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Go 4 TLI

Airframes and lunar tidal forces

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Building one of the most advanced jet fighters in the world is a challenge for any aerospace company – but the one thing you might think you don't have to worry about when you start such a job is the pull of the moon.

But that is exactly the challenge faced by workers at BAE Systems on the Lancashire coast every time the Typhoon build process begins – because the moon's gravitational pull actually causes the ground to move beneath their feet.

So fine are the tolerances now used to build the Typhoon that even the movements of the tide could throw the jet fighter tolerances out.

Spiffy! The whole link is here.

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Wow. That's the first time I've ever heard of the need for such precision. Do you know of other cases, Hannes?

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I don't know if it's still true, but at one point a UK firm manufactured the most accurate lathe in the world. (The UK government had to approve the sale of these units.)

A run of the mill lathe is good to one-thousandth of an inch. If I'm remembering this right, the uber-accurate lathe I'm thinking of was quoted as being at least one-millionth on an inch. (I remember the actual figure being Classified.)

To get this accuracy, these units are installed in facilities that insulate them from everything -- temperature fluctuations, ground vibrations, erratic power supply, etc.

As a final control, these units lift themselves on computer controlled layers of air. This "pillow" is the final control for vibrations and expansion/contraction that are present even in the most controlled facilities.

For something like the unit I'm thinking of:

http://www-eng.llnl.gov/lodtm/about_pg2.html

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Building one of the most advanced jet fighters in the world is a challenge for any aerospace company – but the one thing you might think you don't have to worry about when you start such a job is the pull of the moon.

So now you can't fly when the moon is out? :)

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Phil,

I had known that scientists have used space based radar to measure the bulge on the ocean's surface -- about one meter high -- caused by the moon's gravity. (Or perhaps in the spirit of the the original link I should have written "about one metre high". :)) I had also known that scientists have used satellite laser ranging to measure continental drift to accuracies of millimeters per year. And I always knew that tolerances for optical instruments, particulary those measuring very short wavelengths, are extremely stringent.

But I never have given any thought to what the tolerance for an airframe should be. I always thought the most precsion required in an aircraft would be in the engines and particularly in the compressor and turbine blades. Stumbling on this tidbit was certainly a pleasant surprise.

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This was a fascinating article! I wonder if the tug of Moon's gravitational pull is felt not just at the coast, but far inland as well? It would be interesting to know if the engineers at BAE knew of this phenomenon before they built the factory on the coast.

Thanks for the link, Go 4 TLI.

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The B2 bomber has a tolerance of approximately 1/4 inches over a 172 foot wingspan. It was one of the first aircraft to benefit from computer-networked automated machines. By contrast, the old Mig jets had gaps you could stick your finger in.

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