Blade design

[Glenn D]

Hello. Newbie here again with another elementary question. I understand the basics of an airfoil, but when I look at photos of different wind generators, I cant figure it out. Wind either pushes angled blades "out of the way", or causes a low pressure area on airfoil blades. So if I were to look directly at a wind generator with airfoil blades, the blades should appear to be very thin spokes. Like looking directly at an oncoming airplane, the wings would appear very thin. But looking at different photos of  airfoil type blades, they appear as if the wind were just pushing them aside. Can anyone shed some light on this for me?

[RP]

The effect you are considering is called the "apparent wind".  

Remember: the blades do not just see the wind from the horizon.  Because they are spinning they see the wind coming from a point between the horizon and just around the axis from where the blade is now.

[Flux]

Correct.

If the tip of the blade is travelling 6 times as fast as the wind, the blade sees a vector component of the wind at an angle of attack that is much the same as an aeroplane.

Try Hugh Piggott's web site

http://www.scoraigwind.com/

He has some information that will explain it, with nice diagrams which I don't have time to draw.

Flux

[Glenn D]

Wow! I couldn't have asked for a better explanation. Thanks a million. Hugh Piggott's site really goes in depth; I didn't realize how complex it was!

[finnsawyer]

The equations for lift and drag for an air foil do not tell you how to relate either effects to the power available from the wind.  The simple fact is that the equation for lift could allow a design (width of blade) that would put out more power theoretically than is actually available by the wind.  The result would be excessive drag and poor performance.  I've struggled with this problem from my own perspective and have presented an equation for relating the component of lift in the direction of rotation to the power available in the wind.  I'll not repeat that here.  You can check my comments.  Since all the available wind does not pass through the wind turbine's blades, the TSR issue also seems to be questionable.  I've been in the process of presenting a new way of looking at the problem in my latest diary.  You might take a look at it.  Try to read my comments in chronological order, if possible, to see how the arguments develop.  I imagine I'm going to get a lot of static over the latest development.  But hey, no pain, no gain.