furl point adjust?

[adelaide]

wondering if any one has tied up a tail and pushed back on the centre of the hub with scales, and using figures from say Altons wind calculator thrust column to adjust furl point, has any one tried this to get the furl correct ?

Did you tend to need more or less force?

http://www.alton-moore.net/wind_calculations.html

[Flux]

Nice idea but I doubt if it will get you very close.

The prop has a tendency to seek the wind and this will make it furl later than thrust will predict. If your offset is large you may be near.

If your offset is too small it will never furl. The seeking force depends on the type of prop and is much greater with some than others.

Machines with an offset near the low limit tend to furl suddenly and power drops drastically when they do. Those with larger offset tend to hold much more constant power when furled.

Simple theory isn't good enough to predict some of these things accurately but it gives you a starting point.

Flux

[PaulJ]

   "Machines with an offset near the low limit tend to furl suddenly and power drops drastically when they do".

   Aha, the penny just dropped here, thanks. I have a 2.7m diameter mill with a 125mm offset and have observed just this behaviour. Is this what you would consider a minimal offset?

   I'm not complaining, it works pretty well in moderate winds which we get here most of the time. I'm definitely missing out on the icing on the cake in high winds though, and my next mill will have a bigger offset I think.

   The prop seeking the wind is a bizarre thing to watch, my mill can have the furling tail off the stop and seemingly flapping around almost randomly within a 30 degree or so range while the prop tracks the wind perfectly (in moderate winds). If I could somehow remove the tail in these winds I'm pretty sure the mill would continue to operate mormally.

   Any explanation of this behaviour would be appreciated; I guess it must have something to do with the forces on the blade tips?

   Thanks,

   Paul.

[Flux]

Paul , you are at about 4% and that is normally safe and it seems as though yours does just furl and protect itself. A little more offset would make it better natured.

The effect seems to be related to the tsr at furl point, I suspect that you are well clear of stall at furl point, machines that are somewhat bogged down do tend to furl easier.

You are one of the few who have actually realised that the tail can bend to a considerable angle with the machine pointing straight into the wind. with much less offset it would never furl.

I suspect you will see the tail bend to beyond 30 deg before anything happens , then a gust will pull the prop round out of the wind and it will slow down.

You are quite right, when it is in the seeking mode it will hold into the wind without a tail for quite long periods.

I will have to let others explain the seeking force, but it is real and somewhat unpredictable. Some props show little tendency to seek the wind at all.

Flux

[paradigmdesign]

ok, other than wasting materials, and affecting the wind with an oversized nacell, is there any max offset that one should be worried about? Or is the more the better?

[Flux]

No not more the better, but this is a tricky one.

Even at 4% the tail needs to be much larger than is necessary for a non furling machine. Normally 4% is adequate but it is not an easy thing to change once the decision is made and it proves to be wrong. Perhaps 5% should be a better figure.

Rather than make the tail excessively big I tend to set it a more of an angle to keep things into the wind below furling but it does look odd and makes people ask why it at an angle.

This is another problem of going your own way, it may not work out first time. If you follow proven ideas then they will have sorted this out.

flux

[PaulJ]

   "Machines that are somewhat bogged down do tend to furl easier" - this statement seems quite counter intuitive at first, as surely a stalled or partly stalled prop will have less thrust and therefore less of a push against the weight of the tail, making it harder to furl.

   Thinking about it though, and from my own observations of my machines, the wind seeking effect seems to be at its strongest when the actual TSR (i.e. the TSR of the prop/alternator combination in a given windspeed as opposed to the design TSR of the prop) is high. So, with a partly stalled prop, the furling "push" is reduced, but the wind seeking effect is reduced even further, resulting, as you said, in easier furling. (Whew! Does that make sense?) Interesting stuff!

   "I suspect you will see the tail bend to beyond 30 deg before anything happens, then a gust will pull the prop round out of the wind and it will slow down." Spot on, that is exactly what happens. If I'm hearing you correctly, a bigger offset with a heavier tail would see the furling occur more gradually with less power drop in high winds?

   I was watching my 2.7m machine a bit today as it was seeking the wind independently of the tail, and noticed turbulent sounds coming from one side of the prop or the other  when the wind changed direction slightly. These noises (a bit helicopter-like but nowhere near as loud) stopped as soon as the prop was centred on the wind again.

   This got me thinking, and I've come up with a theory :

  When the wind shifts on a prop spinning at the right speed, the airflow stalls slightly at the tips on the side of the prop that is now slightly upwind (hence the "helicopter" noises). As a result of this, the thrust of the prop reduces on the upwind side and the greater thrust on the downwind side then pulls the prop back into the wind, so the prop is inherently wind seeking at certain speeds.

   Paul.

[Flux]

Paul

Yes I think with a slightly bigger offset and heavier tail you would find that the output held up better in higher winds.

It depends on the prop, the offset and to some extent the angles used in the tail construction, not so much the angle from the vertical but the position of the tail pivot in relation to the wind direction. This alters the way the weight of the tail reacts against the thrust. The controlling force increases at first as the tail lifts and then falls. The point of maximum force depends on these angles.

I have tied a rope to the tail when wind seeking and you can pull it round to about 40 deg without significant effect on the output, then suddenly it makes those helicopter noises and the seeking torque vanishes. It seems an unstable point and by manually furling the tail it is near impossible to run it at about half load.

If you get a chance to look at the power curves of Bergey and AWP you will see that the furling characteristics are totally different. Bergey has a small offset compared with AWP but the prop types are also very different.

Flux

[Smithson]

     What happens if you are worried about the strength of the prop sandwiched between 2 1/2 inch pieces of plywood and add a 1/4 inch plate to the prop?  Do you need to add a similar weight to the tail?

     Also if instead of a 1" pipe inside a 1 1/4 pipe for the tail pivot you use a bushing to make it pivot easier does that effect the Furl?

     And would it help or hinder if bushings were used in the yaw?

     My tail boon is 5' 6 " [ for the Scoraig plans] and I assumed that would be ok as I added a 12" disc on the rotor, as well as a metal nose cone to strengthen the prop.

     I'm finding there is more to this side furl then first thought.   Thank for any comment.     Smithson    

[Flux]

Adding weight to the prop or the alternator has no effect on the furl. Adding weight to the tail does as it alters the controlling force keeping it into the wind.

Adding pretty bearings makes it a nice engineering job that commercial manufacturers are more or less forced to do, but it usually makes it much more prone to yaw about in gusts. There seems to be significant benefit from the frictional damping from simple pipe on pipe bearings.

The only thing I would consider is a steel washer on top of the yaw pipe to increase the area and perhaps a brass washer between that and the yaw head to act as a simple thrust bearing. I have often used a tapered roller bearing here but it works better without it.

Flux

[coldspot]

"The only thing I would consider is a steel washer on top of the yaw pipe to increase the area and perhaps a brass washer between that and the yaw head to act as a simple thrust bearing. I have often used a tapered roller bearing here but it works better without it."

So the friction helps with NOT letting the head swivel as fast or easy?

Thats to bad, I was going to be using one of these in my dual rotor head sey-up.









I had hoped that they would HELP, but if not I'll be using them in a VAWT.

On my Ametek 30 V DC mini mill I used three bearings for YAW and it does chase around looking for the wind very easly but it does have a small tail.

[PaulJ]

   Well, I just had to try it. Seeing if it would track the wind without a tail, that is.

   The method: remove the furling tail, and hook a loop of rope around the tail pivot. From the ground, use the rope to pull the prop into the wind until it picks up speed, then gradually reduce tension on the rope and see if the mill will stay facing the wind, using the nearby 1.8m mill as a wind direction indicator (and taking care that I couldn't get tangled up in the rope if it got caught in the prop).

   The result: strange looks from the wife, and no, the mill did not track the wind by itself. The experiment was flawed though, I'm not sure it was quite windy enough for the seeking effect to be fully operational, although there were definitely a few times when it would have been windy enough for the tail to be off the stop by a few degrees. There definitely seem to be forces pulling the prop into the wind, as it required less tension on the rope once it was flying properly. Releasing all tension saw the prop gradually turn out of the wind. If there were no seeking forces I would have expected it to pull away from the wind much faster.

   I might give it a go when its a bit windier, but I'll have to rig up a relay and a remote shutoff switch so I can stop the mill fast if anything goes wrong.

   Conclusion: don't know! The tail is certainly having more of an effect when it's off the stop than I would have thought, though.

   Paul.

[Flux]

Paul

With your offset It think it will always furl but under some conditions it will take a while. You would have to reduce the offset below critical value to keep it up wind without a tail, but you have seen that even in your case there is not much of the theoretical thrust available at that point to cause it to furl. You might find a critical speed at which it does stay up wind.

Flux

[Smithson]

     Flux.  Thank you.   I forgot to ask in my question about spacing the rotor out about 12 inches from the yaw.  Would that effect the furl?  I noticed that all the designs seem to have the rotor close to the yaw.  Even if you tilt it back wouln't there still be the danger of striking the tower?  On one blade I made the weight of the power sander bent the blade at the tip.  If you could do this by hand wouldn't the wind do the same?  So in furling you answered the question about adding weight to the rotor but would spacing the rotor out from the yaw and putting a bushing in the tail pivot effect the yaw moment?     Thank you again   Smithson

[Flux]

To be honest I am not sure what effect bringing the prop forward of the yaw pivot has.

I don't think that within moderation it makes a lot of difference but I would not want to take it to extremes. I don't think a foot would matter. Normally the thickness of the alternator and a bit of tilt up is enough to keep the blades clear.

Some of my early fast 2 blade props did seem to bend back a fair bit at the tips but I haven't noticed it much on the recent slower 3 blade ones.

Flux

[Ungrounded Lightning Rod]

I would expect positive feedback on furling with the prop further out.

When turning away from the wind the prop's center of effort would move further from the yaw axis, increasing the furling force.  (This might help explain the abrupt furling described above:  Once it starts to go, it gets more force encouraging it to go.  Then again, as it starts to slow it loses gyroscopic resistance to furling, too.)

This would also tend to reduce the stability of the mill when seeking the wind.

Since the stalling of the prop as it turns away from the wind reduces its drag, and whatever the "wind seeking" mode is (one-sided stalling?) provides a centering force, both of which act oppositely to the effect I described above, there may be an opportunity to tune tracking and furling behavior by adjusting the prop's forward offset.