effect you'd want to spin the drive shaft fast, reducing the torque and thus the weight needed to transmit it.)

The problem usually occurs with the funadmantal, i.e. the mode where the shaft is a half-wavelength long and bends in the middle like a bow.  But if you get it to spin up past that you can get higher-order resonances to do the same thing.  Extra bearings - especially with oddball spacings - can be used to "damp the string" at various points to kill lower resonances and get the RPM of the first destructive one up a bit.  (And that's why the early mills with the power transmitted by shafts and belts had LOTS of bearing blocks on the long shafts.)

You really don't want a drive shaft to get into a runaway bending amplitude mode in a storm.  So figuring out how to damp all the modes you'd need to suppress is a design problem that it's easier to avoid than to fix.

Pity.  It would be so nice to put the genny down where you can work on it and keep the wires straight and non-flexing.  But to do that you need to gear the mechanical system down enough at the head to put a lot of yaw reaction torque on the gearbox and blades.  For a one-of it's easier to design getting the electricity through the yaw than to engineer a functional driveshaft system.

[dlenox]

Dave,

Over the last 6 months there have been a bunch of furling issues, pretty easy to see by looking back over the postings.

From my perspective, I know that my wind turbine is wind-seeking, and has apparently done this since the beginning.  I know from watching it that it appeared to be about 90% furled at about 28mph, however 'unfurls' at 36+mph winds.

Along the way I have made a number of attempts to correct this situation including lightening of the tail.  Recently I did extend the yaw side offset, however did not have the opportunity to put back into service.

But in large part I am basically happy with my turbine, however saying this I am obviously not happy with the wind-seeking 'attribute'.  There are other issues that go along with the fact that I have a fixed lattice tower, so maintenance is tiresome at best.

I am redesigning my wind turbine with these thoughts in mind - and many, many other ideas as well.

While I do believe in KISS, simple is not necessarily the best solution for these large machines, and there are many areas that certainly can be improved upon.

I already have a very long laundry list of areas that I will be addressing, will it be the ideal design decisions for everyone? Doubt it as there are many personal decisions that also incorporate things like fabrication skills, timeframe, expenses that only the individual can answer.

My goal is to make a turbine that I can live with, will suit most of my needs and wants, if others care to incorporate any of my design decisions into their particular build, then it is up to them.

Dan Lenox

[poco dinero]

Hello Dan,

I did put it into my post, but I don't mind repeating it.  The "lead", or what you call the "forward offset", of my 5 kw machine is a HUGE 32 inches.  Suits me fine too.  I like lots of "forward offset".

Thanks for being tolerant of what surely must have seemed to you, and what would be considered by a lot of less tolerant people, to be stealing their thread.

Tell you what I like about that rather unique Exmork furling system.  There are no hard stops for the tail boom to slam into.  Furling appears to be limited, not by a hard stop, but by a balance of aerodynamic forces on the cross sectional areas of the forward part of the wind turbine (rotor and alternator) and the rearmost part of the wind turbine, comprised of the tail boom and tail feather.  If they got the balance right, it should just dance around up there, furling a little bit more, then unfurling a bit more, but never slamming into a hard stop.  Just a soft balance of aerodynamic forces, with the rearmost areas "winning" most of the time, thereby keeping the rotor almost edge on into the wind, but not quite edge on.  

Additionally, I like the idea that with a fixed tail boom like this, there is no way that this turbine could run downwind, as Hilltopgrange's 16 footer did.  The long tail boom facing upwind would be very aerodynamically unstable; i.e., the slightest perturbation of that aerodynamically unstable condition would provoke a further movement in the direction of the perturbation, rather than damping out the perturbation and resulting in a continuation of the momentarily existing unstable aerodynamic operating condition.

Now, let me give you my usual caveat.  This thing is sitting in my workshop, not flying on top of a tower.  Everything I've said about it is based on analysis, not observation.  I hope to have it flying soon, though, and I promise to provide a completely honest and objective comprehensive analysis of its performance.  It will be fairly well instrumented.  And if I'm wrong about the furling, I won't have the slightest problem with admitting it.  That's one of the things I like most about this forum:  Folks don't mind admitting their screw-ups, in the hope that somebody else will benefit from them.

poco

[poco dinero]

Hello Chris,

Just a comment on a couple of things we totally agree on.

In general, lots of "lead" or "forward offset" is good.  It

   a.  Stabilizes furling, and,

   b.  Provides tower clearance eliminating concern about which way the rotor rotates.

Secondly, the direction of rotor rotation vs. direction of yaw axis offset is important only in the context of keeping the rotor blades from striking the tower.  If you've got enough "lead", that isn't a consideration.

[ChrisOlson]

Dan,

Dimension B is the offset when the machine is facing directly into the wind - in normal operation.  As soon as it starts to turn the amount of offset begins to increase - the lever that's applying torque to the yaw tube - that torque attempting to overcome the counteracting torque applied by the tail attempting to keep it steered into the wind.

When the rotor gets turned approximately 45° out of the wind the amount of true offset (effective offset- the lever distance applying torque to the yaw tube) is just about doubled with the design in my drawing.  This has proven, on every one of my machines, to keep them flying in a furled position once they furl - it's very hard for the steering system to bring it back around to face the wind.

My spring loaded furling cylinder that I built has also proven itself and works as I expected.  I have the tail stop set on that at 75° so the turbine can't steer downwind with the tail slammed up against the rear of the turbine head.  Since I'm not using an angled hinge anymore I don't have to worry about either tail weight, or tail boom length.  Therefore I've gone to long tail booms that wouldn't work on an angled hinge system, and 12% of the swept area for tail feather area - 8 foot has 6.2 sq ft and 84" tail boom, the one 10 foot machine has 9.5 sq ft of tail feather and 90" of tail boom length - my other 10 footer is going to be retrofitted with the same tail system I have on the first one in the next couple of weeks.

With that big tail and the long boom, even 80 mph winds can't push it around so the rotor goes downwind.

I'm fairly confident that I've solved the furling problems I've had in the past.  YMMV.  I'm just pointing out what I've done - using smaller machines, and more than one, for testing to arrive at a design that I'm comfortable with on my 21.  And what I've gone to isn't really all that radical - take a good look at a Jacobs 31-20 - I've sort of copied what they've done, just going about it a slightly different way (with my own design).

--

Chris

[ChrisOlson]

I'd have to concur on this - on my 10 foot machines, with a 2° up tilt on the shaft axis, I have roughly 18" of room between the blade tips and the tower.  I weigh about 192 lbs and I can lay one of those blades between two chairs and stand on the blade in the middle and it only bends 7-8".

I figure if the wind is strong enough to bend those blades 18" I got bigger problems than what direction they were turning when they hit the tower.

--

Chris

[Hilltopgrange]

Hi Poco,

         I can honestly say my 16 footer did not run downwind, I was on hand and observed it facing into the wind, all three of my turbines and the anemometer where pointing into the wind and within a few degrees of each other. As the wind increased the 2 smaller turbines (12 and 10) furled and yawed out of the wind while the 16 footer stayed full on into it. I just wish I had a camera at hand when it happened, I am still waiting for a calm day to add an extra vane as flux suggested. We have had a few storms since and it has furled and reduced power perfectly, I am convinced it was a freak incident, that day was very stormy with very violent gusts.

Russell

[Dave B]

 I guess it's my point. First there are few (at least who have posted) that either have built, are building or are currently flying machines 17' and larger basically of a scaled up version of the common book built 10' axial machine with furling tail.

 Of that few some have mentioned furling issues. Of that few we have read very little of and or seen very few photos of solutions and or attempts at solving the furling issue (with each individual's machine)

 With machines this large there could be 1000 watts difference in output with only 1 MPH wind speed change at or near furling speed. I have no doubts there can and will be issues with furling as we are working this high on the vertical of the output curve.

 With this very slim margin for error everyone's own home built machine becomes a custom machine and will need to be tweaked accordingly. This is why I am interested in hearing from those who are building, testing and adjusting these larger machines.

 The discussion is interesting and theory, assumptions, calculations etc. etc. can have their place but when I want to move forward I will look for the hands on person and their proof of experiences first everytime.

 It's easy to call a shot after it's been hit or also to be right 100% of the time by predicting a chance of rain. Politicians too are great at saying what everyone already knows which automatically makes you "know" they are right and then they get your vote and confidence besides.

  The range of people on this board extends from the hands on get it done type who couldn't care less what other's think about them to the Political type who only care what other's think of them and who are unlikely to get their hands dirty.

  This is a very interesting place, I'll get my hands dirty thank you.

  Dave B.

   

[poco dinero]

Hello Russell,

I take you at your word that your 16 footer didn't run downwind.  But I still struggle to identify a force that could hold the tail in the fully furled position, hard against its upper stop, with the rotor headed directly upwind.  Hugh Piggott's explanation was the only one that made sense to me.

Did you ever implement Flux's fix?  Or come up with any other ideas?  We'd sure appreciate an update.

poco

[dlenox]

poco,

sorry if I missed the info, thanks for repeating it for me.

since the turbine is on the ground, is there any chance you can take a couple of pictures of the tail and pivot so that it shows what you were talking about.

Dan Lenox

[dlenox]

Dave,

I've experienced wind-seeking fairly early on in my project (over a year ago) and initially people were thinking that the machine flat out was not furling.  But observation has proven to me that it did in fact furl at about 28mph winds, however became wind seeking when the winds increased to 36+mph.

From what I have seen recently on this board is that it seems that a fair number of people that have the larger (over 12') machines and they too have been reporting issues having to due with furling/wind seeking.

Obviously your situation is different as up until recently you have been heating water and not charging batteries, so you may never see this problem with your setup.

From what I see, most of the people that are contributing and watching this thread are all 'hands on people'.  I am attempting to have an open conversation about practical applications and alternatives from those that have more experience and knowledge than I.

For me most of the time the theoretical stuff gets in the way - and a lot of the theory goes over my head.

Dan Lenox

[dlenox]

Chris,

Thanks for the reference to the Jacobs 31-20.  Did a quick google on it and it turned up some interesting photos.

Wow! when I was previously talking about using a 90 degree gear box and putting the alternator below (within 10') of the gear box that is almost exactly what I visualized in my mind!

For others that may want to see it here is a link: http://www.2jbs.com/windturbines/jacobs20kw.html , it is the bottom picture on the page.

Dan

[Dave B]

 There are just too many variables with each home built machine and application to nail down the problem to a specific thing. The best chance anyone has is with the smaller machines as there are so many being built and there is also a type of "standard" that has proven to work for the majority who have built it to the letter.

 If there were as many 17' plus machines being built with the same consistency and to the "letter" of another proven standard then there would be fewer problems reported per unit built of the larger machines also.

 Maybe trying to find that magic bullet is what's happening now with the larger machines. There is much more leeway with the smaller machines so getting things about the same as the book will work for the majority. Tweaking 1000 watts plus for 1 MPH wind speed change is a totally different game.

 Blade profile becomes a huge factor with the larger machines so right there is a major source for inconsistency between home built machines of this size. Comparing apples to oranges (small to big) just does not work, consider yourself very lucky if it all falls together gearing up to a much larger machine.

 My opinion is that many are trying to see the furling tail work as a buffer or variable output device. In the smaller machines it seems to work this way but in the larger machines I think we are fooling ourselves that we can break down 1000 watts into fractions of 1 MPH to make the furling work this way. All or nothing I think is a more realistic goal for a successful and consistant design. Just my opinion based on a lot of run time of only a 16' machine.

 Dave B.

 

[ghurd]

(I have no business talking about furling or 17' machines)

It almost seems like something simpler is being left out to me.

The standard offset per diameter needs increased with larger machines, and I don't recall seeing any fixed amount.

An 8' is more than twice the swept area of a 4'.  Very little posted issues with furling 4'ers.  

Seems to me like a majority of the large machines need re-worked for a larger offset.

As the blades get larger, the required offset gets proportionally much larger.

I will throw this into the discussion.

The concept that the correct offset is based on swept area, more than diameter?

Sort of how a 60 ohm load at 60V takes 1A for 60W.

And the same load at 120V takes 2A for 240W.

Or double the Voltage and double the resistance, for 120V and 120 ohms,

it still is Double the watts, even though that is counter intuitive for most people.

Something like an area cube square kind of thing.

Seems like some study of the offset in functional (and reliably functional) 4 - 10 - 15 - 17 - 20' furling systems would give the required offset in a very basic formula.

And I think that simple formula will incorporate swept area.

G-

[ChrisOlson]

The Jacobs 31-20 is rather unique in that it doesn't use a lot of offset - it uses the torque turning its hypoid gearbox to create the yawing force, balanced by that big spring-loaded tail and the load on the generator.  It's a proven system that has flown all over the world.

Someplace in my computer I have a movie I took of a Jacobs 31-20 running furled in high winds.  If I find it I could send it to you (it's pretty large file size) so you could get an idea of how it works - and it DOES work.

Again, I didn't copy Jacobs' design directly - I took a que from their idea and applied my own design to it, mostly related to their steering system.

--

Chris

[ChrisOlson]

I will throw this into the discussion.

The concept that the correct offset is based on swept area, more than diameter?

In theory I think this could be a case for your idea.  In practice it's all about how much torque a particular design applies to the yaw tube to turn it.  Theoretically, a larger rotor develops more thrust (translated to turning torque on the yaw tube).  So if you take a machine that's set up to properly furl with 8' blades on it, and suddenly bolt 16's to the same machine, but with a bigger generator designed for the 16's, it should furl earlier than the 8 footer did because of more rotor thrust on the same arm.

I think there's a hard limit with how much offset you can use and still get it to fly decent.  I don't know what that hard limit is.  And it obviously is very dependent on how much actual thrust the rotor develops.  In my own testing and experience I've found a LARGE variation in rotor thrust using different design blades of the same swept area.

I've got a setup that I'm going to test on my 21 - I'm mounting a short tower to a flatbed 5th wheel trailer that I'm going to fly it on.  For me, living on the flats here where the wind howls with nothing to restrict it, this works fine as I can get a machine to furl on any good wind day.  I'm going to fly that 21 without a tail on it and measure the torque it takes to keep it turned into the wind with a 250 ft-lb beam style torque wrench with a shaft extending down thru my test tower.

I'm going to eventually use the same tail system on it that Jacobs uses, and what I've gone to on my smaller machines.  But scaling it up to a 21 using calculations is risky, at best.  I'd rather know, with no doubt in my mind, than use calculations and have it not work.

I've already cut my first head apart and am building a different head with more "lead" (I'm going to 30" forward on it with 13.8" of offset).  On my smaller machines 200% of effective offset has worked.  On this one I decided 240% would be better - because once it turns out of the wind I want it to stay there.  On my smaller machines I haven't seen any indication that extending that rotor out front more creates any instabilities in how they fly - in fact it has made them smoother flying and furling.  I think there's a hard limit on the "lead" too, but probably only as it relates to loading on the yaw bearing - and that can be balanced with a long heavy tail that wouldn't otherwise work with a "conventional" angled tail hinge.

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Chris

[ghurd]

I am pretty sure thrust can be directly related to Betz, within reason,

meaning, in my mind, some of the blades worked better than others, were better matched to the output than others, meaning a LARGE variation in rotor thrust using different design blades of the same swept area included some poor blades for the machine.

The swept area, wind speed, watts and Betz are often related as a percentage.

Thrust gets more complicated.

G-

[ChrisOlson]

On my own machines I don't think the 10 foot rotors make any more thrust than the 8 foot.  Here's a photo of one of my 10 footers that I just got done putting my new tail system on last Saturday:





The heads on the 10 foot machines are identical in every respect to my 8 footer, except for the tail length.  This is one of my older heads with the right angle offset bracket on it.  The tail measures 90" from the hinge to the tip of the tail feather on this machine and it's only 84" on the 8 footer.

I set the spring cylinder on this one to the same pressure as the 8 footer - 21 lbs.  That shut my 8 foot down at 400 watts.  I ended up turning the adjusting screw 9 turns (about 3/4" more spring preload) and it shut that one down at 950 watts right on the button after adjusting it.

These 10's haven't really proven that they put any more power than the 8 foot.  If I crank that cylinder down on the 8 footer it'll easily go over 1 kW - BTDT.  It has the advantage that it runs at 580 rpm at full tilt where these 10's only turn at about 430-450.

We haven't had the wind yet so I can adjust this one, but based on my experience with them I'm betting the end spring pressure to shut it down at 1 kW will be about the same as the 8 foot - just because it turns slower and doesn't create any more thrust (or power) than the 8.

The other thing I've done is that the tail flies perpendicular to the rotor plane instead of offsetting it to the anti-furl side.  I didn't want to give it any reason to not furl because the 14.1" of "lead" (forward offset) got on the wrong side of the yaw tube.

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Chris

[ghurd]

"I don't think the 10 foot rotors make any more thrust than the 8 foot"

Exactly my point.

"identical in every respect to my 8 footer, except for the tail length"

Exactly my point again.

"I'm betting the end spring pressure to shut it down at 1 kW will be about the same as the 8 foot"

"except for the tail length"

Exactly my point again.

"just because it turns slower and doesn't create any more thrust (or power) than the 8."

"These 10's haven't really proven that they put any more power than the 8 foot"

Meaning an 8' should make as much power as a 10' without melting?

"The other thing I've done is that the tail flies perpendicular to the rotor plane"

and

"except for the tail length"

I will defer to someone else about detailing those issues, past saying trig and leverage.

[Ungrounded Lightning Rod]

Same here.

The only thing I can imagine is if BOTH the yaw bearing and the tail pivot somehow got stuck - and even then the tail should be out or mostly-out, not folded.

Having the tail FULLY folded up while the mill is facing the wind makes no sense at all to me.  The tail should be essentially downwind at all times - or slightly out on the side opposite the offset to cancel the offset thrust torque.  Folding it up with the mill facing the wind would require a big gust from the SIDE.

[ChrisOlson]

I think you misunderstand.  I'm not using an angled tail hinge here.  It doesn't matter if the tail is 60" or 90", or if it weighs 10 lbs or 20 lbs.  None of that changes the furling point one bit with this design tail.  The tail stays planted straight into the wind and the head just yaws against spring pressure.  Without the angled hinge the tail doesn't "climb" like a conventional furling tail when the machine furls.  Gravity, tail weight, tail length, hinge angle - none of it applies to this design.

The longer tail only enhances steering control of the turbine - steers it into the lightest breeze and gets it fired up, and it doesn't yaw back and forth in gusty winds like machines with short tails will do.

And, yes as I said, neither of my 10 foot machines have proven they can put out more power than my 8 footer when the wind starts to blow.  The 8 footer can be safely furled at much higher wind speed than the 10's and they top out at about the same power level.  Since we're talking about furling here, that's an important issue.

The 10's will put out more power at lower wind speeds of 10-18 mph.  But the 8 will easily match them at the top end when you're looking to see what sort of max watts you can make.  I got three pages of data that I've recorded and collected from these machines on power output, and in 25-30 mph winds the 8 footer can match the power output of the 10's simply because it's still running in the wind and doesn't fly fully furled until around 28-30 mph.

The 10 foot has to be shut down at around 24-25 mph because it can't be safely spun at the rpm's the 8 can turn.  Again, as I said, my best guess is that the end spring pressure required to furl the 10 at 1 kW will be very close to what it took for the 8.

This tail system proved itself, and it worked as I expected on the 8 foot.  I'll know, and have the data, when I get the wind to tune this 10 foot - which sounds like it's coming tomorrow.  As soon as I find that out this same tail is going on my other 10 footer, and I'm going to use what I've learned on these machines and apply it to my 21 that's being built.

YMMV.  But I've gone away from the "conventional" angled hinge tail, looking for a better and more reliable solution.

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Chris