Airfoil profile choice

[David Street]

I am making a small set of blades 1.4 m (4.6ft) diameter, like Hugh Piggot's 4ft machine. I also intend to make an 8ft diameter set and will use Hugh's design.

I am interested to see what other members think about the shape of the profile such as NACA 44xx series or any of the many that are out there.

do you think the windward side should be flat or concave as in some profiles

Stewart Corman has a spreadsheet that I am looking at I have been working in metric so need to do some conversions to see what you have there. Also Hugh Piggot has one on his site but they produce blades of very different shapes.

I look forward to the discussion
David

[Mark Parsons]

Welcome David,

I suspect you will gets lots of opinions. I am not qualified at this time to offer much of an opinion on airfoils.

One of the airfoil selection criteria is your alternator RPM and cut-in voltage requirement.

I made a robust 4' diameter HAWT machine very simply over the course of a couple of weekends and made concave type blades from 4" ABS drain pipe directly mounted on a DC motor. This machine has spun up to over 800RPM in a good wind unloaded. Cut-in for my 12V batteries is about 150RPM. It starts up in the slightest of breezes. In my mind the concave shape should help with starting torque. If you are interested in playing around with this concept the following resources can help.
http://www.reuk.co.uk/Photos-of-PVC-Wind-Turbine-Blades.htm
http://www.fieldlines.com/story/2003/11/11/172721/53
http://www.mdpub.com/Wind_Turbine/index.html

Regards,
Mark

[David Street]

I was more asking about the choice of profile.

http://www.windmission.dk/workshop/BasicBladeDesign/bladedesign.html
uses NACA 2412

http://www.symscape.com/examples/panel/naca4415
uses NACA 4415

http://users.aber.ac.uk/iri/WIND/TECH/WPcourse/page3.html
suses NACA 4412 and clark Y

http://club.cycom.co.uk/windturbinedesign.html
uses:
Wortmann FX63-137, Selig S3021, Selig S2091, Selig-Donnovan SD7032, Selig-Donnovan SD7037, Selig-Donnovan SD8000, ARA-D 6%, ARA-D 10%, NACA 4412, NACA 4415, NACA 63-4-021, E59, NACA A=1.0 Mean line, NACA Reflex Mean line

http://eduhosting.org/windpics/wgfoil.html
uses NACA 4418, a Wortman or 0417 curve and NREL S series

the last site says the hollow pvc/metal pipe are for low power and stall earlier and are noisy

Noise is one of the factors that I wish to keep to a minimum

My thermodynamics knowledge was never good to start with and I will still have to look into finite element analysis, what a mouthful to say and also heavy brain work!

[Stewart Corman]

David,
you have seen some of my posts (since you got my xls caclualtor) and many of the answers are there.

Three simple points ...doesn't really matter WHICH profile you pick and the performance won't be very diff, but construction makes the choice easier.
The Jacob's usnps4 is a flat bottomed, broad cord profile and trivial to make ...so don't get in a dither ...but decide tapered or non-tapered.

Second point is related to:

Noise is one of the factors that I wish to keep to a minimum
Here you can answer your own question. Noise is proportional to rotor speed^5.
Choose a 5 or 6 blade rotor with TSR=4 and you cut down noise by 4^5=1024 vs 6^5= 7776 or 87% and get better efficiency in low WS

Forget plastic blades for 8 ft turbine if you live in winter country ..they very get brittle in sub-zero weather and forces start to get hefty

BTW, you can load JavaFoil and generate your own AOA vs L/D info
This is NOT rocket science ( rockets have super high RE# )

Stew Corman from sunny Endicott

[Mark Parsons]

Here is an idea claiming a 30% improvement of turbine blade power harvesting in low winds.
http://peswiki.com/index.php/Directory:WhalePower_Corp

[Ralph Day]

Hi Mark

Interesting article. I guess we should leave the bumblbees, small birds etc on the leading edges of our turbine blades. What does Rob think?

Ralph

[Stewart Corman]

Mark,
Interesting concept taken from nature
yup, modeling says that a bee can't fly and neither can Allison's turbine!
Here is a whale film clip:
http://quicktime.oit.duke.edu/dstudio/flipper.mov

HOWEVER, doing my DD with Google, I cannot find any followup since three years ago when the articles first appeared ..sounds fishy (whaley?)
<pun intended>

as the old lady said in the Burger King ad .."Where's the beef?"

Stew

[Rob Beckers]

The little article says they measured a decrease in drag and increase in stall angle. Stall angle isn't all that interesting for wind turbines, running one anywhere close to it means very poor lift-to-drag ratio. Decreasing drag (and thus increasing lift-to-drag ratio) would be a good thing.

The other side of the coin is as Hugh Piggott noted (or was it Paul Gipe?), aerodynamic efficiency comes in a distant third for how well a wind turbine performs; First is blade diameter (swept area), second is the average wind speed for your site (that's the fuel), third is efficiency. At that, efficiency is the combination of aerodynamic efficiency, mechanical friction losses, copper losses and iron losses in the alternator, inverter/charge controller losses etc. The moral of this story being that just adding a wee bit to the diameter of the turbine would do a lot more for its production than spending oodles of money and effort on increasing efficiency a few percent.

I agree with Stew, "where's the beef", and disagree with his statement on the efficacy of CFD modeling/simulations... :D

-Rob-

[Stewart Corman]

Rob,
Your point I can take issue with for non-twist blades:
Stall angle isn't all that interesting for wind turbines, running one anywhere close to it means very poor lift-to-drag ratio.As you move down from the tip, the AOA changes dramatically, especially as TSR is reduced at low WS, so some part near the root supposedly comes to stall angle ...increasing the effective stall angle from 15 degrees to 30 degrees is quite a deal! This is exactly why the Bergey XL-1 performs so poorly at low WS with a conventional airfoil.

as far as :
and disagree with his statement on the efficacy of CFD modeling/simulations.Only for conventional two or three blades that sort of work!

If you look at an Allison triangular non tapered,non-twist blade ..it doesn't fly ...note that his pitch angle is 15 degrees and a TSR = 3.5 which means that the 75% station is at AOA = 4.37 degrees ..if you were using models, you'd never even try this setup!!!


Re = 100000
AOA Cl Cd L/D

1 0.50 0.03 18.4
3 0.60 0.03 19.1
5 0.25 0.04 6.0
7 0.07 0.07 1.1
9 0.03 0.12 0.2
11 0.01 0.09 0.1
13 0.01 0.13 0.1
15 0.00 0.18 0.0
17 0.00 0.24 0.0
19 0.00 0.31 0.0
21 0.00 0.40 0.0
AND, BTW ..his first patent references a conventional airfoil that is replaced later by the triangular design ...because?? .... it worked BETTER.

Stew Corman from sunny Endicott