Author Topic: Furling - Rotation direction vs. offset side (Read 3770 times)

Dave B · « **on:** December 05, 2007, 11:55:20 PM »

I remember doing this experiment in school and it got me thinking about our gravity type furling set up. http://science.howstuffworks.com/gyroscope1.htm

Also, I remember as of late Dan B's recent machine was offset to the left side (when looking at the front.) Mine is offset to the right and the rotor rotation looking at the front is clockwise. This experiment indicates that the Precession or desire of the direction of yaw when looking from the top down will be the same direction as the rotor rotation as observed from the front. In other words, in high winds I want my yaw to be counter clockwise looking down from the top how ever my rotor rotation is clockwise. This would indicate that my furling in high winds is fighting the proper direction of yaw (or wanting to stay into the wind). This could be a significant issue I would think. Dan B or anyone, do you have any comments on this ? I would think we would want the rotor rotation to be the same direction as the preferred yaw direction when furling instead of Precession fighting it. I have heard some call this "seeking force". Interesting for sure. Dave B.

snowcrow · « **Reply #1 on:** December 05, 2007, 08:07:16 PM »

Hey Dave B, I think you're right!!! The prop should be turning in a counter-clockwise direction. It might worth doing the bicycle wheel thing on a string yourself before carving blades!!

Blessings, Snow Crow

disaray1 · « **Reply #2 on:** December 06, 2007, 06:27:36 AM »

I'm surprized more builders aren't chiming in on this subject. I've wondered about this also. On a mill with the offset to the right of the tower stub, wouldn't the prop rotation make a difference in seeking force and furl?

David

clflyguy · « **Reply #3 on:** December 06, 2007, 06:46:13 AM »

Hey Snowcrow,

Absolutely- The direction of rotation needs to be determined very early in the

planning stage, as it dictates the shaft/furl tube location-side in relation to

the pivot. Do the bicycle wheel experiment and spin it in the direction you want

your turbine to spin, it will precess toward the side that you will want your shaft on in relation to the pivot and conversely, away from the side that the furl tube

needs to be on. Bear in mind also that when furling in the precessional direction,

the blades move away from the tower at the (gravitational) bottom of their arc

giving you a little added clearance during severe yaw conditions.

If someones turbine is wind seeking and exhibiting curious furling behavior that

tail size/weight/length mods don't seem to give predictable results from, then chances are pretty good that their shaft/furl sides are backwards for the choice of

rotation.

-too much fun- Gus

dinges · « **Reply #4 on:** December 06, 2007, 07:10:42 AM »

Googling yields http://www.fieldlines.com/story/2007/7/27/154838/185

A more extensive search is left as an exercise for the reader.

Peter.

snowcrow · « **Reply #5 on:** December 06, 2007, 08:45:58 AM »

You know, this could probable solve the infamous stator burn-out problem people have been dealing with!!!! The faster the rotation is in the wrong direction becomes, the great the force against the normal furling action!!!

Blessings, Snow Crow

Dave B · « **Reply #6 on:** December 06, 2007, 12:09:40 PM »

To those who have mentioned this before I apologize for a less than thorough search. With my latest issue of my blade strike on my 18' and my thoughts previously on my 12' basically overspeeding this experiment hit me broad side. I am not stating that this is an issue with all the machines out there built with the offset on the right looking at the front and clockwise rotor rotation looking at the front but I believe it could be something to consider. If so and it makes a significant difference in furling performance and predictability then it's too bad it is such a major modification to change to see if it would be helpful to those who maybe have had overspeed, burnout, and or just plain weird furling action that has been tough to work out with weight, length, tail area etc. I am fortunate, I believe Dave Moller (my fabricating friend and blade carver) can carve my next set of blades with opposite pitch for counter clockwise rotation, if not we're sure in for some work changing my offset to the other side, I will be changing things for sure. Maybe for Dan B. it will prove a fortunate coincedence that he has changed his offset to the other side also. Dave B.

Dave B · « **Reply #7 on:** December 06, 2007, 12:55:33 PM »

It looks like Dan B has changed his blade rotation to counter clockwise looking at the front now also. I guess with his offset changed to the left now besides it seems as if his could be precessing against the furling also only in the opposite direction. There are so many variables this may not be an issue for many but for some I think it might be something to consider especially if you are having a tough time adjusting the furling. Just my thoughts, I will make a change and post updates as I go. This could be a while, outside I don't get as much done in the Winter. Dave B.

Royalwdg · « **Reply #8 on:** December 06, 2007, 02:08:31 PM »

Hey Dave, I haven't talked to you since the day after "the incident" but have been watching everyones comments on blade rotation and furl direction. If you look at the generator from a bats eye view(top) or the rats eye view (from the bottom) you will see both counter-clockwise furl and clockwise furl with the same blade rotation. Gravity would be the only monkey wrench in this study. The faster the rpm the less gravity would affect the condition. Am I missing something you guys have seen. It seems that there should be no difference.

Experimental · « **Reply #9 on:** December 06, 2007, 05:25:42 PM »

Hi Folks,

One thing I think you may be confused on -- it isen,t the tail that take,s your mill out of the wind -- it,s the force applied to the blades thru the fact that you have offset, that makes the blades turn out of the wind !!

The tail "weight" has a restoring effect to the blades , when the wind force against the blades, allows it to turn back into the wind...

Gyroscopic effect only trys to keep the axis, to remain in the same plane it was in (resists being turned, either left or right)

This is how a DG -- directional gyro in an aircraft works...(artificial compass)

Spinning clockwise or counter clockwise, makes no difference, it will still resist being turned left or right !!!

Wish all , happy holidays and happy building, Bill H

Dave B · « **Reply #10 on:** December 06, 2007, 09:04:27 PM »

Bill,

Yes, we wish to have the blade force of the wind push the rotor about it's yaw axis and the tail be the restoring force to keep it into the wind. This until the wind force against the blades over comes the restoring force of the "gravity tail", hence furling. I also agree that gyroscopic force will resist rotation left or right of the axis of what is spinning no matter clockwise or counter clockwise rotation. Precession enters another axis, vertical in this case as demonstrated in the video. It is the direction of rotation about this axis that will be dependent upon the direction of rotation of the spinning wheel. Dave B.

Capt Slog · « **Reply #11 on:** December 07, 2007, 02:51:19 AM »

Perhaps I'm missing something, but I can't see what difference it makes.

The experiment in the video works because the wheel is only supported by one end. Because of this, the opposite end of the axle is allowed to drop and it is in response to this drop that the axle then precesses. In response to this precession, in turn the gyroscopic forces in the wheel keep the wheel upright.

Surely in a wind turbine situation, this doesn't occur. It is true that will be a force experienced on the shaft as the spinning mill yaws one way or the other, but this will be either be up or down (seemingly in a plane parallel to the tower) depending on direction of spin.

This becomes a problem if the axle is worn (and indeed is probably partly responsible for the wear) as this causes the blades to tilt in towards the tower.

But as being responsible for not letting the mill furl on the basis of that video, sorry, I can't see it.

clflyguy · « **Reply #12 on:** December 07, 2007, 08:23:22 AM »

Dear Capt. Slog,

It really does apply sir. The rotational mass is in front of the pivot, and only supported at one end. A perfect scaled up example of the bicycle wheel experiment.

If you were to eliminate the effect of air pressure against your turbine (windmill

in a vaccum chamber) and spin the blades with a motor, you unit would pivot away from

the direction the blades were spinning at the top of their arc. Gus

DanB · « **Reply #13 on:** December 07, 2007, 08:46:53 AM »

I don't think it makes much difference at all...

in one direction, furling will tend to pull the blades towards the tower which may be a minor issue, but yawing will also pull the blades towards (or away) from the tower. My choice to turn counter clockwise was arbitrary - and my machine being setup 'opposite' of most others (with the alternator on the other side) has only to do with the fact that it was a large machine - I knew it would be in my shop for a long time and it had more to do with the 'layout' of my shop (and fitting it in here) than anything else.

Capt Slog · « **Reply #14 on:** December 07, 2007, 08:53:21 AM »

It is supported at one end, BUT not in the same manner as the bicycle wheel, the end of which rests on a fulcrum, and that is a major difference.

If you were to clamp the bike wheel axle in the same manner as a wind turbine axle and repeat the exp, there wouldn't be the precession that you see in the original. The precession only occurs when the wheel tries to drop under gravity.

Flux · « **Reply #15 on:** December 07, 2007, 10:54:01 AM »

Dan is right, the only difference it makes is that one way the blade tips are thrown towards the tower during a furling yaw and the other way it is the unfurling that throws the tips towards the tower.

If you have a situation where gusts drive the thing rapidly into yaw then the other way from the normal construction may be slightly better, but during violent turbulence the tail is just as likely to slam it away from the furl direction.

It is with vertical furling that things really screw up as the gyroscopic force aids or bucks the furling force. and not just adds pivot friction as with a side furl unit.

Flux

Dave B · « **Reply #16 on:** December 07, 2007, 11:52:07 AM »

Flux,

Could you explain what you mean by "prop seeking force" ? Maybe I am getting this confused with precession. Is there prop seeking force without an offset vertical axis on which to pivot ? Does the direction of rotation of the prop change the direction of the prop seeking force ? I realize a blade profile is different than a solid disk when spinning, there is the air resistance to consider. In fact I may be on to something here. If you spin a blade set that is designed to rotate clockwise when wind blows against the front you are looking at then if there is no wind blowing what you have created is a fan. The air displaced will blow toward the back. This will want to move the blades forward (if the offset is to the right.) This would simulate precession. So, if the wind is really blowing and our rotor is spinning clockwise... then the wind stops blowing but our rotor keeps turning, we have now created a fan blowing air to the rear and the rotor wants to pivot "toward" or against what was the direction of the wind. I would call this "coasting seeking force" and this would simulate precession as shown in the video. Interesting stuff for sure. Dave B.

Flux · « **Reply #17 on:** December 07, 2007, 12:24:45 PM »

I can't really explain this properly.

In most respects a spinning prop behaves as a disc and the thrust on it could be related to that on a smaller disc.

There is another phenomena with a spinning prop that tries to keep it pointing directly into the wind. I think it comes down to the same thing that makes a vertical axis machine work on both sides when one blade is going what looks to be the wrong way.

Under the right conditions a horizontal axis up wind machine will keep running up wind without a tail. The condition is unstable and when it reaches a critical angle the seeking force vanishes and the thing turns down wind.

With the usual alternator offset type furling scheme, we normally think of balancing the torque due to thrust about the offset against the tail steering moment. If there is a force trying to keep the prop into the wind then this seeking force needs to be subtracted from the thrust. If the seeking force exceeds the thrust then it will never furl. Normally there is a state up to about 30 deg to the wind where there is no significant reduction in power and the thing looks to be furling with the tail partly bent. Beyond this angle the seeking force drops off and the then thing becomes dominated by the thrust and true furling takes place.

Changes in operating conditions can sometimes delay furling to a high wind speed or cause it to fail completely.

Because the seeking force vanishes at some angle, most furling schemes cause a significant drop in power in the higher winds. Unless you produce controlling forces by more complex methods than a hinged gravity controlled tail, I strongly suspect that any machine that doesn't drop power in high winds is failing to furl.

Under turbulent conditions a furling machine will occasionally get caught out and deliver a high burst of power but the general power level should fall significantly when the tail goes beyond about 45deg.

This is an aerodynamic issue and is not related in any way to the gyroscopic things that others seem to be in a muddle about.

Flux

Dave B · « **Reply #18 on:** December 07, 2007, 03:33:11 PM »

Let me try again. You have a blade that will spin clockwise looking at the front (tail furthest away) when the wind blows. Stop the wind but now observe the spinning blade. The pitch is such that you now have a fan blowing wind toward the tail. The counter force is trying to move the blades forward (screwing itself forward as any prop would) If the blade axis is offset to the right of the vertical yaw axis then the tendency will be for the blades to yaw forward (clockwise looking down). Different profile blades could act differently I'm sure. This is all aerodynamic stuff I'm talking about here that in my case seems that it could fight against furling also. I think the precession argument will go on for a while but my vote is that that gyroscopic force makes a difference also, CCW or CW blade rotation VS. offset side to help or hinder furling. Maybe taking a pole would be interesting ? Dave B.

jmk · « **Reply #19 on:** December 07, 2007, 07:07:29 PM »

Just build two tiny machines toy size and test the theory.

Flux · « **Reply #20 on:** December 08, 2007, 01:40:06 AM »

Gyroscopic force only occurs during change of direction. At best any effect would only slow the furl, it will have no long term effect.

I haven't even noticed any transient differences in furling with a reversed rotation blade, but things are so turbulent on my sites that it wouldn't be noticed.

I think I may have over simplified things when I said that the seeking force is subtracted from the thrust. To be honest about it, I don't think there is a thrust component involved, just a tendency for the prop to want to run symmetrically about the air flow. The larger the offset the less your chance of it remaining up wind with no tail. With no offset it may run for a long time up wind with no tail. Even down wind machines sometimes get caught out and run up wind.

At best furling is a rather imprecise method of control, simple and adequate in most cases, but mainly suited to small machines. It will never compare with a well designed pitch controlled machine. Normally it is good enough as long as the alternator dominates the situation or where the alternator is self protected by reactance limiting.

With your heating control I still think that you will need to slam on a large load in higher winds to pull the blades below the peak of their power curve if you want a defined upper limit on speed.

Someone here was discussing a grid tie battery connected scheme and found that by clamping the volts you can induce stall and improve control, this is effectively doing the same thing.

Flux

TomW · « **Reply #21 on:** December 08, 2007, 07:07:50 AM »

Flux;

With no offset it may run for a long time up wind with no tail. Even down wind machines sometimes get caught out and run up wind.

Heres one that was even weirder. My 2 meter [6.5 foot] Zubbly [R.I.P.] conversion did an about face in gusty conditions. It ran tail into the wind for a good 10 minutes. Pretty weird to see and I would be skeptical it was possible if I had not seen it myself. It did not run very well but it did run.

Just a report on the odd things than can occur that make little logical sense.

Cheers.

TomW

jmk · « **Reply #22 on:** December 08, 2007, 08:07:00 AM »

Yes, the blades spin to slow when backwards. Plus they spin backwards too. Once in a great while the wind gets calm and when it starts back up, usually in the next morning, and when the wind is 180 deg from where it was, my mill will try to work backwards till the tail catches the cross wind and spins it back around. It was wierd to see the first time.

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Author Topic: Furling - Rotation direction vs. offset side (Read 3770 times)