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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"?

Peter Mortensen
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ljwobker
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5 Answers5

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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.

Phil Sweet
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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

LazyReader
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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.

WillKeogh
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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.

Jack Grahl
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Just visited a traditional windmill yesterday in the Netherland and had the same pressing question in mind.

After reading other explanation, I think "clearance" from the supporting pylon makes more sense to me, at least for those old windmill.

The long blade will bend in the direction on the wind blow due to resistance and clearance is need from the supporting pylon. Also, the blade is twisted by the wind, bringing closer to the pylon.windmill balde deflection

NiPe Pen
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