Blade theory 102

[imsmooth]

When using a blade calculator I see that when the thickness gets smaller (everything else is unchanged) for different blades the RPM goes higher.  Is this because there is less drag?  When I enter numbers, I see that a blade that tapers to 1/2" tip thickness spins slower than one that tapers to 1/4" thickness. I am looking for the theoretical answer.

[wdyasq]

I hope you have a good thinking cap, a very good math education and a bit of time.

Airfoil thicknesses are a percentage of the length. An "XX12" airfoil is 12% thickness while a "XX18" is 18% thick... with many airfoils having similar numbers. Personally, I find little difference in the drag of a 12% airfoil to the same family at 18%. And, as many thicker airfoils in a 'family' have better low speed performance I'm not sure the quest of the thin airfoil is a valid hunt. Also, once a wind-turbine is producing a lot of power, the problem is handling the power, not a minuscule difference between two similar but slightly different thickness airfoils.

Start your search of the proper airfoil with "Selig Airfoil" you will learn more about Reynolds numbers, Angle of Attack, Coefficient of Drag and laminar flow than you ever cared too.

After a few hundred hours of research, it will probably occur to your well-washed mind that the last few percent of efficiency with the near perfect airfoil can be matched by a mediocre airfoil, a couple of inches more length and and two more turns of wire.

Have fun  ,

Ron