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#21
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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" |
#22
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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" |
#23
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"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. |
#24
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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 |
#25
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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" |
#26
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"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" |
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