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Forum Index : Windmills : Q from Fundamentally Challenged

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montyLalor

Newbie

Joined: 17/12/2008
Location: Australia
Posts: 21
Posted: 12:46pm 05 Jun 2009
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Just a quick question about how blade feathering actually happens. Is this image how it works?



The arrow indicates direction of rotation (viewed from the outside looking in towards the hub).
"So are you doin' this project to make us money or cost us money?" she asks again...
 
Janne
Senior Member

Joined: 20/06/2008
Location: Finland
Posts: 121
Posted: 12:59pm 05 Jun 2009
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Hello,

No, The pictures you posted shows the blade being turned towards stall. Feathering is when you turn the blade angle steeper.

Turning the blade towards stall can be dangerous, because on the first few degrees the blade is being turned, power might actually increase, before the blade finally hits stall. Feathering is better in many aspects, with it the power limiting happens more smoothly, no danger of power increasing etc..
If at first you don't succeed, try again.

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montyLalor

Newbie

Joined: 17/12/2008
Location: Australia
Posts: 21
Posted: 01:33pm 05 Jun 2009
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Okay, thanks. I don't doubt what your saying as I know next to nothing, but you say that feathering (real feathering, not my stall image) turns the blades out of the wind in a smooth, predictable fashion, but doesn't the inherent increase of attack angle as the blades turn out of the wind increase the power and/or torque extracted from the wind as they feather? Or is my knowledge of aerofoils just that vague? Which I am okay with, I just need some help getting the preverbial penny to drop.
"So are you doin' this project to make us money or cost us money?" she asks again...
 
Janne
Senior Member

Joined: 20/06/2008
Location: Finland
Posts: 121
Posted: 02:07pm 05 Jun 2009
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Hi,

When you feather the blades, the angle of attack decreases. The angle of attack is the angle between the blade aerofoil, and the "virtual" wind direction. This virtual wind direction is a vector between the real wind direction, and blade rotation.

I hope this makes sense.
If at first you don't succeed, try again.

My projects
 
Greenbelt

Guru

Joined: 11/01/2009
Location: United States
Posts: 566
Posted: 03:02pm 05 Jun 2009
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Monty;
This is my understanding of featherd, when the angle of attack is zero to the wind, then the blade has no force to drive it.
"Roe


Time has proven that I am blind to the Obvious, some of the above may be True?
 
Gizmo

Admin Group

Joined: 05/06/2004
Location: Australia
Posts: 5119
Posted: 10:50pm 05 Jun 2009
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Yes Greenbelt's drawing is spon on.

Glenn
The best time to plant a tree was twenty years ago, the second best time is right now.
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