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. 
