So, has accosting mentally unbalanced rednecks on isolated country roads so lost its charm that ET has taken to randomly knocking blades of wind turbines in Lincolnshire?

Nah, fuck off.

Sorry to disappoint any budding ufologists looking in the but the real villain of piece here isn’t a vistior from outer space, merely plain old physics.

To explain what did cause one of the turbines to shear off sometime on Sunday morning we need to start with a photograph of the damaged turbine, preferably one that hasn’t been photoshopped by a moron at The Sun newspaper. in fact, this one will do very nicely…

So, what you’re looking at there is a Horizontal Axis Wind Turbine (HAWT) with a blade length of approximately 22 metres situated on top of a 90 metre tower. As for the location of the wind farm, its about 7 km inland from the Lincolnshire coast, as the crow flies, at an approximate altitude above sea level of only around 2-3 meters. In addition, there are also two other important things to note; its an upwind turbine (i.e. the rotor faces into the wind ahead of the tower to minimise turbulence) and, in operation, the rotor turns in a clockwise direction.

The other important information we need relates to the weather conditions at around the time that the damage occurred and, luckily, there’s a Met Office weather station about 5-6 km NE of the windfarm at Donna Nook and the data from this stations tells us that its a was a fairly cold night on which the air temperature ranged from 0C to -1C and there was a steady 7-9 kn westerly wind (but no data on any gusts, sadly). So, as the turbine was operating at the time, it must have been facing inland.

Now, assuming that the turbine was operating correctly prior to the damage occurring, there are three main things that could lead to serious damage.

The first is a brake failure. Wind turbines of this type typically operate at between 10 and 22 rpm and are fitted with a braking mechanism which prevents them turning too rapid and placing the turbine under too much stress, remembering that in free rotation (without the brakes engaged) the tip of the rotor will typically be moving at a speed of around 6 times the wind speed. As it turns out, there an incident of just this type in Denmark only last year, one that was actually captured on video, which means you can see for yourself what kind of damage can arise from a brake failure.

[youtube]http://www.youtube.com/watch?v=c3FZtmlHwcA[/youtube]

On the face of it, a brake failure seems unlikely given that it was operating in what would only be classified as a gentle breeze on the Beaufort Scale and the damage is nothing like as extensive as our Danish example.

Secondly, there’s turbulence, which turbine designers seek to minimise by using an upwind turbine, at the cost of it needed a motorised drive system to position the rotors facing into the wind, as opposed to downwind design, which doesn’t require an additional mechanism to keep them in line with the wind, and also the reason why wind turbines are placed on top of such high towers, keeping them away from any ground level turbulence.

In this case, its difficult to see quite what would generate turbulence sufficient to snap off a blade in such a light wind, given that the blade length in use is fairly modest given the height of the tower on which the turbines was situation – on a 90 metre tower its possible to use blades of up to around 40 metres in lengthy:

With that in mind and nothing unusual in the weather to suggest that the turbine was operating under unusually turbulent conditions, turbulence alone seems an unlikely suspect.

Finally, there’s the problem of cyclical stresses which comes in two parts.

First, even using an upwind turbine you have some degree of turbulence to contend with because the tower hinders the airflow at the lowest point in the stroke, as each blade sweeps down and through past the tower. This results in a cyclical twist on the main bearing which could wreck the turbine, but this can be compensated for by allowing the turbine shaft to rock through a few degrees in order to allow it to avoid having to resist the worst of the peaks in torque, and its wouldn’t necessarily cause the blade to shear – if anything, the turbine shaft would go first and the turbine would lose the entire rotor assembly.

Second, and this is where things become much more interesting, facing a turbine into the wind causes its rotor to act like gyroscope with the result that the blades are subject to gyroscopic precession, which tries to force the turbine to ‘wobble’ around its main axis much like a spinning top wobbles as it turns at speed. The cyclical twisting that this generates places significant stresses on the turbine’s axle, bearing and – most interestingly – at the root of the blade where its attached to the axle assembly, which is precisely where the blade sheared off on this turbine.

To make matters even more interesting, the stresses arising from gyroscopic precession are at their weakest when the blade is horizontal, but at their strongest when the blade is vertical and they apply a backwards force against the rotational direction of the blade. So, as the blade turns clockwise through the vertical position, gyroscopic precession is trying to force the blade in an anti-clockwise direction, which means that if a blade were to sheer off at or close to its highest vertical position, the forces that caused to shear would also force it backwards towards the next, oncoming blade.

And if you look at the photograph you’ll that its the blade that located in an anti-clockwise direction from one that sheared off that’s bent in the middle just as you would expect it to be bent had it been struck about halfway between the middle of the blade and tip, just where you can see a slight kink in the photograph, by large metal object. Say, for example, something like a 22 metre long metal blade falling backwards at a reasonable speed from a near vertical position at the top of the tower having just sheered off at the base.

As for why a blade like would shear off, a combination of metal fatigue or a structural flaw in blade plus the cyclical stresses the blade’s subjected to as the turbine rotates would do the job very nicely, or it may simply not have been fitted correctly when it was installed…

…and there’s your explanation for the damage caused to the turbine and there’s not an alien spacecraft anywhere within light years of the Earth let alone buzzing a wind farm in Lincolnshire.

You see, it is just like they used to say on the X Files. The truth really is out there and its very simple, straightforward and requires nothing more complex than a halfway decent understanding of some fairly Newtonian mechanics.

Sadly for our would-be ufologists, ET will not be trying to phone home, yet again, because ET was never here in the first place.

RES IPSA LOQUITUR

Footnote.

Despite The Sun having been the paper to ‘break’ the entirely laughable ET angle on this story, by far the best coverage you’ll read today is to be found in the Daily Telegraph which included this in its coverage:

At least half a dozen Lincolnshire residents reported seeing the orange-yellow spheres, which some witnesses claimed were trailing octopus-like “tentacles”.

Mmm… makes you wonder doesn’t it… were they really tentacles or could they have, perhaps, have been noodly appendages..?

Oh, and before I forget – if anyone from the Healthy and Safety Executive swings by could you please pay a bit more attention to reports like this:

A wind turbine blade came crashing to the ground Wednesday, halting energy production at a small-scale wind farm southwest of Wyanet because of what may be a defective design.

In all, four turbines on Richard Shertz’s property – all part of the AgriWind facility in central Bureau County – have stopped turning after a blade on top of one of the towers broke off about 9:30 a.m.

Before saying things like this the the press…

The Health and Safety Executive described the damage as a “unique incident”

Particularly in light of this…

A Suzlon company representative said the blades on all four turbines were scheduled to be replaced next week.

The representative also said the point at which the blade separated from the stem is a spot where cracks have been found on other blades. The cracks are the result of a design issue, and the company is currently retro fitting blades throughout the country.

And do get a dictionary and look up the meaning of the word ‘unique’ as well.

UPDATE

Moving swiftly past the latest daft theory, which suggests that our non-existent UFO could, in reality, have been the MOD covertly testing a new BAE systems UAV, the Taranis, more than a year ahead of schedule, I’ve had a couple of slightly puzzled e-mails from friends who can’t quite see how a blade that shears off a wind turbine that’s rotating in a clockwise-direction could hit and damage a following blade on the same turbine.

The assumption, put simply, is that the angular momentum of the blade that shears off should cause it to be thrown clear of the turbine, possibly hitting a neighbouring tower/turbine in the process, all of which sounds perfectly reasonable provided you neglect to consider all the forces acting on the blade at the point at which it breaks off.

The assumption my friends are making is that the blade, when it shears, will continue moving in more or less the same direction it was travelling just before the failure occurred and fly off at a tangent to the circle described by the rotation of the blade around the the turbine’s axle, as it were being launched by something like a trebuchet.

This is what would happen if a blade shears after its passed the vertical, but if it breaks off on the way up to vertical that you also have to factor in the effect of anti-clockwise forces acting on the blade as a result of gyroscope precession, which are significantly larger at the tip of the blade than at the root, where it attaches to the axle. Provided that the blade shears off on the upstroke, as the blade is moving from the horizontal (9 O’Clock) to the vertical (12 O’Clock), the difference between anti-clockwise gyroscopic forces acting on the blade at the tip (strong) and the root (weak/negligible) will impart backspin on the blade when it breaks loose (and cause it to twist in the air as well), preventing it from being thrown clear of the turbine (and the following blade) by its forward momentum.

Simple.