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DaViD

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Joined: 14/01/2009
Location: United States
Posts: 120

Posted: 03:57pm 04 Mar 2009

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I've been talking to a physics prof. from our local college and he is amazed by this and has asked me if the wind started back up and continued to spin the prop. backwards? and if not how long did the prop. continue to spin until it came to a stop?

If your not living on the edge your taking up to much space!

GWatPE

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Joined: 01/09/2006
Location: Australia
Posts: 2127

Posted: 11:29pm 04 Mar 2009

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Hi DaViD,

The wind did not actually stop fully. The precessional forces turned the rotor around to face backwards to the normal wind direction, and the blade continued to keep spinning.

This does not happen with the wooden blades I made, that are beeing tested currently.

The chinese blades are of very thick airfoil section, and almost symetrical profile, with only a slight taper and twist.

Gordon.

become more energy aware

DaViD

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Joined: 14/01/2009
Location: United States
Posts: 120

Posted: 12:00am 05 Mar 2009

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Hi GWatPE,I'll pass that info. on to the prof. when I talk to him and see what he comes up with and let you know on this thread. He seems really interested in why it happened.

If your not living on the edge your taking up to much space!

SparWeb

Senior Member

Joined: 17/04/2008
Location: Canada
Posts: 196

Posted: 09:17pm 05 Mar 2009

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David,
Remember to mention that the prop was unloaded, because the generator was open-circuit at the time. That's the key to understanding what happened. Not impossible, but very improbable.

Steven T. Fahey

DaViD

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Joined: 14/01/2009
Location: United States
Posts: 120

Posted: 09:21pm 05 Mar 2009

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Hi SparWeb, do you know the pitch or angle of attack on those blades?

If your not living on the edge your taking up to much space!

Bolty

Regular Member

Joined: 03/04/2008
Location: Australia
Posts: 81

Posted: 11:57pm 05 Mar 2009

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Hi David!
I am the proud owner of the turbine in question. I thought that it may be best if I gave you the information direct. The blades are 1.3m radius fibreglass that weigh 2.7 kg each. They are of Chinese manufacture and were part of my 1KW turbine that I imported. The tips of the blades are very flat having possibly 1 degree of positive pitch. The blades use an aerofoil section that is not far from symmetrical. To expand the area of the blades, I have 3 solid steel plates 0.5m long on each blade (at the centre) These extenders weigh about 4 kg each, making the total weigh of blades and extenders about 20kg (not desirable to be this heavy, but an experiment in the effect of extenders) I do not have my camera available at the moment, but if needed I could post photos later!

The only time that they turn around and downwind, is after a stong gust that accelerates the blades to a high angular velocity. Then and only when the wind drops, it appears that the precessional forces cause it to rotate around (since the tail no longer has sufficient force to keep it into the reduced wind). It then stabilizes as a downwind machine, with the blades continuing to spin in the same direction that they were before turning. ie at no time do the blades stop spinning and turn in what you would expect to be the correct spin direction. Another way of stating this is to say that the leading edge of the blades now becomes the trailing edge??? It will continue to spin in this annomolous state and continues to generate power to my system, even though it is probably at a reduced level.

The phenomonen has been observed with no extenders, and to a much greater degree with the extenders. Due to the extra weight with the extenders, the moment of inertia is clearly a factor, as it appears to increase the precessional force. At present there are lightweight wooden blades on the extenders, and it no longer exhibits the turning downwind. The wooden blades are also much more profiled. ie they have a very significant twist and are much more asymmetric in aerofoil cross section.

If you need any more info, I would be pleased to provide it!

DaViD

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Joined: 14/01/2009
Location: United States
Posts: 120

Posted: 09:40pm 06 Mar 2009

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Hi Bolty I'll pass that info. along to him. We are supposed to get together and talk this coming monday so he might have some info. on what he thinks could cause it right off the top of his head he usally does that when we shoot the crap over coffee.

If your not living on the edge your taking up to much space!

GWatPE

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Joined: 01/09/2006
Location: Australia
Posts: 2127

Posted: 10:52pm 06 Mar 2009

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Hi sparweb,

I would not say that the mill was open cct, but merely the loading was not engaged. This is a grid tied system. There are times when the inverter had not connected to the grid yet and the winds are variable, so the mill was not correctly loaded. This is not like a battery connection.

On some occasions, with the blade extensions, and the chinese blades, the rotor was seen to turn and face the wrong way and the inverter was grid connected. This suggests that the precessional forces with the higher inertia in the rotor was the contributor.

The tail may need to be sized to allow for rotor inertia as well.

Gordon.

become more energy aware

SparWeb

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Joined: 17/04/2008
Location: Canada
Posts: 196

Posted: 07:22pm 09 Mar 2009

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[quote=GWatPE]
I would not say that the mill was open cct, but merely the loading was not engaged.[/quote]

I guess you had to be there. If the tail is too small, then it swings around. If the blades work backwards, then it can turn backwards. There's too much mental gymnastics required to imagine which way the air goes in this state, and I must be caffeine deficient this afternoon because it isn't coming to me...

Even if the trailing edge is knifing through the wind, the airfoil can still produce lift if it has a positive angle of attack. Reverse flow on an airfoil works, that's how VAWTs with blades work, for example. Weakly, but not zero.

Steven T. Fahey

DaViD

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Joined: 14/01/2009
Location: United States
Posts: 120

Posted: 03:09pm 10 Mar 2009

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------------------------------------------------------------ --------------------

Well fellas I guess I've opend a subject that has a lot of professors grinding their teeth and scratching their heads. There is a prof. from M.I.T supposed to call today. This guy should know he is a teacher and lecturer on rotordynamics design and development.

If your not living on the edge your taking up to much space!

Bolty

Regular Member

Joined: 03/04/2008
Location: Australia
Posts: 81

Posted: 11:26pm 11 Mar 2009

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Hi David.

This is a very interesting phenomonen to observe. It certainly has had Gordon and myself thinking!

I hope that I have not incorrectly given an impression to you due to what I now realize was a mistatement!

"It then stabilizes as a downwind machine, with the blades continuing to spin in the same direction that they were before turning. ie at no time do the blades stop spinning and turn in what you would expect to be the correct spin direction. Another way of stating this is to say that the leading edge of the blades now becomes the trailing edge??? It will continue to spin in this annomolous state and continues to generate power to my system, even though it is probably at a reduced level."

The part of this statement that I now realize is incorrect is "Another way of stating this is to say that the leading edge of the blades now becomes the trailing edge??" On reflection this is clearly wrong!!! It conflicts with the adjoining statements ("ie at no time do the blades stop spinning"

If I have confused you with this I sincerely apologize. At the time that I wrote the above, I was not thinking clearly about the spacial alignment.

To summarize more clearly the following rewritten paragraph may help.

The only time that they turn around and downwind, is after a stong gust that accelerates the blades to a high angular velocity. Then and only when the wind drops, it appears that the precessional forces cause it to rotate around (since the tail no longer has sufficient force to keep it into the reduced wind). It then stabilizes as a downwind machine, with the tail pointing into the wind. The blades continue to spin, but of course as they are now downwind they are rotating in the opposite direction to what they would be with the wind from that direction (relative to the blades)ie at no time do the blades stop spinning and turn in reverse. Another way of stating the effect is that it simply appears as if the wind has quickly reversed direction and the turbine has followed the wind around 180 degrees. Of course there has been no change in wind direction. It will continue to spin in this anomalous state and continues to generate power to my system, even though it is probably at a reduced level.

I hope this has not confused you!

pancho43

Newbie

Joined: 27/02/2009
Location: Australia
Posts: 10

Posted: 06:35am 13 Mar 2009

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Hello fellers, the "pitch" of a propeller,windmill blade,or even a bolt is either right or left hand. if you turn it around to the opposite direction the "pitch" is still either left or right hand, the same as it was in the first direction, so it would still turn the same way with the wind in the same direction, but as previously stated "with reduced power". You cannot turn a right hand pitch to a left hand just by turning it around. Have a look at the thread of a bolt pointing away from you, on a right hand (normal) bolt the thread will wind from the lower left, rising to the right. Turn it around and it will still be the same.
Cheers, Peter.

Pete

GWatPE

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Joined: 01/09/2006
Location: Australia
Posts: 2127

Posted: 11:53am 13 Mar 2009

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Hi Pete,

I think we get what a bolt looks like, especially Mr Bolt above.

Gordon.

become more energy aware

DaViD

Senior Member

Joined: 14/01/2009
Location: United States
Posts: 120

Posted: 12:29pm 13 Mar 2009

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Hi guys, I am still waiting for the fellows to get back with me. They must
be banging heads trying to figure it out.
I was wondering in my little head (about the size of a marble) if it could
be possible that the extra weight and inertia when spun around 180 deg.
If it could cause an area of turbulance on the tail side that causes air to
slip around the prop. and then there would be enough turbulance on the
side of the prop. with the correct angle of attack to keep the thing
spinning at a reduced rpm? ( just an idea, slap some sense into me if it is
Dumb Thinking

If your not living on the edge your taking up to much space!

DaViD

Senior Member

Joined: 14/01/2009
Location: United States
Posts: 120

Posted: 03:49pm 13 Mar 2009

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Lets think of it this way, If a rock was sticking up out of a stream the water would start to flow around it ahead of the water that has pooled in front of it. The water would flow around both sides of the rock and swirl back upstream toward it on the other side causing an eddy current pushing leaves or debris back up against it causing force on the rock as the water traveled in the upstream direction?

If your not living on the edge your taking up to much space!

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