747 weight distribution

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"

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"

"Brian Whatcott" wrote in message
...
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

Reminds me of a funny but true story involving a 727. No, a 727
does not have an HST. (only our 747-400 did )
As many may know, some model 727s require 'tow fuel,' which
is a minimum amount of fuel to prevent it sitting on it's tail when
the airstair is raised. Normally an easy operation at the terminal
gate.
In this case, a charter 727 for one of the '49ers away games
parked way over in our cargo area to offload the team to their
busses as they usually did. On this day they were having a problem
rounding up a fuel truck to upload 'tow fuel' so the word went out
over the radio for about a dozen or so bodies to substitute for fuel
during the short time it would take to reposition the aircraft.
Didn't take much persuasion since most of us are aware of how
well a #1 sports team is fed. In no time the front of that 727 was full
and the party had begun. I guess I should mention that our company
had a well published (and sometimes ignored) rule that an employee
caught raiding the galleys would be fired. :-(
It happens that the Ops. Manager heard the call go out as he was
driving around in his truck and decided to lend his substantial girth.
Last to arrive, as soon as he reached the top of the airstair it was raised
but as soon as he realized what was going on up front he realized
what a compromizing situation he was in. Being a good person
by nature he decided to stay aft instead of coming up front and
firing all of our asses! Even when somebody yelled out, "Hey George,
you're cancelling half of us out!"
As usual that ice cream and cake tasted gooood.

On Tue, 21 Jun 2005 04:36:40 GMT, Gord Beaman
wrote:

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.


Let me put it this way Gord: in a conventional airplane layout,
loading the aircraft near aft CG limit ALWAYS reduces drag.

Now what do you say? CG aft = drag reduction........

Brian Whatcott Altus OK

Brian Whatcott wrote:

On Tue, 21 Jun 2005 04:36:40 GMT, Gord Beaman
wrote:

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.


Let me put it this way Gord: in a conventional airplane layout,
loading the aircraft near aft CG limit ALWAYS reduces drag.

Now what do you say? CG aft = drag reduction........

Brian Whatcott Altus OK

Oh yes indeedy...it certainly will, the reason is that now the
horizontal stab doesn't need to be trimmed for so much
'downforce' BUT the a/c tends to be unstable because the
stabilizing effect of having quite a lot of AERODYNAMIC 'tail
down trim' is reduced...this is what I'm querying, is fuel weight
induced tail down trim still used in cruise for economy?...
--


-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"

"Jim Knoyle" wrote:
snip
Reminds me of a funny but true story involving a 727. No, a 727
does not have an HST. (only our 747-400 did )
As many may know, some model 727s require 'tow fuel,' which
is a minimum amount of fuel to prevent it sitting on it's tail when
the airstair is raised. Normally an easy operation at the terminal
gate.
In this case, a charter 727 for one of the '49ers away games
parked way over in our cargo area to offload the team to their
busses as they usually did. On this day they were having a problem
rounding up a fuel truck to upload 'tow fuel' so the word went out
over the radio for about a dozen or so bodies to substitute for fuel
during the short time it would take to reposition the aircraft.
Didn't take much persuasion since most of us are aware of how
well a #1 sports team is fed. In no time the front of that 727 was full
and the party had begun. I guess I should mention that our company
had a well published (and sometimes ignored) rule that an employee
caught raiding the galleys would be fired. :-(
It happens that the Ops. Manager heard the call go out as he was
driving around in his truck and decided to lend his substantial girth.
Last to arrive, as soon as he reached the top of the airstair it was raised
but as soon as he realized what was going on up front he realized
what a compromizing situation he was in. Being a good person
by nature he decided to stay aft instead of coming up front and
firing all of our asses! Even when somebody yelled out, "Hey George,
you're cancelling half of us out!"
As usual that ice cream and cake tasted gooood.


Good man!...reminds me of 'manning pool' (basic in the RCAF). we
used to have 'lights out' in the barracks at some ungodly hour
like 10 PM or something and had a tough decip Warrant Officer
named Duke, who'd show up damned near every night at midnight for
a 'surprise bed check'...every night seconds before Duke stepped
into the room someone'd hoarsely whisper "Duke's comin!"...we
found out months later that the whisperer was Duke...
--


-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"