Evening, all!
Fuzzy, here is my take at explaining.
First let me say that even if stalling a single element might not sound sensible, it is just a matter of referential.
The only thing that needs to be done is to breakdown the actual force applied on that secondary profile between a component parallel to the ground (drag) and a perpendicular one (downforce).
That might sound trivial, but we just need to agree that drag and downforce are not defined relative to the profile chord or whatever.
With the secondary profile being so inclined relative to the ground (carefull, I am just talkig about inclination relative to the ground, angle of attack is not the same here, as the incoming flow as been bent upwards by the first element), when the flow is attached, the resulting force contains a drag component that is quite strong.
When you need maximum downforce from your wing (say, when you turn
), that drag component is not a major issue for you: you want max df, pay the price for it.
However, in straight, you don't care really about your downforce past a certain point. Therefore, if you could prevent this second profile from working properly, then you should be able to save on the drag component. From there comes the idea of stalling.
In the books, stalling a wing is not great, because you look at forces in the wing referential. Projecting them on the ground referential, and again because of that high inclination of the second profile, the actual drag penalty is quite reduced.
And in the same time, the drag penalty you were taking from creating maximum downforce from your wing is gone...
I saw in one of the previous pages a graph where the forces on the second profile were represented in the profile referential. I will not take it and project them back in the ground referential, but have a look at that and you'll understand.
Hopefully, I was clear enough!