Re: 747 EUD CG change. Stab change?

Date:         01 Feb 99 02:37:17 
From:         "Michael F. Lechnar" <>
Organization: nams
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MCL757 wrote:
> Someone asked me if Boeing enlarged the horizontal stab on the 747 when they
> extended the upper deck to compensate for the CG change.  I don't think the
> stab was changed.  I figure that considering the high weight of all the fuel
> that could be in different places affecting the CG that the extra weight of the
> extended upper deck really is relatively small.  Does anyone know?  Was there
> any stab change or did they recalibrate the trim's zero setting?

The external lines of the horizontal didn't change with the stretched
upper deck.  For the 747-400, provisions were made to carry fuel back
there.  In fact, you need the have that tank active to use the full
875,000 lb MTOW.  The fuel isn't kept there for cg control, but rather
it is moved into the CW tanks as soon as there is room.  This is
typically over by the time the airplane reaches cruise altitude.  The
drag benefit for keeping the cg aft is too small to justify the
complexity of a fuel management system to keep the fuel back there.

Mike Lechnar
Aircraft Performance Engineer
"If I was speaking for Boeing, I wouldn't be doing it here."