Fuel Savings with aft C of G

[Flyintrim]

It is known that aircraft operate more efficiently with an aft C of G.

But does anyone know how to calculate the savings attained by aft movement of the C of G? Or for that matter the reduction in drag?

[CJ Driver]

Aft C of G IS favourable

Sorry Crossbleed, but the original post was quite correct - it is always the case that a conventional (wing and tail) aeroplane is more efficient the further aft the C of G is. In case you forgot your basic aerodynamics, in most aircraft the wings give lift and and tailplane presses down (negative lift) to balance the whole shebang. (This doesn't necessarily apply to canard types, exotic military aircraft with inherently unstable fly-by-wire designs, and so on, but the average joe's aircraft all work like this). The further aft you move the C of G, the less the induced drag as a result of the tailplane (because it needs to do less work).

Flyintrim: I suspect it would be fairly straightforward to calculate the effects, if all the W&B information is available to you. In particular, if you know the centre of lift (downforce) for the tailplane, and you know the moment arm of the change you have made to the C of G in the cabin, you can directly calculate the reduction in downforce required and hence the change in induced drag from the tail.

[galaxy flyer]

My current plane, the Global, says 1.5% change in specific range for each 5% of CofG change. Aft is an improvement in range, fwd the opposite. But this varies by type. The C-5 was closer to a 1 for 1 number, but I cannot remember.

GF

[BOAC]

Page bottom - SIMILAR THREADS

[5 Greens]

All moden gliders have fin ballast tanks. The more water you put in the wings the more ballast is required in the tail, also depending on the weight of the pilot.

[amber 1]

would operate the 757 with the front hold containing 1/3 of all baggage and freight. The remaining 2/3 split equaly between the 2 bays in the rear hold.

Does anyone know the reasoning behind this method of loading?
I know several companies operate their 757's this way but no-one has been able to tell me why. The usual response is that they have always done it that way.
Loading everything in holds 4 and 3, if they will take it, should improve the fuel efficiency as Saskatoon says and I have never come across a trim problem doing this.

[dixi188]

Airbus A300-B4

Rules of thumb:- +1% MAC = -0.2 to -0.3% fuel burn.

Any use to you?

[crewcostundercontrol]

Interesting topic!

The A320 Family do not have a fuel advantage with an aft C of G, or to put it more bluntly loading the aircraft with all the bags at the back does not help fule burn. It does on the larger Airbusessses and on other aircraft. Read Airbus "Operational help on fuel saving" and the data for 320 family shows no advantage!

[Mad (Flt) Scientist]

Originally Posted by crewcostundercontrol

The A320 Family do not have a fuel advantage with an aft C of G, or to put it more bluntly loading the aircraft with all the bags at the back does not help fule burn. It does on the larger Airbusessses and on other aircraft. Read Airbus "Operational help on fuel saving" and the data for 320 family shows no advantage!

Airbus's "getting to Grips with fuel Economy" says this about the A320 series:

The A320 family does not show the same SR variation with CG as the other aircraft. The aft CG produces worst SR at FL290, crossing over to show an improvement at higher flight levels. The SAR variation is much smaller also. This is due to a complex interaction of several aerodynamic effects. The SAR can be considered effectively constant with CG position. Loading is therefore not critical for fuel economy for the A320 family.

So its more a case of no consistent effect of cg, rather than no effect at all. I think this unusual A320 effect has been discussed here before. On most aircraft you'd get to that "no more use moving the cg aft" state only if you'd already effectively eliminated the tail trim lift and moving further aft led to an increase in trim forces instead. That would be an awfully far aft cg in most cases. The comment about it changing the effect with FL makes me suspect there's some kind of strange Mach effect, perhaps there's a pronounced CM shift with Mach and they actually have got an (almost) unloaded tail.

I also have a suspicion that what's being discussed as negligible is the effect of moving from nominal cg to the aft limit. Perhaps the aircraft nominal is already quite far aft? I find it hard to credit that the full cg range shift from fore to aft would be negligible.

[kijangnim]

Greetings

Aft C of G reduces the stall speed by 5%, whereas Fwd C of G will increase your take off weight (gives better control in case of engine failure)

[hawk37]

MFS, your recent post:

"I think this unusual A320 effect has been discussed here before. On most aircraft you'd get to that "no more use moving the cg aft" state only if you'd already effectively eliminated the tail trim lift and moving further aft led to an increase in trim forces instead. That would be an awfully far aft cg in most cases."

I tried a pprune search, but haven't found any discussion of A320 wing unusual C of G drag considerations. Can anyone?

Secondly, while remaining within the certified CG envelope, is it really possible to move the cg aft to the point that the trim drag has been eliminated, or even starts INCREASING? Surely at this point we would be talking unstable flight characteristics where computers would continually be making fine adjustments to the pitch in order to keep the aircraft under control, as in F16 and other fighter designed to be manoeurrable but not stable.

[spongebob]

Is this also true with turbo props....

Currently moving to turbo props and just wondering if this also the case with them, Im moving to the ATR72 and it seems to be very trim sensitive, especially the -500's.

Regards.

[LYKA]

A320 Series Fuel Savings With An Aft CG

All:

There is no simple explanation because we have a very complex interaction of the following aerodynamic effects:

Wing trailing edge camber effect on wing load distribution changing aerodynamic efficiency as a function of lift coefficient.

Downwash on the horizontal tail plane as a function of angle of attack

Aerodynamic efficiency of the horizontal tail plane as a function of the required trim

CG effect on the required trim

The combination of these parameters leads to the result that the optimum CG position in terms of overall aerodynamic efficiency is more forward for low lift coefficients (low angle of attack) and more rearward for higher lift coefficients (higher angle of attack).

Brgds

[Hunter58]

I somehow faintly remember that on the MD-11 the fuel burn variation over the total cg range is something like 4.5%. However the first 1% is over the forward half the CG range and the last 3.5% over the last half. The trim tank helps you move the cg by some 15 points aft so you have to be aft biased already.

As I said, I faintly remember. Any MD11 driver having fresher memories please to correct.

[cats_five]

5 greens, there are plenty of single-seat glider designs which can take water in the wings without it affecting the CoG.

[Fitter2]

(The topic has strayed a bit from fuel saving, but minimising drag at cruise speed is equally relevant to powered aircraft and gliders.)

Gliders carrying water ballast have it in tanks forward of the spar; inevitably there is a forward CG shift. Whether the user has the ability to put it back to the optimum with tail water depends on the glider (my ASH25 doesn't have the ability) but it certainly does have an effect.

[kijangnim]

Greetings
Forward means more stable, makes sense for a glider

[Keefie]

CJ Driver is quite right--the less the tai downlift the less the required wing uplift. It is not possible to generalise , you need to know relative arm lenghts and sizes of wing and tailplane

[Mike773]

Quote kijangnim:

Fwd C of G will increase your take off weight

Have I missed something or are you suggesting that the TOW calculation is influenced by the CG? TOW remains the same no matter what the CG.

Fwd CG will increase the amount of stab trim required. Is that what you meant?

[kijangnim]

Greeings,

Sorry badly written , a forward C of G will enable you to increase your takeoff weight since the arm C of G tail (rudder) will be longer thus provides better control in case of an engine failure, we tried it on the B767, and the difference can be up to 3 tonnes.