Wow this got long. Hope something in here helps somebody.
Shorter blades of the same design tend to turn at a higher RPM.
Wooferhound and Zubbly collaborated on a PVC blade design. Fast and easy to make.
The dimensions are given only in angles. The blade length and pipe diameter don't seem to matter for the design. (naturally a larger machine should use longer blades and bigger pipe) But you know that.
I had bad luck changing their design.
With 4" PVC, the blade in front of me has a root width (chord) of 1.75", tip width is 3/4". This blade happens to be 9" long, but others from 4" pipe go from 6" to 18". They all have the same tip and root chord of 3/4" and 1.75" (determined by the angles on / and the diameter of the pipe).
In the same wind speed...
The Longer blades have more power, but turn slower. Start turning in less wind because they overcome the cogging easier.
The shorter blades turn faster, but have less power. Need more wind to overcome the cogging and start turning.
Blades too long will not be turning fast enough to make charging voltage when they should.
Blades too short won't be turning at all when they should all ready be going fast enough to charge.
There is somewhere when everything gets a nice balance. The blades are long enough to start turning in a wind below where they will make charging voltage, and short (fast) enough to make charging voltage in the low wind.
I make a press fit hub for the stepper, and try different length blades until one works. Blades are held on the hub with sheet metal screws. 4 'seperate' blades, meaning not one piece. If it can't get going fast enough, try shorter blades. If it can't get started, try longer blades.
Do this. Really! Wish I would have done this as the very first thing.
For your stepper try a 4 blade hub and prop, 4" PVC, a set of 8", 10", 12", and 15" (each) blades. (make all 4 sets, you will want them for the next stepper)
Hand held 15" (30" dia) blades can be frightening in a big wind, so be ready.
It is VERY easy to see, feel and hear how everything is working. Or not working. A few blade changes and you will have a feeling for what is best.
And a much better understanding of "why".
For you next question, a high percentage of my smaller steppers do best with 9 to 11" blades. Even 1" makes quite a difference. But make all 4 sets anyway!
G-