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DocLightning
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747 Upper Deck And Balance

Fri Nov 13, 2009 10:30 pm

A question I've been wondering about:

On various models of the 747, the upper deck has changed size and has covered varying portions of the total fuselage length. The 748 has the shortest upper deck with respect to the fuselage of all models, while the 748i has the longest upper deck of any 747 (not sure about the ratio of upper deck length to overall fuselage length, though).

Yet with all these different upper decks, the planform of the aircraft is unchaged. For example, the wings on the 748 are the same distance back from the nose as they are on the 748i, even though the 748i has a much longer upper deck.

How does this work? Wouldn't this mess up the CG of the aircraft for one of the variants?
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zanl188
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RE: 747 Upper Deck And Balance

Fri Nov 13, 2009 11:35 pm

The easiest place to make changes to payload or structure with minimal effect to center of gravity is in or near to the middle of the fuselage - which is where Boeing has made changes to the upper deck.

Now if they made changes to the forward end of the upper deck....
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tdscanuck
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RE: 747 Upper Deck And Balance

Sat Nov 14, 2009 1:14 am



Quoting DocLightning (Thread starter):

How does this work? Wouldn't this mess up the CG of the aircraft for one of the variants?

It will move it around...the empty CG of a 747-8F is almost certainly different than a 747-8i. But that's not the same as "messing it up". The CG is pinned down, to a large extent, by how you load the cargo and fuel. That would be more than enough variance to cover up model-to-model differences due to the hump.

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lowrider
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RE: 747 Upper Deck And Balance

Sat Nov 14, 2009 5:16 am

I remember hearing somewhere that the extended upper deck adds about 18000 pounds to the airframe. With BOW's around 380k lbs on the 400, it is not that much to account for.
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747classic
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RE: 747 Upper Deck And Balance

Sat Nov 14, 2009 11:16 am

Each variant of the 747 has a different W&B computer (for checking the CG manually).
747-100/100B/100SF/SP/200/200M/200C/200SUD/F/SF/300/300M/300SF/400/400M/400F/400BDSF/400ER/400ERF

The influence of the stretched upper-deck weight is relative minor, compared to the overall weight distribution over the length of the fuselage. And the upper deck change is close to the OEW CG point.
The largest (movable) weights are the payload and the fuel. Especially the center-wing fuel creates a large fwd shit in CG of the 747. The 400 series (except freighters) have tail fuel tanks installed.

All 747-100/200/300/400 variants have comparable CG characteristics.
747-SP is different due reduced length.
Also the 747-8I/8F are different because of the increase in length and different gear positions.

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747classic
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RE: 747 Upper Deck And Balance

Sat Nov 14, 2009 2:23 pm

The only 747 variant with a little trouble on the CG (on the ground only) was the 12/13 pallet combi variant.
Before the tightening of the combi main deck fire fighting regulations after the Heidelberg crash(SAA 747-200M), due main-deck fire, it was possible to operate the 747-200M aircraft in 13 pallet configuration.
In this configuration it was possible that after landing (fuel tanks almost empty) and after disembarking of the passengers the aircraft would tail tip, due all the weight installed on the main deck cargo positions in the aft section of the aircraft.
In that case, first some main-deck cargo had to be off loaded, before the passengers could disembark.
Normally this was prevented by a correct load sheet at the departure station, but if due a rerouting or not so favorable headwinds more fuel was used, this could happen.

After installing main deck fire suppression systems on the main-deck it was only possible to operate 747 combi's in 6/7 pallet configuration, with no CG problems at all.
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Zkpilot
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RE: 747 Upper Deck And Balance

Mon Nov 16, 2009 1:59 pm



Quoting DocLightning (Thread starter):
For example, the wings on the 748 are the same distance back from the nose as they are on the 748i, even though the 748i has a much longer upper deck.

How does this work? Wouldn't this mess up the CG of the aircraft for one of the variants?

No because loading of payload onboard is done to balance CG to specifications.
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klemmi85
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RE: 747 Upper Deck And Balance

Mon Nov 16, 2009 5:40 pm



Quoting 747classic (Reply 4):
creates a large fwd shit in CG of the 747

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