Fuel Savings From Cg

[j_davey]

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]

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]

sounds good i`ll continue as such ... anyone got any ballpark figures?

john?

[FE Hoppy]

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]

2%

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

[galaxy flyer]

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]

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]

2% ? Wow that is quite significant! Similar to the fuel saved by winglets.

John

[FE Hoppy]

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]

Yeah your welcome Hoppy...feel free to go to that Nasa site link I posted..have a nice day..

[FE Hoppy]

The pdf you linked talks about a 2% reduction on an A310. so you are welcome.

[Down Three Greens]

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]

ssg might be better off reading this one: http://www.flightlevel400.com/document_files/grips/GettingToGripsACPerformance.pdf

[FE Hoppy]

or this:http://www.smartcockpit.com/pdf/flightops/flyingtechnique/41

[galaxy flyer]

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]

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]

GF

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

[Angels 60]

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]

Angels,

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

[Angels 60]

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]

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]

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]

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]

That makes sense..thanks Exeng.

[OutOfRunWay]

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]

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]

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