Background:
Almost all of my blades have twisted to some extent after carving them. Good, low moisture, knot free wood is hard to come by here and if it can be found, it's expensive. I carved a set of tapered blades from birch plywood. No twisting or warping but low strength. Broken blade count now: 3 sets. They're breaking at the rotor.
In an effort to increase the strength, I selected a NREL symetrical blade shape and made lots of modifications. I'm not an aeronautical engineer, so I did a LOT of guesswork.
The new 4 foot blades began as a double thickness of 3/4" birch plywood glued together. Finished dimensions are 1 inch thick, 5.25" chord, straight, with no taper or twist and are set with 10 degrees as angled from the hub. The second angle takes up about 12 the blade width on the windward side of trailing edge is an additional 5 degrees. They are covered in 3 layers of 2.3 oz. fiberglass per side, which amounts to about 18" additional thickness. 2 ounces of lead is all it took to balance them. If I'm not explaining this so it's understandable, just look at the pic....
The blades start turning in the slightest breeze. Cutin is about 75 rpm. I have no idea about efficiency. They don't seem to stall under a heavy load from the batteries. One thing I really like about the blades is they don't seem to want to overspeed. I can hear them start to stall just before furling at around 40 mph (too high) That's around 470 rpm by my freq. meter. They need a lot more testing and I'll be asking a bunch of newby questions about testing them. For now, I'm glad to have a set of blades that are still together.
