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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"?

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    $\begingroup$ I think it's worth adding that side views of traditional windmills at e.g. en.wikipedia.org/wiki/Windmill similarly show the axis tipped upwards, so this design feature has a long history. $\endgroup$
    – Mark Morgan Lloyd
    Nov 22 at 7:41

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Short answer- because the pylon causes too much interference otherwise. Tipping the rotor up increases the clearance to the pylon and reduces flow perturbations and their associated vibration problems. For some designs, they can also exploit the vertical velocity gradient. That requires a bit of rake on the bladeset. There is more specific energy up high, so you reduce the swept area in the top half of the disc by tilting a raked rotor upwards. A third use is to try to deflect the wake in a way that minimizes the impact on other turbines in the farm.

And when in doubt, ask NREL - https://www.nrel.gov/docs/fy17osti/68004.pdf

BTW, the power loss due to tilting is much smaller than you might think for small angles. It runs about as the cosine of the tilt angle cubed. So a 5 degree tilt is still about 0.9889 of the power.

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  • $\begingroup$ The clearance is probably the most important. I think the tilt was introduced long before interference between wind turbines became a problem. $\endgroup$
    – Peter Mortensen
    Nov 21 at 23:23
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Most airfoil blade shapes; forward orientation is slightly bent upward 10-15 degrees because this creates the most lift with the least amount of drag. enter image description here

The now slightly turbulent air flow isn't just blowing forward, it's coming up against the edge of the blades. enter image description here

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The blades bend backwards under wind forces and if they hit the tower its frequently catastrophic - the tower wall buckles, weakening it, and the blade disintegrates causing the fast spinning rotor to become highly unbalanced, and the turbine collapses. Wind turbine design standards have strict rules on minimum tip-to-tower clearance allowed in extreme wind gusts. The nacelle tilt that you observed, and coning of the rotor (blades are not in a flat plane), keep the blades further away from the tower, at the expense of a small energy loss.

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  • $\begingroup$ Wouldn't it be easier (and with less loss) to just push the blades forward more? Longer shaft to the nacelle, I mean. $\endgroup$
    – cthulhu
    Nov 21 at 11:23
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    $\begingroup$ @cthulhu, that would shift the centre of gravity and, more importantly, it would affect the moment of inertia and the ability to adjust yaw. You want to hold the propeller close to your chest! $\endgroup$
    – Transistor
    Nov 21 at 12:27
  • $\begingroup$ Great answer! Follow question, in extreme winds, quite simply can/do wind turbines stop? Can/do they feather? What happens to wind turbines in extreme winds, do they just spin (and generate power!) like hell? $\endgroup$
    – Fattie
    Nov 22 at 12:44
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    $\begingroup$ Why not have the rotor further forward of the tower ('overhang' is the technical term)? Because the rotor and nacelle are heavy - several hundred tonnes on a multi-MW turbine - and having that sort of mass cantilevered 10 or more metres out in front would require a lot of extra steel. I'm confident that the turbine designers have carefully studied this and the geometry you see is the optimal tradeoff. $\endgroup$
    – WillKeogh
    Nov 22 at 13:10
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Because under heavy winds the blades can bend back slightly. The angle ensure that even if they do so, they won't be fouled by the mast.

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