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Airfoild and angle of attack of said Airfoil
By wdyasq, Section Homebrewed Electricity Posted on Sat Jul 05, 2003 at 04:19:40 PM MST
What is the best design

I have been going through some NREL sites and have noticed the airfoils they use are "twisted" quite a bit more and have a lower angle of attack than those being built by those on websites I have visited. A flat bottom airfoil is a lot easier to carve than those shown on the "government" sites and they use several airfoils depending on where on the blade the airfoil is (be?).
If one had the ability to carve multiples of the "exotic" shapes, would it be worthwhile to research and cut the foils? DOE claims "The increases ranged from 10% to 35%". And, I wonder if there would be an increase in the low-speed output of a mill?
Ron

Airfoild and angle of attack of said Airfoil | 13 comments (13 topical)

Re: Airfoild and angle of attach of said Airfoil (none / 0) (#1)
by Mike on Sat Jul 05, 2003 at 04:52:29 PM MST

I've been playing with airfoils since I got interested in model airplanes
as a kid. I even built a wind tunnel once as a school science project.
There are hundreds of thousands of airfoils out there, and as best as I
can tell, every single one is supposed to be ten percent better than any other
one. It's similar to the Mexican army, where everyone is a general.
The bottom line is that if you have one specific speed, wing loading and
angle of attack in mind, you can computer-design an airfoil tailored to those
particular conditions. In the case of small wind gennies working over vastly
changing range of conditions, a simple flat-bottomed Clark-Y will give better
results over the average.
Adding undercamber to an airfoil increases low-speed lift, but it also increases
drag considerably and it makes the angle of attack much more critical. I don't
think there's enough to be gained at the hobby-genny level. You could make up
for any lack of efficiency by adding another 6" of blade.

- Mike

http://www.bioelectrifier.com

Re: Airfoild and angle of attach of said Airfoil (none / 0) (#2)
by wdyasq on Sat Jul 05, 2003 at 07:49:36 PM MST

"In the case of small wind gennies working over vastly
changing range of conditions, a simple flat-bottomed Clark-Y will give better
results over the average.
Adding undercamber to an airfoil increases low-speed lift, but it also increases
drag considerably and it makes the angle of attack much more critical. I don't
think there's enough to be gained at the hobby-genny level. You could make up
for any lack of efficiency by adding another 6" of blade."
*****
While I will agree the "Clark Y" is a proven and easy to carve airfoil, there are airfoils designed for slow speeds with camber, a wider range of "angle of attack" and more theoredical lift. There may not be much to gain at the "hobby" level. On a 6' blade, 6" is a 15% or 17% variance in swept area, depending if 6'6" is the standard or 6' is the standard, using a 6' blade as an example.

I think the only way to find out will be to have a few mills running with different airfoils, "twists" and angles of attack.

Ron
Adventure is just bad planning." -- Roald Amundsen
[ Parent ]

Re: Airfoild and angle of attach of said Airfoil (none / 0) (#3)
by Mike on Sat Jul 05, 2003 at 08:31:20 PM MST

An undercamber gets pretty tricky when you reach the tip speeds of a six foot prop
cranking at 800 RPM. The big commercial gennies can regulate their RPM, while
most of us have to take what the wind gives us and deal with it.
I've found the Clark-Y to be great at the root, while my favorite shape at the tips is a
little thinner, but still flat on the bottom.
I think if anyone is interested in getting the last drop of energy out of a blade, there's
more to be gained with a proper low-drag tip design than trying to come up with a
supercritical airfoil that's super-efficient at all speeds. Tips loose energy in two ways...
they travel faster than any other part of the blade, so the drag is the highest. Also, they
spill air off the end of the blade and loose lift.
Honestly, I don't think our simple blades do all that bad. My six footer has enough
efficiency to hit it's design TSR of 7+, and it spins 20 amps out of the Leeson, which
is only rated for 14 amps continuous. As Scotty would say, "I'm givin' 'er all she's got,
Captain! She cannae take any more!"

- Mike

http://www.bioelectrifier.com

[ Parent ]

Re: Airfoild and angle of attach of said Airfoil (none / 0) (#4)
by wdyasq on Sat Jul 05, 2003 at 09:23:12 PM MST

" there's more to be gained with a proper low-drag tip design than trying to come up with a supercritical airfoil that's super-efficient at all speeds."

Mike,

I think we agree more than disagree. "Supercritical airfoils" are super-stupid IMO. Foils relying on "laminer flow" are upset by the slightest inperfection. The proper foil for "hobby" wind turbines in my estimation should, in aviation terms, have a wide "bucket" and have the ability to be replicated. The Clark Y fits the bill for a hand carved wheel.

One factor seldom considered is the power one can harvest from the turbine. If you have a generator (alternator, steam engine, air-compressor, etc.) that can only put out a certain amount of energy and there is much more energy available than the generator can "gin", one had better be furling the generator.

The late Charles Whitman carved Simitar props for his planes. These would actually change pitch due to loading and speed giving a "variable pitch" to a fixed propeller. Wood is one of the few materials with the properties to work this way without desrtuction due to fatigue. Few folks have achived the efficiency and speed Whitman did even with variable pitch props and much more elaborate construction.

I like you attitude of using multiple small turbines. If I can locate some of the "tape drive" motors you use, I might just make a few of the little monsters - with different airfoils- of course.

Ron
Adventure is just bad planning." -- Roald Amundsen
[ Parent ]

Re: Airfoild and angle of attach of said Airfoil (none / 0) (#5)
by Mike on Sat Jul 05, 2003 at 09:47:07 PM MST

A lot of the long, wide props with the curving blades used in WWI era
aircraft were designed to flex also... not quite a constant-RPM prop, but
close enough, considering the low end torque of those Gnomes and Hissos.

Electronic Goldmine has a 99V Amtek motor in their latest flyer
for $19.99 in "used but good" condition. Part number B1511

12V is stated at 300 RPM, similar to the 72 volt one I just put up.

- Mike

http://www.bioelectrifier.com

[ Parent ]

Re: Airfoild and angle of attach of said Airfoil (none / 0) (#6)
by wdyasq on Sat Jul 05, 2003 at 10:15:38 PM MST

Thanks for the links Mike.

BTW- if you look for "Charles Whitman".... well, better look for Steve Whittman. His "Tailwind" is still a remarkable plane over 50 years after it was designed.

I need to order some things for electronic projects and may as well get one or two of those tape drive motors while I am at it.

Thanks,

Ron
Adventure is just bad planning." -- Roald Amundsen
[ Parent ]

Re: Airfoild and angle of attach of said Airfoil (none / 0) (#7)
by Mike on Sat Jul 05, 2003 at 10:35:18 PM MST

I built a model of the "Tailwind" over 35 years ago. It's a real testament
to a good aircraft design when you can scale it down to a 48" wingspan and
it still works. Nobody has yet figured a way to scale down the air, so the
reynolds number of the scaled-down wing generally sucks.
Which is another point to consider with our props... a super airfoil designed
for a 200 foot rotor may not be worth a darn when scaled down to six feet.
I've seen some airfoils that really make a person wonder... For example,
the Grob II high-performance sailplane has a better L/D ratio when INVERTED!
I've never seen anyone try to stretch a glide that way, but you never know.

- Mike

http://www.bioelectrifier.com

[ Parent ]

Re: Airfoild and angle of attach of said Airfoil (none / 0) (#8)
by wdyasq on Sat Jul 05, 2003 at 10:50:13 PM MST

Your point of "scaling down" airfoils is well taken. NREL has a "medium" and a "large" airfoil design. The "medium" is for an 11 meter mill - even scaling that airfoil down to 30% - the Reynolds numbers get quite a shake.

I'm rather fond of Riblett's "GA" series of airfoils. The lower speed ones would probably work well for turbine blades and the medium speed ones for the tips. I ahve an idea for a "laminated" blank with a wider mid section and thin tip I would like to try. With a $20 generator and suing "lumber" for the props, it may be possible to test a few concepts I have wondered about.

Later, I may be able to say, "I blundered about in that field and found the Clark Y......" Or, maybe not...

Ron
Adventure is just bad planning." -- Roald Amundsen
[ Parent ]

Re: Airfoild and angle of attach of said Airfoil (none / 0) (#9)
by DaveR on Sun Jul 06, 2003 at 01:22:02 PM MST

Mike, Whats the date on the flyer you have from Electronic Goldmine? Its not listed in the July flyer that I could see. I found the motor on the web site Part number G13464 but its now $54.95. Would love to snag a few at $19.99. Thanks, Dave

[ Parent ]

Re: Airfoild and angle of attach of said Airfoil (none / 0) (#10)
by Mike on Sun Jul 06, 2003 at 01:28:05 PM MST

Yep... you're right. The sale ended on the 1st of July, and the flyer that just
came doesn't have it listed. It's the same motor that's normally $54, which is
way too much.
The same motor is listed here at a better price:

http://www.surpluscenter.com/item.asp?UID=2003051913523979&item=10-1840&catname=electric

Frankly, I'd buy a 2 hp Leeson from American Science & Surplus before I'd spend
the same money or more on a smaller Amtek.

- Mike

http://www.bioelectrifier.com

[ Parent ]

Re: Airfoild and angle of attach of said Airfoil (none / 0) (#11)
by DaveR on Sun Jul 06, 2003 at 01:34:13 PM MST

Thanks Mike! Dave

[ Parent ]

Re: 2HP Leeson (none / 0) (#12)
by Norm on Sun Jul 06, 2003 at 08:03:17 PM MST

Mike, After you talking about the 2HP Leeson...I'm thinking ...I just gotta give my 1HP Leeson about a 600RPM spin one of these days real soon and see what I get...Thanks for the inspiration. Your enthusiasm carries over loud and clear! (:>) Norm.
( :>) Norm
[ Parent ]

Re: 2HP Leeson (none / 0) (#13)
by troy on Mon Jul 07, 2003 at 03:03:45 PM MST

Enthusiasm and hard work count for a lot.

Excellent discussion of airfoils. I have built a few RC sailplanes and from my modest exposure to 'foil design, it seems like a black art. Sure, you can manipulate the major variables to get the "perfect" airfoil for your needs, but change any little outside factor and your precious ten percent improvement turns into a 30% loss.

I'll stick with a nice generic flat bottom airfoil that looks a lot like my 2 meter sailplane wing.

Good luck and have a lovely day!

troy

[ Parent ]

Airfoild and angle of attack of said Airfoil | 13 comments (13 topical)

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