We said now in BEM theory, we’re actually looking at individual blades. That is important. What’s happening at an individual blade section or a strip that cut out of the blade? Wind is coming from the left from left to right hitting a turbine blade. I cut out a section, wind is coming from the left. There’s an airfoil. Look at your picture, the wind is hitting the lower surface or the upper surface of the airfoil? Lower. What does a propeller do? Upper. What would you have expected? Lower or upper? If I had asked you before class? Lower? I found that weird when I saw that the first time, that the wind hits the lower surface. Because you would expect, like for a propeller, that it would hit the upper surface. Well, there is a fundamental difference. The propeller puts energy into the flow and what the propeller does, it sort of tries to swim through the air and push the aircraft forward with it. So the lift vector is going to be directed forward. For a wind turbine, we want to do the opposite. We don’t want to put energy in, we want to get energy out and sell it to the electrical grid and make money with it. The forces along a section of a wind turbine blade, they look differently. In the end, what you want to do is you want to generate torque. Because that torque times the rotor speed, capitol Omega, is going to produce the power to the generator that you can sell. The rotor thrust is a side effect associated with it. You can do anything with the rotor thrust. The rotor thrust is just annoying because it’s a large force that’s acting on top of an 80 meter tower and creating a humongous bending moment to the foundation. So it’s a side effect that you have to live with. And if you look at that figure well to call it a thrust is a little bit of a stretch because it’s acting in the air speed direction. For a propeller the thrust would be defined opposite , because that’s what pulls you forward. For a wind turbine, the thrust is actually a drag. But we call it a thrust. Let’s keep that in mind. But these torque what we want and thrust what we have to live with unfortunately. Locally they are generated by the airfoil somehow. Depending on the angle of attack that the airfoil operates at, if the drag is caught in this direction that means the local velocity vector is in the direction of the drag. The lift is perpendicular to that. And we define the angle between the thrust direction and the local lift vector as the blade flow angle phi.