Remember that:
- the farther in on the blade
- the less swept area per unit of lenghth
- so the less power per unit length.
The inner quarter of the blades collct 1/16th of the power (while the outer quarter collect 7/16ths - nearly half of it.
As you get really near the hub the optimum shape gets very long and very angled - requiring a lot of wood - while the amount of power collected drops to nearly zero. So when the shape the calculators tell you to make starts getting really long as you approach the axis you might as well stop bothering - optimizing its length, getting its angle right or even having it be an air foil at all (in order of progressively less ideal). If you stop at 1/8th of the radius and just turn it into a solid block you throw away 1/32 of the power - about 3%. Not all that big a deal.
Now there is another reason to continue moderately far in toward the hub:
The blades have a lot of torque while they're spinning at roughly the design TSR (which will be in the ballpark of 6). All the parts of the "twisted" blade are seeing the same angle-of-attack from the "apparent wind" (the vector sum of the real wind with the wind from their motion). The air is all attached, all the sections of the blade have lots of lift pulling them both downwind and around the circle, producing lots of torque.
But when the wind first starts and the blades aren't moving yet the "apparent wind" is just the actual wind. Out near the tips the blade is nearly flat-on to it. So you get lots of drag but virtually no torque. You need something to start it. But near the hub, if you continued your twist, you have a section that's at a more reasonable angle to the wind. This acts as a sail, much like the designs we (loosely and improperly) sometimes refer to as "drag turbines". Think "air screw" (versus "worm gear working backward" for the ends of the blades). While it's not operating in the way the shape was designed to fly, it DOES deflect the wind in a way that produces a reasonable amount of torque. So it's great for starting.
If you have an airfoil section there, it's good to have it at the proper twist angle. That way once the blade is running at the design TSR it's not acting as an air brake and fighting the rest of the turbine. But it's not as important to make it as long as the optimum chord length for the slower wind it "sees". If the inner 1/8 is only collecting 3% of the available power, making it shorter than optimum so it only collects, say, 2% means you've only lost 1/100th of the power you could have collected.
One other thing the longer chord length, more closely alligned with the axis, does is provide strength against the forces trying to fold the blades back against the generator. You're slowing the air down to about 1/3 its previous speed and this shows up as drag, trying to bend the blades backward. Yet you're only supporting the blades at the very center. The blades are a very long lever, greatly magnifying the bending stresses near the axis, so the stress is greatest there. Having the blades thick in the along-axis direction at that region strengthens the blades against this stress. But again there's no point in using a BUNCH of wood (most of which ends up as shavings) just to make that section REALLY LONG. Just make it somewhat thick and call it a plan.
