Brian Whatcott wrote:

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.

IIRC a Russian airliner became unmanageable and crashed when the
Captain's teenaged son wrestled control from the autopilot while
the a/c was trimmed like this.

Is this what you meant Brian?


Well...I also recall hearing crew asking passengers to move on
account of unexpected cargo distribution. That was pre-take-off.

Brian Whatcott

Yes, and that was strictly a CG manipulation thing...this other
isn't, it's a drag reduction thing only. I'm sure most of us know
that lateral stability of an a/c is supplied by the wing's
dihedral and fore/aft stability is a function of having the
aircraft quite nose-heavy which is balanced with quite large
down-force provided by the empennage...if most of that downforce
can be supplied by moving fuel aft then the aerodynamically
supplied downforce can be zeroed out saving considerable
drag...mind you, stability suffers and you need an autopilot to
handle the a/c.
--


-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"