Uh oh... You may think I know more about it than I do.
(I had my wood blades made by another member)
Have a look at Rover's posts about his. A good place to start anything like this is with any info about what worked, what did not, any why.
The ratios are built into the angles.
Length is not a factor to the design. I used the same angles with 5 gallon pails, and they worked fine (until medium-strong wind bent the flimsy blades back).
(no laughing)
I set a squarely cut end on poster board (ie: 12 pack cardboard).
Which brings up the issue of squareness. I only cut from one end, as square as I possibly can, leaving the other end factory square for reference.
I trace the OD, then draw the 7 and 25 degrees on the cardboard.
I put the pipe on the cardboard, and mark either 7 or 25 degrees on the proper end, with a super extra ultra fine Sharpie permanent marker.
I measure the OD of each as close as I can, then use that size to mark the other blades.
Being off from the plans from perfect 1/32" is not usually a problem, as long as all the blades are actually the same.
For balancing, I start taking off some of the square part of the root, maybe some off the hub (my plywood hubs usually end up egg-shaped), then I shorten the heavy one.
I had some that were over 1.5" difference from the longest to the shortest after balancing, and they ran smooth as silk.
That 4 blade sketch was for a stepper motor question. Tiny 15~18" diameter stuff. The square at the center kept the blades from twisting on the single screw. It allowed me to change out blades for different motors, etc.
I just keep trying different set ups until satisfied. Don't cut up blades to make the new blades, because eventually you will end up making more blades like the ones that were cut up.
If it was mine, I'd probably start with 5 blades, about 4' diameter. And 5 blades 4.5' diameter. And I'd expect to make a lot of changes as things went along, because I always do.
I believe 4 blades is generally frowned upon. Even number (2, 4) of blades can create issues. 3 or 5 is the 'usual' number.
Pipe blades are slow compared to typical carved wood blades, so I think 5 is usually good.
I believe after cut in that motor will create some pretty serious drag, and 5 PVC blades may get past it fine.
And if I needed 5 blades, I'd start out making 6-8... One will be too heavy/light, one will be damaged when 'the tools' do something wrong
, etc.
G-