PDA

View Full Version : Fuel Savings From Cg


j_davey
20th May 2008, 07:50
Hi all,
Can somebody quantify the fuel savings of an aft Cg as opposed to an extrem Fwd Cg? i would really be interested to know what the difference is in real numbers.

we have always been taught that an aft Cg saves fuel and some w&b systems actually suggest an optimal LIZFW (usually around 10pt from aft limit)

Is it worth my while trying to achieve an optimal Cg on a short sector eg dub-fra ??

thanks, john.

BOAC
20th May 2008, 08:02
I have no figures, but whoever 'taught' you was correct and it is worth it on EVERY sector, and more so with every upward $.

j_davey
20th May 2008, 13:43
sounds good i`ll continue as such ... anyone got any ballpark figures?

john?

FE Hoppy
20th May 2008, 20:48
We had correction factors in the old L1011 AFM but I don't recall them off the top of my head. But for sure it was real and on long sectors when loaded aft of that used in the jet plan the savings were considerable.

ssg
20th May 2008, 21:50
2%

http://www.nasa.gov/centers/dryden/pdf/88338main_H-2002.pdf

galaxy flyer
20th May 2008, 22:30
1 1/2% for every 5% of CG shift on a GLEX. Had charts for large types like the C-5, 744. It is significant.

FE Hoppy
20th May 2008, 22:47
2% on the data from the 310 tests from AB but the figures are different for each type as G flyer states.

GF do you know Mike McCook?

j_davey
20th May 2008, 22:59
2% ? Wow that is quite significant! Similar to the fuel saved by winglets.

John

FE Hoppy
20th May 2008, 23:27
conversely some manufacturers offer Forward CoG schedules to improve take off perf. Ideally you would like to have it forwards for take off and then shift it back in flight which is what many jets do now.

ssg
20th May 2008, 23:59
Yeah your welcome Hoppy...feel free to go to that Nasa site link I posted..have a nice day..

FE Hoppy
21st May 2008, 00:49
The pdf you linked talks about a 2% reduction on an A310. so you are welcome.

Down Three Greens
21st May 2008, 01:08
http://iata.org/NR/ContentConnector/CS2000/Siteinterface/sites/whatwedo/file/Airbus_Fuel_Economy_Material.pdf

Pages 10-12

All airbus types

DTG :ok:

FE Hoppy
21st May 2008, 01:13
ssg might be better off reading this one: http://www.flightlevel400.com/document_files/grips/GettingToGripsACPerformance.pdf

FE Hoppy
21st May 2008, 01:18
or this:http://www.smartcockpit.com/pdf/flightops/flyingtechnique/41

galaxy flyer
21st May 2008, 03:15
FE Hoppy

Nice link to Airbus V1 philosophy. Mike McCook doesn't ring a bell, where might I have met him? I'm an exile from some of ssg's threads.

john_tullamarine
21st May 2008, 04:06
I recall a presentation from a QF fellow at the RAeS some years ago .. relating to a study (which was driven by a pilot's interest, rather than the engineering folk at it turned out) which looked at rescheduling the tailtank usage on the 744 to constrain the aft cg a little more ... long range it was worth a couple of (fare-paying) extra passengers .. over a year, something in excess of a lot of money.

FE Hoppy
21st May 2008, 10:37
GF

He was a Galaxy FE and Instructor. But I guess there are many.

Angels 60
23rd May 2008, 17:51
I checked out the Nasa link for aft CG...can't say it applies to all planes, but they seem to make it clear that if you can leave the fuel as far aft for the duration of the flight....2% savings is the approximate result.

Begs to ask the question of why we still hang our tails on the end of the plane the way we do...

FE Hoppy
23rd May 2008, 18:11
Angels,

Don't forget that aft CoG is bad for Vmc so it's not always appropriate to have an aft CoG.

Angels 60
23rd May 2008, 18:24
Couldn't agree more Hoppy...I guess in some cases trying to save a buck on fuel, or engines, or whatever, we could end up reducing safety...defeats the purpose if the plane goes in.....

FE Hoppy
23rd May 2008, 19:18
doesn't make it any less safe, just restricts max weight. The rules are set by those who's job it is to oversee safety. Our job is to maximise profit within those rules. This is a business not a sport.

Angels 60
23rd May 2008, 19:48
Can't argue with that...if the numbers don't add up right, no one flies, it has to make sense financialy to fly a plane.....

Not being familiar with moving fuel forward and aft...do you try to move the fuel center of CG for take off, then aft once at cruise?

exeng
24th May 2008, 03:29
They were fairly slick at moving fuel about.

For the rest of us on slower aircraft such as the 747 the fuel management was predicated on wing bending moment. So you got rid of the fuel in the centre tank as soon as you could (which also had the effect of moving the C/G aft) then burnt from the inners and then from all four mains.

Happy days they were:)


Regards
Exeng

Angels 60
24th May 2008, 03:55
That makes sense..thanks Exeng.

OutOfRunWay
25th May 2008, 16:28
I always thought the bigger busses, from the A310 onward used an automatic pumping system to keep CG aft (within limits) pumping fuel into the horizontal stabiliser?

Isnt that why the stab tanks are called trim tanks? Any big bus drivers out there who can correct me..

Im only a little bus man...

regards

OORW

groundfloor
26th May 2008, 14:10
This is what the Airbus 34-6 book says:


The trim tank transfer system controls the aircraft's Center of Gravity (CG).
The system either transfers fuel to the trim tank (aft transfer), or from the trim tank (forward transfer).
This movement of fuel changes the aircraft's CG.
When the aircraft is in cruise, the system optimizes the CG position to increase fuel economy by reducing drag.
Normal operation is automatic, but the crew can manually select a forward fuel transfer.
The Fuel Control and Monitoring Computer (FCMC) calculates the aircraft's CG and compares it to a target value. (This target depends on the aircraft's actual weight. See AFT CG Target Graph below).
Based on this calculation, the FCMC determines the quantity of fuel to be moved aft or forward in flight.

It does it very effectively.:)

Admiral346
26th May 2008, 23:38
All the A330/340 family do that kind of transfer, usually after takeoff - there might be fuel in the stabilizer tank (trim tank) during takeoff, but that depends on the overall quantity taken.

If this particular system is inoP (the aft transfer), there is a fuel penalty according MEL of something like 1.5 - 2% (I forgot, it's been a while now).
So this percentage should be what is saved by having an aft CG.

Nic