S822 Profile

[windy]

After looking through different profiles that are used on windmills, I decided on trying the S822 profile, using the table saw method.http://fieldlines.com/board/index.php/topic,145306.0.html. After looking at the profile using the QBlade program, I am not sure as to how to pitch the blade on the wood blank.
Here are 3 view of the same profile but at different pitches. The rectangular box represents the blank.


[ Specified attachment is not available ]
 After analyzing the profile with QBlade, the first view has a lift to drag ratio of +39, the second view has a lift to drag ratio of 0, and the third view has a lift to drag ratio of -21. Looking at the way bcalmed and ChrisOlsen carved their profiles, which is like view 3, it would have a lift to drag ratio below 0. That's what is confusing to me. You would think that the lift to drag ratio should be above 0.
 The QBlade program is for designing airplane wings, so I am not sure if this program can be used for looking at profiles used on windmill blades.
 Any comments or explanations would be greatly appreciated.

windy

[Flux]

I think you are confusing things with wind tunnel tests and with aeroplane wings. If that section was in a wind tunnel then the wind would be from West to East.  Your wind turbine will have the wind blowing from South to North.

The pitch needs to be set relative to the apparent wind when running, which is a vector determined by the relative wind and tip speed velocities.

If you can find it, Hugh Piggott has this nicely explained on his web site, probably under blade theory or blade carving somewhere.

For your turbine you would have zero pitch with the chord line of the blade set parallel to the blade tip direction as it rotates. Positive pitch will mean that you set the trailing edge down wind relative to the leading edge.

Flux

[Royalwdg]

Windy,  This profile would not be optimal for a wind turbine blade.  I agree with Flux in that your picture number three is the correct orientation.  Set at 10 degrees you would have a geometrically calculated TSR of about 5.7 or even 6.  This profile with the same curve top and bottom will create no lift at 0 degrees angle af attack.  Lift is a little better at 3, 4, and even better at 5 degrees. This would be where it will run with a work load.  At 6 degrees this profile stalls .  As compared to other profiles like the Clark-Y, which is similar to the Dans tip profile, does not stall until 12 degrees and the GEO222 stalls at about 9 degrees. In high winds blades run really well work load so much and the angle of attack approaches 0 degrees.  This is where your proposed profile runs out of lift.  It may be somewhat self limiting.  It's fairly easy to carve a Clark-Y profile which performs better by a long shot.    Dave M

[windy]

Royalwdg, Flux,
 Thanks for the replies.
Looked at Hugh's website on blade theory and lots of information on building blades.
 I looked into carving a GOE222 profile, but a few people who did carve this profile stated that it would not reduce
power even after it was fully furled. I get some crazy downburst winds that can go from 30 to 80 mph in a few seconds and thought about trying a profile that may be self limiting. Was concerned that the top and bottom of the S822 profile were similar.
 I looked at the Clark-Y profile and would definitely be easier to carve. My 8.5 degree pitch was just a guess. How do you go about figuring the optimal pitch? The generator I built is a 17 footer from Dan B and Dan F's book, Homebrew Wind Power. Had a 16 foot diameter blade on it until it flew apart in a downburst, so I put my 10 foot diameter blade back on until I get another set built.

Thanks!
windy

[Royalwdg]

Windy,  I've been running the GOE for a while.  I've built 3  20 foot machines and have never seen a runaway when furled.  If your machine is on a constant electrical load such as heating it is th tamest.  Might see 10 to 20 rpm in the high winds of 40 to 50 mph.  Shorting your machine at this point will give you 1 or 2 rpm.  If you are charging batteries then its free wheeling until battery voltage and that  will cause some higher rpms until battery voltage.  I'm not saying a freak wind change wouldn't follow your furled turbing around and kick that rpm up a bit. The Goe222 does paddle in the furled position probably more than the Clark-Y.  The clark-Y has a higher stall angle of attack and will start powering up a little sooner than the 222.  The 222 will have more torque in it's power band and Clark-Y will run better at high rpm.  Clark-Y is definitely easier to carve.   I wouldn't shy away from good performing blades for these reasons.  I does sound like you are located in a fairly wind violent place.  I find it a challenge to try to take advantage of the available power. 
 Dave M

[windy]

Royalwdg,

 I decided to try and carve the Clark-Y profile, but am still struggling with finding the optimal pitch angle. Looked at Hugh's website but his formulas are for twisted and tapered blades. Not sure if that would work for straight blades. I see that you carve your GOE222 blades at a 10 degree pitch. Do you think that a 10 degree pitch on the Clark-Y profile would be optimal, or would you carve it to a different degree.

Thanks for the help!
windy

[wdyasq]

Royalwdg,

 I decided to try and carve the Clark-Y profile, but am still struggling with finding the optimal pitch angle. Looked at Hugh's website but his formulas are for twisted and tapered blades. Not sure if that would work for straight blades. I see that you carve your GOE222 blades at a 10 degree pitch. Do you think that a 10 degree pitch on the Clark-Y profile would be optimal, or would you carve it to a different degree.

Thanks for the help!
windy

I'm sure any angle you cut will be optimal for one small region at one wind speed.  Some airfoil profiles have a wider AOA (Angle of Attack) tolerance.

Ron

[Royalwdg]

Wincharger used the 10 degree pitch for a good reason.  I've carved all the blades i've made at 10 degrees because of the already proven performance.  I've recently altered my carving machine to be able to make replacement blades for the small 650 watt Winchargers which ran an 8 foot single bade carved at both ends sith a smaller profile very similar to the GOE222. Since this machine would run up to 900 RPM I had to reduce the pitch to 8 degrees.  If you carved your ClarkY to a 10 degree pitch you will be very happy with the performance.  The 8 degree pitch will give you a little more speed and a little less torque.  !0 Degrees is a good starting point.    Dave M

[windy]

Royalwdg,

Thanks again for the reply!
 I did some calculations and with a 17 foot diameter blade and with a TSR of 6, the outer chord width comes out to 7.5 inches wide. Does this sound about right? What would you recommend for a TSR.

windy

[Royalwdg]

Windy,  That sounds about right.  Tip speed ratio will change according to the load that is put on the generator.  A good performing blade will be able to hold a good tip speed under load but it will be less than a free wheeling blade.  A genny that charges batteries will soom up in rpm until it hits battery voltage then its true qualities ar revealed under load.  My carving machine is limited to 6.5 inches chord length.  For my 10 foot blades (20' diameter)  I would prefer another inch on the width.  You are on the right track.  If you carved a constant chord constant, constant 10 degree pitch, constant profile you would have a pretty good running blade.  Remember that most of the work is being done at the outside 1/3 of the blade diameter.   Dave M