While blasting across the northern French countryside I noticed that the wind turbines along the rail line are installed such that they point slightly "nose-up". I don't have a more technical term for that, or said another way the disc of the blades does not appear to be perpendicular to the ground / horizontal.
I snapped a photo of one, and it's not incredibly obvious, but I looked at several and they all have a distinct "few degree" attitude.
I would have expected that the maximum wind vector over any period of time is horizontal to the ground, so you'd want the blade disc to be perpendicular to that, but no one ever engineers something like this without a reason, and I'd love to know what that reason is.
Another thought I had is that maybe the hub and blades are tilt-able in this particular axis, but that again begs the same underlying question which is why would being able to change this angle improve the efficiency?
Yet another thought is that maybe having the angle slightly nose-up reduces a mechanical load somewhere in the system? But if that's true... also "why"?