It's been a while since I've posted and done anything with my windmills, but I wanted to get back to making some new blades over Christmas break. I think everyone has their talent or favorite part when making a windmill and mine is definitely the blades.
This time around I wanted to do a bit better and make a set of blades with something that closer resembles a high lift airfoil instead of having a flat wind facing face and a sharp front edge (school textbook airfoil). The rest of blades were have been historically pretty good with an exponential taper and twist, all done by hand. For my new blades, I would like to make a mini set on my CNC mini-mill (5"x6"x12" + rotary axis) as well a hand carved larger set for my treadmill motor. The mini set will probably be 10" long blades and the larger blades will be in the 25-30" range to hit a desired 455 RPM cut in. If the mini set goes well, I may CNC the large blades in several steps.
I found in the past my blades would struggle to get going and then would rocket up quick in speed and then seemingly not do good at high speeds. I was reading Sparweb's blade aerodynamics page and realized I made the same mistake he mentions in that my blades had way t0o high of an angle of attack.
http://www.sparweb.ca/1_Blades/Aero.htmlSo I did a bit of math to figure out the apparent wind angle on the blade based on the TSR and where you are along the length of the blade. Then I picked an angle of attack I wanted based on looking at a few dozen lift coefficient graphs. I picked an angle a bit below the peak lift such that the blade would provide more lift once it really ran into the load from the motor or the wind speed picked up... maybe this is the wrong way of thinking, but I liken to having more torque down low in the RPM's from a turbo charged diesel engine. If did this wrong, let me know. I have some time since I'm still trying to get my CAD software back up and running due to a Windows 7 update issue.
Hopefully I can break my old record of 13.3 amps going into a 12v lawnmower battery. I now have a ~5ah, 14.4 volt LiFe battery that used to be in my daily driver as a replacement for the 65 lb Pb battery to play around with as well as a pile of other 3S 5ah LiPo packs.