On Thursday, December 4, 2014 10:02:17 PM UTC-8, GM wrote:
On Thursday, December 4, 2014 4:54:49 PM UTC-5, Steve Leonard wrote:
Not quite sure what you are trying to describe, but here are my observations.

In planform, all seem to have an eliptical shape at the tip.

In airfoil sections used, most seem to have a concave section on the top (side away from the mast) that is visible under proper light conditions (front 2/3 or so of the blade lit up by sunshine, a portion that is dark, then the last 10-15% of the chord is back in sunshine).

Many of the large diameter turbines now have, for lack of a better term, a "dihedral break" in them out near the tip. Near as I can tell, this is primarily for providing blade to mast clearance, as the blade flexes back towards the mast and the designers had a choice between adding mass (to get stiffness), putting a "kink" in the blades, to bend it out away from the mast, or making a new top end with an even longer overhang. Two of those three options add cost and weight and reduce efficiency. The third gets the clearance needed and actually increases the swept area over a blade set that does not have this "dihedral" break in the blade. This "break" is in the range of 15 to 20% of the balde length in from the tip of the blade.

Some of the earlier wind farms in Kansas had roughly 75 foot long blades, and these did not have this feature. The newer ones going in have nearly 100 foot long blades, and they do have this feature. It may look like a "dip", as inboard of this, the blade is bent back towards the mast, and outboard of this, the blade is much more nearly parallel to the mast.

Hi Steve,
your explanations are pretty much spot on. The blades are being built with a natural dihedral so they straighten out under load. There are typically around 12-13 airfoils in a wind turbine blade - at least in ours. To further increase the blade tip to tower clearance (we don't call it 'Mast'), the entire drive-train is tilted back by a few degrees.
Not sure which wind project in KS you are referring to but our current generation blades are 47.8m long which makes for a 100m rotor diameter. 120m and 130m rotors on 140m tall towers are already being used in Europe. Our aerodynamisists tell me that these blades have an L/D of around 100:1.
Uli

So Uli,

The windpower aero group must be as interested in localized AOA changes and drag effects due to gusting as we are. Has there been any crossover between the fields. I expect at full power they're at higher reynolds numbers that we are. They certainly have more funding at their disposal than the soaring community.

Cheers,
Craig