Airfoil Theory

[FishbonzWV]

Hi all,

I was looking for info on thin cambered airfoils to see if I could tweak my plastic blades and happened across this site. Thought it make good reading.

Hope the link works

http://www.desktopaero.com/appliedaero/airfoils1/airfoils1.html

Here's a few of my experiments with thin blade design.

http://www.otherpower.com/images/scimages/1725/Red_blades_001.jpg

http://www.otherpower.com/images/scimages/1725/front6blades.jpg

http://www.otherpower.com/images/scimages/1725/Plastic_Blades_001.jpg

http://www.otherpower.com/images/scimages/1725/Green_Blades_004.jpg

Fishbonz

[scottsAI]

Hi FishbonzWV,

Nice find!

Can't say I understand it yet...!

Thanks.

Have fun,

Scott.

[wdyasq]

Two programs to confuse budding wind-turbine designers ....

http://www.mh-aerotools.de/airfoils/javafoil.htm

http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html

And one to finish the struggle ...

http://www.ae.uiuc.edu/m-selig/propid/

No hair restorer is supplied with any of these programs.

Ron

[Boondocker]

Thanks for posting the links.   It is great playing around with the simulations.  I have to ask the question, is really worth getting that involved in foil design to tweak out that last bit of efficiency?  When more energy can be captured by just a small increase in prop diameter.

[finnsawyer]

For someone serious about analyzing the performance of the limited twist blades one finds people that use this site to be building, the NASA program would appear to be the one to use, as it has a Stall Mode.  One would need to calculate both lift and drag, however, and it's not clear whether the program can calculate drag when the the air foil is in stall.  One would have to calculate lift and drag for a sufficient number of stations along the blade from the tip to the root right through those parts of the blade that are in stall.  From that data one could then determine the performance of the blade and how much the region of stall affects it.  This does, of course, assume that the program gives accurate results for the air foil in stall.

[finnsawyer]

It depends on what you are trying to do.  For most of the users of this site the answer is no.  For someone looking to manufacture wind turbines it may make sense to try to wring the last bit of efficiency out of the design.  In fact, it will happen.  Competition and economics dictates it.

You may have noted my other comment on this thread about the blade going into stall.  That's fairly obvious when you are going to make the blades out of say 2 by 8's (actually 1.5 inches by 7.5 inches).  You can't get much angle out of such a piece of wood, even if you use a wedge at the root.  At some point toward the root the blade section will go into stall.  From that point toward the root the lift coefficient drops severely and the drag coefficient rises greatly, so the blade is simply dragging along a chunk of wood.  If you were to replace the three blades by six made from 2 by 4's you could push the stall point further toward the root and improve performance slightly, since each blade could now have more twist.  The best solution would be to carve the blades from laminated wood and put in the proper twist right down to the root.  Then one could use the NASA program to help design the blade.

Personally, I would go further and put in that hemispherical "nose cone" I've mentioned and design the laminated blade around the resulting increased air flow using the known solution for fluid flow around a sphere.  If you can get an extra kick, why not get it?  This extra kick, by the way, would be in addition to the Betz Limit.  That's the beauty of a sped up air flow.    

[thefinis]

I feel that if you start with a good blade design and want a small increase in power go with a better blade so tower loading remains at or near the same. If you need much of a power increase go with a bigger blade. You need to match turbine size to tower capacity and match blades and wind conditions to generator needs for power and rpms.

You do the best you can with what you have and try to improve on it next time.

Finis

[scottsAI]

Hi wdyasq,

Thanks for the first link, had the other two!

How do you take a flat foil and twist it and calculate the power out using these programs?

I found one wind blade simulator cost $3k, outside my cost for a play toy.

Have fun,

Scott.

[wdyasq]

I think only two folks in the world know and understand how to use PROPID. And, they aren't talking.

I think there is a lot to be gained with the proper airfoil and twist in the whole scheme of things. For the 'hobby wind-farmer' I think time would be better spent other places for the small gains one would get in performance.

It is difficult to properly carve an airfoil correctly over several feet and repeat it three times. It is even more difficult if one uses a complex airfoil with reverse curves in it. It is difficult to even properly model and write the code to efficiently CNC carve the things.

Once someone understands the complexity of the problem, they will avoid carving for other peoples machines. Any mistake is likely to be blamed on 'the blades'. and to be correct, each blade set needs to be matched to the machine. That means each set of blades needs to be designed FOR that machine. And, few will say 'I want this' with a proper description.

They will say, 'I want a TSR of X' and have no flipping idea of what they speak. Or, ask the TSR of an airfoil ... I will guess they will be reluctant to pay a qualified aeronautical engineer to design the airfoil. They want it for X magnets and Y Turns of Z dimension wire and W volts ... but can't answer how much torque it takes.

At some time I hope to be able to understand and run PROPID. I wish it was a LINUX program as I might have it run in the background while I thought of how to play with it but, alas it is a windoze program. I don't suffer windoze well.

Ron