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Bede
PostPosted: Thu Jul 19, 2012 2:57 am 
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Question:

Why are W&B datums in newer aircraft far forward of the aircraft ? The datum used to be the firewall or LE. What is the advantage of a datum ahead of the nose?


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Goodwrench
PostPosted: Thu Jul 19, 2012 3:15 am 
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For one, it removes the requirement to deal with negative numbers in the calculations.


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Bede
PostPosted: Thu Jul 19, 2012 5:04 pm 
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I thought of that, but so would using the front of the spinner. The B737 has the datum somewhere around 100" ahead of the nose. Apparently this is standard practise now.


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Go Juice
PostPosted: Thu Jul 19, 2012 5:52 pm 
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So when are ya'll starting to use the new 703 weights?


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ChallengerDan
PostPosted: Thu Jul 19, 2012 5:55 pm 
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What I was taught in school was that for engineering and drawing purposes it provides a mean for installing test probes and can also accommodate in change in design before certification without making the FS numbering cumbersome. On the CL600 series, it is 144 inches in front of the radome bulkhead. The math and the drawings are simplified because of it. I don't know if this practice trickled down to GA aircrafts though.

Terms of weigh and balance though, on these, you will be using MAC % for CG calculations, and if you plumb-bob it for any reason (symmetry, mapping, etc) you have a REF FS installed on the belly. FS439 IIRC. Not sure how it works out on the B737 but should be similar.
For us mechanics and for pilots, really doesn't make much difference. Do the math and check the charts, right?
12402340 lbs-in @ 765.2 inches doesn't mean a whole lot to me once I get out of the hangar. 24% MAC shouldn't either once you come out of the cockpit.

Streamlining the calculations was a big improvement because now you are just adding and substracting moments depending on weight added and removed, which is instinctively easier than considering the position relative to the datum to determine POS or NEG. Maths are not the strong point of Average Joe. And while putting it up front, why not make some room for that fancy Test Pitot and AOA vane that are sitting 8 feet in front?


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cpta2020
PostPosted: Fri Jul 20, 2012 12:00 pm 
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Bede wrote:
I thought of that, but so would using the front of the spinner. The B737 has the datum somewhere around 100" ahead of the nose. Apparently this is standard practise now.



Thats probably where the Test Pitot Tube was sitting during Certifications.

at least thats what I was told before

Cheers,

:)


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