Kajs,
I'm not cruel, but I did want to get your attention. Those pictures from Royal Fabrication are interesting. It's a good example of a solid wood blade breaking, but your blade isn't solid. It's more complex, so its strength is also a more complex question.
There's no way to do a proper analysis through the internet, and one isn't called for anyway. By eye alone, it looks to me like you've missed a crucial structural element: the spar, or some equivalent beam, that carries the structural load from the skins of the blade, to the root, to the attachment. How will this blade be attached to the hub? In your second picture I now see some spacer blocks between ribs, but they are far from being a continuous member that take the bending from tip to root.
Ron isn't quite correct about the exterior shell taking the bending load, but he follows up that comment with his concern about the lack of a spar, like mine, so he asks the same thing, just in a different way.
Lift is the force a wing/blade/airfoil makes, perpendicular to the oncoming wind.
In you mind's eye, look at a 3-bladed rotor face on. The wind is at your back, and it turns (clockwise or counter-clockwise). Each blade is sweeping through the air as it rotates, and the wind is pushing air through the swept disk. For the sake of argument, the angle of attack of the blade is 15 degrees from the plane of the disk. The lift force made by the blade point, then, about 15 degrees away from the axis of the rotor. This makes for a lift force on that blade that is partly pushing the blade back toward the tower, partly making the blade turn.
Whenever I try to work out the thrust load on wind turbine blades, I get a force that increases with increasing wind speed and power captured. Figuring out exactly how much is like throwing darts in a dark room! I can only guess, and use a bit of math, but I usually come out with about 400 pounds for an 8-foot (2.44m) diameter rotor.
If this is the only blade made this way, then finish it up, find a way to mount it (you would have to do that anyway to attach it to the rotor hub) and put weight on the tip, gradually, until you see wrinkling of the lower surfaces between the ribs. Do not put more load on after you see the wrinkling: there won't my much more strength after that effect appears.
Okay, so maybe I do sound cruel.
If you can put 100 kilograms on the tip, and you see no wrinkling, then you are darned lucky and you can forget about my tirade! The joints of the ribs to the skin and the closed plywood cell may provide enough shear resistance to keep the blade from collapsing.
Please report back if you do decide to carry out that test!
By the way, this is the perfect time to explain what my alias means:
A "spar web" is the flat sheet between the top and bottom members of an I beam. The web can be thin if the beam is not subject to crushing, or loads out of the plane of the beam. The web is normally loaded with shear and buckling, and it can be very complex to analyze. Unfortunately, it is often ignored, at the designer's peril!
So: the skins of your blade can provide tension and compression strength, but the ribs lack the SparWeb to carry the shear. In another time and place that could be funny.
Please don't be discouraged. I can make all kinds of suggestions, but it's ultimately always in your hands to satisfy yourself with your project.
Good luck.
