747 weight distribution

On Mon, 20 Jun 2005 20:42:54 GMT, Gord Beaman
wrote:

Brian Whatcott wrote:

On Mon, 20 Jun 2005 03:28:37 GMT, Gord Beaman
wrote:

Peter Clark wrote:

On Sat, 18 Jun 2005 08:07:11 -0400, Jack Davis
wrote:

On Sat, 18 Jun 2005 03:25:15 GMT, Gord Beaman
wrote:

Jack Davis wrote:

Huh? I've not flown a jet with a "small fuel tank in the tail". Am I
missing something?

-Jack Davis
B737

Only on larger a/c Jack...747 have them (some at least). What I
find interesting is how few people seem to know much about them,
is it that they aren't used much now?...

I see. Obviously you can count me among the many who didn't have a
clue, and I used to fly 747s (-100 and -200).

FWIW, the 400 has them. I believe the 777 and A300 are similarly
equipped. From what I understand they pump to the CWT and help keep
things in the trim envelope when you're near MTOW and high fuel loads.

Yes, I'm certain that some (at least) of the 747's do have them
but this reason isn't what I'm talking about...it seems that the
Concorde article in this regard is all to do with the critical
fore/aft balance and to keep the CG where they want it for
different phases of flight and this post from Peter sounds
similar BUT it's not what I'm referring to which is using fuel
weight INSTEAD of aerodynamic trim to reduce drag for more
economical cruise.


Hmmmm...maybe I'm missing something: trimming near aft CG limit is
aero drag favorable in sub sonic transports

Brian Whatcott

Yes it is IF it's done by transferring fuel aft. This has to be
done ONLY at stable cruise though because it drastically reduces
the fore/aft stability and requires operation of the
autopilot...I understand that some a/c are unmanageable without
an autopilot in this condition due to the reduced stability.

But that's not what I understand stab tanks are for. With MTOW and
lots of gas you are pretty heavy forward, and can be out of the
acceptable takeoff trim range, so having fuel in the back helps put
you back in the envelope (basically acting as ballast). As you fly
off the fuel in the main tanks, you replenish with the stab tanks by
pumping forward (range). They don't replace (or get used for)
elevator trim.

I believe the Concorde didn't have elevator trim due to the delta
wing, so the only way to make adjustments was to pump the fuel around
and physically shift the arm.

Peter Clark wrote:
snip

But that's not what I understand stab tanks are for. With MTOW and
lots of gas you are pretty heavy forward, and can be out of the
acceptable takeoff trim range, so having fuel in the back helps put
you back in the envelope (basically acting as ballast). As you fly
off the fuel in the main tanks, you replenish with the stab tanks by
pumping forward (range). They don't replace (or get used for)
elevator trim.

I believe the Concorde didn't have elevator trim due to the delta
wing, so the only way to make adjustments was to pump the fuel around
and physically shift the arm.

Yes Peter, I fully understand what you're talking about 'but this
ain't that'. I do understand that some a/c do have large CG
changes as fuel is burned off. The a/c that I'm probably most
familiar with is the CP107 Argus (4 engined ASW patrol aircraft).

They held 6640 Imperial gallons of high octane fuel and their CG
changed only slightly between full fuel and dry tanks. The CG
started off more or less in the middle of the allowable range,
slowly moved forward till down to 3/4 full then reversed and
moved rearward slowly ending up near the aft limits near dry
tanks. (an odd pattern indeed)

Anyway I'll keep looking for info on this 'drag reduction thing'.
--


-Gord.

"I'm trying to get as old as I can,
and it must be working 'cause I'm
the oldest now that I've ever been"