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dimensioning the tailvane for a Hugh's Pigott windturbine
By blueEnergy group, Section Wind Posted on Tue May 08, 2007 at 06:16:49 AM MST
We manufacture Hugh's Pigott windturbine in Nicaragua and recently encountere problems with our tailvane after a small change in the surface.

We have built a 1kW Hugo Pigott's machine and wished it to furl early (something like from 700 - 800 kW). In fact we will install the machine in a windy, isolated community and wanted to make sure the stator wouldn't overheat.
So we changed the design of our tail vane, wish is now a trapeze with a surface of about 9300 cm2. All angulars are 2" * 2" * ¼" and the plywood is ½" as in Hugh Pigott's manual. We added an aluminum angular (1.5" * 1.5" * 1/8"), 94cm length to help maintain the plywood.
To protect the plywood, we covered it with resin and fiberglass. The resulting weight is about 7kg. The metal part weight is about 21kg. The weight of the whole tail assembled is about 30.5 kg.The furling moment measured for the whole tail is 20 kg*m.
We put the tail at an angle of 110º to the turbine in the non furled position. The angle of the tail, dimensions of the body are like in Hugh Pigott's 12' manual.
Now our turbine doesn't take the right orientation, it's far from being front to the wind. It looks like the tail isn't sufficient anymore to compensate the action of the wind on the blades. Curiously when we stop the turbine short-circuiting it, the tail aligns with the wind as it should.
Could someone help us dimension correctly our tail vane? If someone has already made calculations for tail vanes or tested similar ones, it would be great to have some help to redimension our one... Or maybe someone has an idea of another possible reason why our tailvane doesn't work as supposed...

dimensioning the tailvane for a Hugh's Pigott windturbine | 14 comments (14 topical)

Re: dimensioning the tailvane for a Hugh's Pigott (3.00 / 0) (#1)
by Flux on Tue May 08, 2007 at 01:35:05 AM MST

This is a problem with that type of furling system, the tail has to balance the thrust of the prop against the alternator offset.

With the machine braked then there is little thrust and the tail trails directly down wind. When running you want the prop to face square into the wind, the tail will have to run at an angle to the wind to balance this.

If I have converted your metric correctly you have about 10 sq ft of tail, which seems reasonable.

"We put the tail at an angle of 110º to the turbine in the non furled position. The angle of the tail, dimensions of the body are like in Hugh Pigott's 12' manual."

Sorry I can't follow that. Without offset on the alternator you would need the tail directly behind the axis of the alternator shaft. To balance the offset force the tail needs to be moved away from the straight back position, away from the furling direction.

I assume this is what you mean but I can't imagine where you are measuring your 110deg.

I find that with my tails I have to use angles up to 30 deg away from furling direction but I tend to keep tails small and light. It looks a little odd having the tail set over at such an angle but it has no other ill effect.

What you need to observe is the wind direction in relation to the prop axis, add a little vane or streamer below the machine so that you can compare these directions.

If you have the prop heading directly into the wind, then the tail will be at an angle to the wind, that is how it develops the balancing force.

If you are still wrong when you have done this test then you have to increase the offset angle of the tail even more or find a way of increasing vane size or boom length without increasing the tail moment and raising your furl speed. I have never used plywood, I find it far too heavy for my methods and it will be even heavier by the time you have covered it in fibreglass, but others seem to use it with no problem.

Unless the prop is badly at an angle to the wind it makes very little difference to the power out, I doubt you will see much difference up to 30 deg. If you are worse than that you need to change something.

You could manage a heavier tail for the same furling point by changing the tail hinge angle but that is not an easy change to make.

Flux

Re: dimensioning the tailvane for a Hugh's Pigott (3.00 / 0) (#11)
by blueEnergy group on Thu May 24, 2007 at 01:47:26 PM MST

Thank you for your help.

When I spoke of the angle I meant 110° away from the plan of the turbine.
We had tried to increase it a bit up to 120° (30° for you) but it was still not sufficient...

We compared the prop with the wind using a vane, and it's about 30°, so it's pretty bad. But it's already a good new to know that till 30° it doesn't change a lot the production.

For this tailvane we might let it as it is for the moment but we are trying to improve the design, so I wondered which material do you use? Plywood is quite hard to protect from water so we were thinking to change the material.

Change the tail hinge angle is a solution too, but that's not so easy to make tests to find the right one, as you say...

Anne-Claire, blueEnergy
--
blueEnergy
www.blueenergygroup.org
[ Parent ]

Tail vane for a Hugh's Pigott windturbine (3.00 / 0) (#2)
by David HK on Wed May 09, 2007 at 06:57:30 AM MST

An angle of 110 degrees seems too much. But as Flux commented, we do not know your method of establishing this angle.
On Hugh's 4 feet machine, Page 62 of his booklet, the angle is 95 degrees - 85 degrees plus 10 degrees. I nearly fell foul of this measurement and had to read the instructions several times to get it right.

On page 56 there are two angles to consider in each elevation - one is 6 degrees from the side and the other is 9 degrees as viewed from the rear.

If you have the booklet or drawings try to read them again very carefully.

David HK

Re: Tail vane for a Hugh's Pigott windturbine (3.00 / 0) (#12)
by blueEnergy group on Thu May 24, 2007 at 02:00:52 PM MST

Hugh Pigotts recommands 100° for the 8'turbine, nothing for the 12' in the version of the manual I have at least. But it looks more like we need more force on the tail to equilibrate the forces on the blades, a bigger angle then, no?

I'll check the angles of the hinge.

Anne-Claire, blueEnergy
--
blueEnergy
www.blueenergygroup.org
[ Parent ]

Re: dimensioning the tailvane for a Hugh's Pigott (3.00 / 0) (#3)
by Dave B on Wed May 09, 2007 at 09:22:57 AM MST

I use a chain to adjust the low stop on my tail. If you have plenty of angle built in then it is a simple matter to do as Flux suggests and to observe the angle of the blades to the actual wind direction by using a ribbon or other simple means. Between adjusting the tail weight and the low stop you should be able to balance the offset of the blades to run proper. Nothing is etched in stone with the design no matter how close each is built to copy another, same goes with each site besides. Dave B.

Re: dimensioning the tailvane for a Hugh's Pigott (3.00 / 0) (#4)
by libra on Wed May 09, 2007 at 01:44:33 PM MST

I am looking for the amout of weight it should take to furl the tail.
I have seen a reference to dan using a fish scale and someone said about 10lb pull.

I made my angle adjustable but found that when I set the angle to 120 degrees it took alot of force to pull the tail up.

I have to make new blades and will change the angle but would appreciate info about the pull to be expected..

I searched with yahoo but couldn't find much

Libra

[ Parent ]

Re: dimensioning the tailvane for a Hugh's Pigott (3.00 / 0) (#13)
by blueEnergy group on Thu May 24, 2007 at 02:02:51 PM MST

We use a chain too, and tried angles from 95° to 120° but it never looked better... More angle seems a lot no?

Anne-Claire, blueEnergy
--
blueEnergy
www.blueenergygroup.org
[ Parent ]

Re: dimensioning the tailvane for a Hugh's Pigott (3.00 / 0) (#5)
by Ungrounded Lightning Rod on Wed May 09, 2007 at 01:54:34 PM MST

The tracking of the mill when the tail is unfurled is controlled by:
- The turbine's wind moment, composed of:
- The turbine's effective area (swept area times a fudge factor), times
- The turbine axis offset from the yaw axis.
- The wind moment of the tail, composed of:
- The area of the tail, times
- The distance from the yaw axis to the tail vane's center of effort, times
- A function (roughly sin) of the angle of the tail when on the unfurled stop.

The onset of furling is controlled by:
- The same function (roughly sin) of the angle of the tail when on the unfurled stop.
- The sin of the TILT angle of the tail pivot
- The wind moment of the tail (see above) divided by:
- The weight moment of the tail, composed of:
- The weight of the tail, times
- The distance from the tail pivot to the center of gravity.
- (A fudge function relating the offset angles of the unfurled
stop and the tail pivot if they are not equal.)

The slope of the partial-furling curve is mainly related to the
tilt of the tail pivot.

Because the weight of the vane is most of the weight of the tail,
the tail pivot is near the yaw bearing, and the tail's center of
effort is near its center of gravity:
- Changing the tail's area without changing its thickness, and
- moving the tail vane in-and-out
will each have a strong effect on tracking. But (provided the
offset between the unfurled stop and the tilt angle isn't TOO
pronounced) their effects on the onset of furling will be
small, because each has two opposing effects that nearly cancel.
- Moving the vane in or making it smaller makes the mill fall
off from the wind.
- Moving it out or making it larger will make the mill track
closer to the wind (or even past it toward the tail side).

The weight of the tail only appears in the furling side of the
equation. So adding or removing weight without changing the
tail vane area or location will adjust the onset of furling
without affecting the tracking.

Because the weight of the tail appears in the furling onset,
adding or removing weight from the tail, or sliding an
additional weight along the tail boom, can adjust the furling
onset without affecting tracking.

I'm not sure exactly what changes you made to the tail compared
to hugh's design (since I don't have his book in front of me).
It would be helpful to give both the unchanged and the changed
dimensions/weights/angles of each of the things you changed.

I note that you said:

Now our turbine doesn't take the right orientation, it's far from being front to the wind. It looks like the tail isn't sufficient anymore to compensate the action of the wind on the blades. Curiously when we stop the turbine short-circuiting it, the tail aligns with the wind as it should.

The tail is not SUPPOSED to be aligned with the wind. It is supposed to be off at an angle on the side opposite the offset of the turbine's axis from the yaw axis. This provides the restoring force to keep the blades pointed at the wind. What you want is for the blades to be face on to the wind - the turbine axis to be aligned with the wind direction - as long as the tail is sitting on the unfurled stop.

You have lift type blades. When they're spinning and under load the lift produces a drag that is close to that of a solid disk the size of the swept area. When they're stopped they have roughly the drag of flat sheets of their own size - tiny by comparison. So when you stopped the mill most of the yaw-away force from the offset turbine axis went away. Without that the tail tracked about straight downwind. This is normal but immaterial, because the only thing affected by this mode is startup.

Re: dimensioning the tailvane for a Hugh's Pigott (3.00 / 0) (#6)
by Flux on Thu May 10, 2007 at 12:05:06 AM MST

Nice explanation. For the few that understand the basic idea that covers just about everything.

Flux

[ Parent ]

Re: dimensioning the tailvane for a Hugh's Pigott (3.00 / 0) (#7)
by Ungrounded Lightning Rod on Fri May 11, 2007 at 01:44:41 PM MST

Thanks, flux.

I figured that the basics of tail-pivot auto-furling had been explained so often that anybody who would get them already had. B-) Of course that leaves newbies out. B-(

So for newbies:

- You mount the genny so the turbine's axis is a few inches off to one side of the yaw axis. ("Yaw axis" is the pivoting to face the wind.) This makes the drag try to turn (yaw) the blades away from the wind - and try harder the harder the wind blows.

(Details of how far are posted elsewhere. It's not too critical since you can adjust the tail to match whatever it is. Except for a pitfall: If there's too little offset there's a fluid-dynamic goofiness that makes the blades "grab" the wind and stay pointed into it rather than trying to turn away.)

- To keep the turbine pointed into the wind you mount the tail at an angle off to the other side. This provides a compensating force - also increasing with increasing wind - trying to yaw in the other direction. When the turbine is facing the wind they balance out. So the turbine tracks the wind. (But the tail is off at an angle so it looks goofy.)

- To make it furl you put a hinge in the tail boom mount. Of course you can't just let the tail swing - then it would be useless. So you add a restoring force to keep it straight out most of the time. You can use a spring - but that's an extra part to wear out and goof things up. A better alternative is to tilt the top of the hinge away from the mill. That way when the tail tries to pivot away from straight out its weight provides a (progressively increasing) force trying to push it back out straight. (The "equal-and-opposite-force" from this works against the hinge and mount, transferring the tail's wind-driven forces to the mill. Thus the tail still points the mill into the wind - sorta...) The spring force, or the force from the tail's weight DOESN'T go up with the wind, so the mill no longer points straight at the wind for all wind speeds.

- You put in a stop to keep the tail from folding all the way into the blades and destroying things. (Some people do this with cutaways in the hinge assembly - but that puts a LOT of force on the stop due to leverage. Putting a bumper on the back of the genny catches the arm farther out, reducing the required forces enormously. If your tail pivot is a pipe-over-pin with the tail held on only by gravity, you need to angle the stop down a bit, or provide a guard above where the boom hits it. Otherwise, when the tail hits the stop, it might jump up and off the hinge pin.)

- When the mill is tracking the wind there is a side-force on the angled-out tail, which folds it back (and up in the tilted-hinge case), increasing the restoring force and thus the force trying to turn the mill. This reaches an equalibrium with the force from the offset blades and the mill tracks at some angle to the wind.

- If you get everything adjusted right (which includes having the spring or the tail hinge's offset angle pulling the tail further out than the position that makes the mill point straight into the wind), a range of winds high enough to overpower the mill if it WAS pointing straight upwind will also make it track at a range of angles away from the wind where, at each wind speed, the mill will end up operating under (but near), its maximum safe output.

- Downside to this is that, at lower winds than where you NEED to furl, the mill may track a bit off the OTHER way rather than facing the wind squarely. So you add a second stop to keep the tail from moving outward further than the angle where the mill points straight into the wind.

Result: The mill faces the wind until the wind is getting strong enough that it's approaching the mill's safe limits. Then as the wind rises further the tail folds up more and more, and the mill turns farther and farther away from the wind - at first tracking close to the maximum safe power point, then falling off from it (to avoid overspeeding in gusts before it can react).

How's that?

[ Parent ]

Re: dimensioning the tailvane for a Hugh's Pigott (3.00 / 0) (#8)
by Ungrounded Lightning Rod on Fri May 11, 2007 at 01:58:00 PM MST

Of course this represents an ideal - and a lot of tuning to get it right.

The MAIN point of furling is to protect the mill from destruction by excessive current generation (burning out the genny) or overspeeding (tearing up the structure, starting with the blades). (The first, by the way, will quickly lead to the second.) Tuning needs to approach from the safe side or you lose your mills when you overshoot.

So don't sweat trying to track the maximum power point. It's better to err by falling below it (losing some - maybe a lot - of the power) than by going above it (destroying the mill and losing ALL the power).

If your genny is big enough for your blades and wind conditions, most of the time your mill will be running below furling speed anyhow. So the bit of power you lose by furling more than "optimum" and thus not maxing out your potential generation during storms is a small price to pay for the survival of your equipment.

It's perfectly OK to over-furl so much that the mill shuts down almost completely in a storm.

[ Parent ]

Re: dimensioning the tailvane for a Hugh's Pigott (3.00 / 0) (#14)
by libra on Sun Nov 02, 2008 at 10:54:49 PM MST

Good to be part of the group again.

I built a 15' wind generator with a 7" offset and I used the furl design by Hugh where the tail was pinned at the top and bottom of the mast at what I thought was suppose to be 20 degree side tilt. It kind of felt heavy when I tried to pull it into the furling position. The tail was 75 inches long and the tail fin was about 7 sq feet.

Well it lasted for about 3 weeks and when it furled it seemed to hang in the furled position (fully furled) and the blades would almost stop rotating and then the tail would fall into the proper position.

I would see 40 to 45 amps (24 volt system) and then it would furl and the amps would drop off to almost nothing and then it would start up again.

I learn't the hard way that the angle of the tail was too steep and when the tail fell back into normal position it hit the stop so hard that it slammed the generator into the face of the wind so that the centrifugal force caused the blades to twist 38 inches into the side of tower. Now i feel like part of the group!!!

I have made new blades and have added a strip of birch in the middle of the cedar laminations to maybe stiffen them up.

I have redesigned the tail angle quite a bit so it pulls into the furled position quite easily. I have reduced the tail weight hy using 9mm plywood rather than 3/8 "
and have made adjustable stops with rubber belting to cushion the stops.

Have I got it right this time I sure hope so as it is an effort to get it up to the 140' level of the tower.

I would appreciate comments from those that have used this furl design.

I can post a picture if required.

Libra

[ Parent ]

Re: dimensioning the tailvane (3.00 / 0) (#9)
by Seaspray0 on Sun May 13, 2007 at 11:48:26 PM MST

I'm no expert on tailvanes, but I do know quite a bit on sail designs for sailboats. I believe the tails should be taller with less chord. Our sails are roughly 28 feet tall with a chord of about 8 feet. We'd design them to go taller with less chord to gain more force out of the wind but it would move the center of force on the sail higher above the waterline and cause the boat to heel more. For a tailfin that is ballanced above and below the boom this doesn't present a problem. Short sails with a long chords just do not work well in wind speeds between zero and 30mph. Most of the tail fins I've seen for the wind generators tend too be just that. Just my humble opinion.

Re: dimensioning the tailvane (3.00 / 0) (#10)
by Flux on Tue May 15, 2007 at 02:04:22 AM MST

Yes a tall narrow vane is better than a long short one. Within reason the difference is not that great for a given area. Wind in the wake of the blades is probably very odd anyway, sometimes you need a bit of extra boom length to gain stability, possibly by going back into cleaner air.

Probably with a boat the wind gradient is greater with height and the effect is much greater. Very few commercial manufacturers have adopted tall narrow fins for wind generators, the wind vanes used for meteorology certainly do use this shape. It needs more supporting on a wind generator to avoid fatigue and falling off but I would certainly avoid very long and lean horizontal shapes.

Flux

[ Parent ]

dimensioning the tailvane for a Hugh's Pigott windturbine | 14 comments (14 topical)

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