747 weight distribution

[Robin]

Hello, this is an odd question of curiosity from an amateur: Two days
ago I was flying from Dulles to LHR on a United 747-400. The aircraft
taxied to the end of the runway, then the pilot announced there was a
problem with the weight distribution on the aircraft wings, and we
would have to move passengers to compensate. For the next half hour,
cabin crew moved passengers around, apparently from economy to
business, and then we took off. Is this normal? Can passengers body
weight really make a difference on a huge aircraft like a 747?

[Dean Wilkinson]

Yes, it certainly does... nose to tail is where the weight distribution
matters. Sounds like you had too many people at the tail at not enough
toward the nose...

"Robin" > wrote in message
om...
> Hello, this is an odd question of curiosity from an amateur: Two days
> ago I was flying from Dulles to LHR on a United 747-400. The aircraft
> taxied to the end of the runway, then the pilot announced there was a
> problem with the weight distribution on the aircraft wings, and we
> would have to move passengers to compensate. For the next half hour,
> cabin crew moved passengers around, apparently from economy to
> business, and then we took off. Is this normal? Can passengers body
> weight really make a difference on a huge aircraft like a 747?

[Brian Whatcott]

On 29 Mar 2005 06:11:36 -0800, (Robin)
wrote:

>Hello, this is an odd question of curiosity from an amateur: Two days
>ago I was flying from Dulles to LHR on a United 747-400. The aircraft
>taxied to the end of the runway, then the pilot announced there was a
>problem with the weight distribution on the aircraft wings, and we
>would have to move passengers to compensate. For the next half hour,
>cabin crew moved passengers around, apparently from economy to
>business, and then we took off. Is this normal? Can passengers body
>weight really make a difference on a huge aircraft like a 747?


loaders like to place the CofG near the aft limit. Fuel and cargo
make huge invisible loads.
Still, when someone gets it wrong, you can still shuffle passengers.

Brian Whatcott Altus, OK

[Roy Smith]

Brian Whatcott > wrote:
> loaders like to place the CofG near the aft limit. Fuel and cargo
> make huge invisible loads.
> Still, when someone gets it wrong, you can still shuffle passengers.

I was under the impression that most big jets had a small fuel tank in
the tail, and they could pump fuel back and forth to trim the CG. Am
I mistaken?

[Gord Beaman]

(Roy Smith) wrote:

>Brian Whatcott > wrote:
>> loaders like to place the CofG near the aft limit. Fuel and cargo
>> make huge invisible loads.
>> Still, when someone gets it wrong, you can still shuffle passengers.
>
>I was under the impression that most big jets had a small fuel tank in
>the tail, and they could pump fuel back and forth to trim the CG. Am
>I mistaken?

No you're not but they use it for economy...most a/c are built to
be quite nose heavy so they need quite a lot of nose up trim in
level flight, this increases the 'fore and aft stability'.

It's done with the moveable horizontal stabilizer and creates a
considerable amount of drag. Long range a/c (used to at least)
pump fuel back to the tail tank to replace the 'down force'
needed thus removing that drag...this removes the fore/aft
stability and mandates the use of autopilot (I think)
--

-Gord.
(use gordon in email)

[Jack Davis]

On Wed, 30 Mar 2005 04:09:15 GMT, Gord Beaman >
wrote:

(Roy Smith) wrote:

>>I was under the impression that most big jets had a small fuel tank in
>>the tail, and they could pump fuel back and forth to trim the CG. Am
>>I mistaken?
>
>No you're not but they use it for economy...most a/c are built to
>be quite nose heavy so they need quite a lot of nose up trim in
>level flight, this increases the 'fore and aft stability'.

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

-Jack Davis
B737

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[Gord Beaman]

Jack Davis > wrote:

>On Wed, 30 Mar 2005 04:09:15 GMT, Gord Beaman >
>wrote:
>
(Roy Smith) wrote:
>
>>>I was under the impression that most big jets had a small fuel tank in
>>>the tail, and they could pump fuel back and forth to trim the CG. Am
>>>I mistaken?
>>
>>No you're not but they use it for economy...most a/c are built to
>>be quite nose heavy so they need quite a lot of nose up trim in
>>level flight, this increases the 'fore and aft stability'.
>
>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?...that they've gone out of
style?...that the risks of using them isn't worth the
savings?...what?. They're called 'Trim Tanks' I believe and the
theory is to replace the aerodynamically derived 'down force' of
the tailplane with fuel weight at some expense of fore/aft
stability which is compensated for by use of very capable
autopilots.

This system is only used in stable cruise. IIRC this system
caused the crash of a large Russian airliner which was handled
roughly while in this mode by the captain's son.
--


-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"

[Jack Davis]

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).

Thanks!

-Jack Davis
B737

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[Brian Whatcott]

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).
>
>Thanks!
>
>-Jack Davis
>B737

Hmmm...in case we dismiss Jack out of hand, better mention that
Concorde used tank redistribution of weight in flight and the KC135
which has been shunting fuel round the sky for a long, long time, can
place fuel between various tanks for CofG purposes.

Brian Whatcott

[Roy Smith]

Gord Beaman > wrote:

> 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?...that they've gone out of
> style?...that the risks of using them isn't worth the
> savings?...what?. They're called 'Trim Tanks' I believe and the
> theory is to replace the aerodynamically derived 'down force' of
> the tailplane with fuel weight at some expense of fore/aft
> stability which is compensated for by use of very capable
> autopilots.

The Concorde used trim tanks. There's a fascinating explanation of it at
http://www.aircraft-info.net/aircraft/jet_aircraft/aerospatiale/concorde/.
Sounds like pumping fuel around to maintain proper CG during different
flight conditions was a full time job.

[Jack Davis]

On Sat, 18 Jun 2005 13:15:37 GMT, Brian Whatcott
> wrote:

>Hmmm...in case we dismiss Jack out of hand, better mention that
>Concorde used tank redistribution of weight in flight and the KC135
>which has been shunting fuel round the sky for a long, long time, can
>place fuel between various tanks for CofG purposes.

I knew about the KC135 (that one was obvious because of it's special
mission) and the Concorde, but when the OP wrote about "most large
aircraft" having trim tanks I had to ask. I really don't consider the
Concorde a large (or normal!) aircraft.

>Brian Whatcott

-Jack Davis
B737

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[Tim]

In article >,
says...

> The Concorde used trim tanks. There's a fascinating explanation of it at
> http://www.aircraft-info.net/aircraft/jet_aircraft/aerospatiale/concorde/.
> Sounds like pumping fuel around to maintain proper CG during different
> flight conditions was a full time job.

There's a double DVD from www.itvv.com - over 5 hours of Concorde flight
deck material from the Captain & Flight Engineer. Lots of explanation
of procedures; the 2nd DVD particularly covers a lot of the fuel
management. As an interested non-pilot, I found it fascinating.

T.

[Gord Beaman]

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).
>
>Thanks!
>
>-Jack Davis
>B737
>
Ok Jack...I haven't ever flown the 47 (not even as a pax) but
I've read a lot about this feature of large a/c. Perhaps it's
only certain marks of 747's that have it?...as I said before, I
find it very odd that so few have heard of it. That article about
the Concorde was interesting but wasn't what I meant, this
feature that I'm speaking of is solely an economy measure which
reduces drag for more efficient cruise.

It's only purpose is to replace aerodynamic tail down trim with
fuel weight therefore reducing drag. I don't know what the hell
it's called but it makes sense (to me at least)...frustrating
that not many know about it, even some 747 pilots/engineers.

Thanks for your courteous input Jack
--


-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"

[Peter Clark]

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.

[David Lesher]

Isn't there a 747 model with fuel inside the vertical stab? I seem
to recall reading of same...

--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

[Gord Beaman]

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.
--


-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]

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

[Gord Beaman]

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?
--


-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"

[Peter Clark]

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.

[Brian Whatcott]

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

[Gord Beaman]

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"

[Gord Beaman]

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"

[Jim Knoyle]

"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.

[Brian Whatcott]

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

[Gord Beaman]

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"

[Gord Beaman]

"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"