tales of tails

[jacquesm]

The blades are well underway, nowhere near done yet, but I can see that it's getting there. Maybe another two weeks or so (if that cloth and the resin arrive in time that is). See my diaryfor more information on that.

Time to start thinking about the next phase of the project, the tail.

I have looked at lots of windmills, and from what I have seen yaw control exists in a few forms:

Passive tails

Active tails

Tailless designs (downwind machines)

Servo systems, no tail just a sensor and a motor to do the work




The passive tails are the most common in homebrew machines. From what I see they are simple, reliable, can be integrated with the furling system and require little or no maintenance. That's all really good.

But they do have one drawback. At high rpm a sudden windchange leads to huge coriolis forces on the rotor and subsequently the blade roots. So strong sometimes that blades get wrenched clear off their attachment points and tossed away like spears. This is not good. The larger the windmill the more pronounced this gets, because these forces are directly related to prop diameter.

My machine is an 'upwind', 5 m prop machine, I don't have excess power to run a servo. So my choice is pretty much dicated, an active tail of some sort.

Active tails come in many shapes and sizes, chain drive, shaft drive, belt drive, you name it, it probably has been done.

Here's a new one: A few days ago I was toying around with a pair of stepper motors and connected them 'back to back', and noticed that they generate so much power that the one will turn the other, totally in lockstep (and vice versa). Quite a bit of torque too, not as much as I was putting in, but pretty decent.

Now, if I place one of these motors on my tail, with a little fan attached (90 degrees to the rotor), the other on a bracket around the yaw bearing, with lots of downgearing I might be able to crank the machine around the tower automatically whenever there is a sidewind, nice and easy, no coriolis forces to damage the rotor.

Let me know what you think of this, and if it is impossible why (better to get shot down early than to try it and fail, there is a lot of work tied up in this machine).

[DanB]

Just my opinion... and you probabably realize at this point I always lean towards simplicity and resist change ;-)

"The passive tails are the most common in homebrew machines. From what I see they are simple, reliable, can be integrated with the furling system and require little or no maintenance. That's all really good"

Yes... I agree!  It's wonderful...

"But they do have one drawback. At high rpm a sudden windchange leads to huge coriolis forces on the rotor and subsequently the blade roots. So strong sometimes that blades get wrenched clear off their attachment points and tossed away like spears."

I wonder...  if that can be a non-issue if the hub of the blades are strong.  There are a few factors there... if the alternator, and hub is heavy then the centrifugal forces make the machine resist moving.  True - there are some intense forces on the tips of the blades, but Im not sure if its a big issue.  Tail size is also a factor here.

With the gravity furling tail system, the tail can swing in 1 direction (when it furls).  On very gusty windy days - Ill notice times when the wind changes suddenly and hits the tail - the blade is spinning fast, so it doesnt wanna yaw quickly.. so the tail simply gets lifted.  Of course, it only does this in one direction.  My guess though, is you could use a furling tail, and it would be simple and it would work with a 5 meter machine so long as the tail is not oversized and the hub is strong... and the outer parts of the blades are not unreasonably heavy.  Lots of weight in the alternator would make it resist fast yaws too I think.

 This is not good. The larger the windmill the more pronounced this gets, because these forces are directly related to prop diameter."

Im not sure they are directly related to prop diameter... since larger props run much more slowly and tend to have much larger hubs, but I could be wrong.. I'd have to think/read about that statement.  I notice my smaller windmills get slammed around a lot harder it seems, the larger ones rarely move quickly.

If you were the type who felt comfortable making active electronic controls (I guess there are simpler mechanical ways to do it perhaps) that would be totally reliable for years.. perhaps I'd go for it.  The furling tail system has 1 moving part and it's been used on machines much larger than 5 meters, although I cannot say I know how well they worked.  There are definitely some strange and undesirable forces involved with it - but Im not sure if they are strange, or undesirable enough to really be an issue.

If your not having a pitch control of some sort to govern the speed, I think the furling tail is the best bet... it should furl fast enough not to overspeed.  I think if you were turning it out of the wind with  a motor - you'd be taking some chances.

[JB]

any kind of furling is better than none. That is from experience. Ive seen those antenna rotators people have put on them also to turn them out of the wind. I use the gravity systym. The wind comes from too many directions . I dont want to baby sit mine more than I have too. If something goes wrong with the electronics plus the extra weight of the servos and if it gets out of hand you might have to shoot it and put it out of its misery. They can get spooky. JB

[jacquesm]

I think there may be a bit of a misunderstanding here, I understand about furling, I have a variable pitch prop that is adjusted by centrifugal forces:

Also, the steppers don't need any electronics, one stepper drives the other directly.

[Flux]

You could use the steppers, but you will need a lot of gearing to produce a slow yaw, I should think that a worm gearbox between the fan tail and the yaw would be simpler and cheaper.

You could still use a servo, there is no need to move it continuously, this is introducing the trouble you are trying to avoid. If you move it every time the average wind direction moves more than 30 degrees that would be good enough. A well geared windscreen wiper motor would be big enough and the consumption would be negligible.  It would also enable shut down, which would be wise until you have proven your pitch control to be reliable.

If you are on a very turbulent site, I share your worries about the gyroscopic force  but if you are well above obstructions a simple tail ought to be ok, I doubt that the gyroscopic forces are much greater than the wind load bending force on a pitch controlled blade if you pitch to feather, if you pitch to stall, the wind loading will almost certainly be the limiting factor.

Jacobs managed with a tail and as DanB said, the larger machines tend to be less frantic in their yawing.  The tail can be a lot smaller than with a furling scheme and you may be able to arrange a power steering pump or something as a yaw damper.

I would make sure you can stop the thing either with electrical braking or some way to turn it out of the wind

Flux

[jacquesm]

Thank you flux, that's execllent input. My site is not very turbulent (clean in all directions for 1000 ft except for the southeast where my house and some trees are situated, they are 150 ft away from the tower. The tower is on a hill, and it's 60 ft high, the house projects about 10 ft above the top of the hill, the trees a little bit more.

A yaw damper is a good idea. The main reason why I'm worried about coriolis is that I have never built blades before, and even if the little test blade I have made seems quite strong I figure the attachment point near the root (where the foam ends and the 'hard' stuff begins) is the weak point. I intend to use some 'fingers' to ease the transition, but my worries remain.

In the beginning I intend to run the machine with a very light setting on the spring that controls the governor, to make sure everything works and to keep RPM down, then as I get more confident I'll crank it up, eventually limiting the rotor speed to about 270 RPM. The blades can be turned 20 degrees, initial angle of attack is 4 degrees, so with the governor at its maximum the tips will be at 24 degrees attack, the root will be at 44.

Worm gearboxes are expensive, I checked into that and they go for about $180 a piece, and are pretty heavy. That's why I was looking for alternatives, a buddy of mine has a chain drive active tail on a 3Kw machine, but it's very ugly and needs a lot of  maintenance (that chain is exposed to the weather).

I'll have a 'short out' switch at the bottom of the tower, my 2 Hp driven lathe will cut out at even the lowest gearings when I short the windings directly, but I'm not sure how that will work with 60 ft of cable attached to them. They are pretty big blades after all, and I really wouldn't want to fry my windings.

So, passive with a pump as an oid damper is a good alternative to an active tail, the only possible failure point that's got is if the pump loses it's oil but you will notice that soon enough because of the increased speed of rotation.

I'm trying to design this thing in such a way that parts will be easily manufacturable by whoever wants to make one, the basic idea is 'raw materials in', 'windmill out'. No exotic stuff, other than the blades, and they could be replaced with a wooden alternative without a problem.

That's why I don't like gearboxes and such, specific brands of driveline components are not always easy to get, it was hard enough to pick bearings that are available everywhere.

A power steering pump on the other hand has almost the same design everywhere, even between brands they are sometimes interchangeable, so I don't see too much of a problem with that.

[jacquesm]

I *do* have variable pitch blades, and no electronics would be needed at all, just two steppers windings of one connected to windings of the other.

I see your point about the reduced RPM, but the blades are 8 times as heavy for a 2 times increase in size !