Collared tips for incresed effeciency

[Usman]

I have noticed tips of a small wind turbine and a larger (commercial) one "collared" or feathered outwards as in the picture.

The commercial manufacturer was claiming such a tip design has less noise as well as reduced vibrations and helps the tip to stay stable in opertaion. Any idea how valid is such claim?

[Usman]

Here is a photo:

[Usman]

Photo of the large commercial blade:

http://www.enercon.de/www/en/rotorblattkonzept.nsf/mainView/1?OpenDocument

[SparWeb]

This is not too different from an aircraft "winglet".  Winglets are designed to improve the distribution of lift over the span of a wing, usually to improve fuel effeciency.  If commercial turbine builders are using them, then it's obvious from where they're borrowing their ideas.  For a wind turbine, these tips can only go so far before being likely to hit the tower!

For our miniature purposes, you will not see a performance improvement worth the investment in time.  Designers use a computational fluid-dynamic model to simulate the airflow.  There's really no other way to optimize a winglet on an aircraft, so I don't see why it would be different for a wind turbine.

About the noise... don't know about that one.  Get the incidence wrong on your winglet and the wind turbine could easily make more noise.

Then again, they look wayyyy cool!

Cessna boosted sales of its 172 aircraft when they swept the tail fin backwards.  The airplane flew 1 knot slower as a result of the change, but thousands of customers didn't care...

Boeing now puts winglets on its 737 series of aircraft.  These winglets are proportionally bigger than the winglets on airbus aircraft, but it affords SO MUCH more room for the company logo!

[Usman]

Thanks for the very interesting response. I do agree its aesthetically very pleasing apart from making the rotor effecient, if at all.

[wdyasq]

http://tinyurl.com/2nyjpn

From Lonnie Prince - CNC carved of hard maple with Carbon-fiber overlay.

Ron

[crusader13]

If these are properly designed, and again properly, they should increase the blade efficiency by 6-10%. The basic idea is to prevent the loss of "lift" generation that is  produced by the blade from "rolling" off its end and/or wing. Winglets need not be large but just enough to  Considerations, yes, from structure.

Crusader 13

[Ungrounded Lightning Rod]

The basic idea is to prevent the loss of "lift" generation that is  produced by the blade from "rolling" off its end and/or wing.

The loop of air moving around the tip of the blade (tip vortex) lowers the pressure on the upwind side and raises it on the downwind side, partially depowering the outer section of the blade.  The winglet interferes with this flow, letting the outer part of the blade stay essentially fully-powered.  The winglet also adds drag, which costs you some of the power.  But if properly designed the winglet can pay for its drag many times over.

But much of the same effect can be had by bringing the end of the blade to a knife edge (smoothing it into the leading edge bulge and the upper and lower airfoil surfaces, of course).  this causes the air trying to do an end-run to jet off the end of the blade.  There are two ways to look at this:

 1 It "throws" the tip vortex outward, effectively lengthening the blade.  The physical end of the blade remains nearly fully powered while the depowering is applied to the virtual extension of the blade (which couldn't apply torque to the shaft anyhow.)

 2 By the time the air gets turned around to come back and raise the pressure behind the blade, the blade has moved on.  So while you still lose some pressure on the front of the blade you don't also lose vacuum on the back.  (Note that a backside-only wingtip is also only blocking the return of the air to the downwind side of the blade, not hampering the air leaving the front side.  So the sharp wingtip does the same job without the extra material and its associated drag and design and construction complexity.)