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]

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]

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) :ok:

[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]

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]

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 :ouch:, 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. :ok:

[kijangnim]

Greetings

The TAB TRIM is to trim the aircraft for n-1 engine at speed V2 +10 kts :ok:

[gulfairs]

On a dc10-30

the difference in fuel burn between full forward CofG and max Aft Cof G is a a whisker over 1%.
Aft is 1% less.
Does not sound like much but an 11 hr flight @ 10 tonne /hr
= over a tonne saved.
It has nothing what so ever to do with weight off the runway, or VMCg or even VMCA
BBG

[wulfend]

Current MD-11 info is that an optimum aft CG, about 32% aft of the mean aerodynamic chord saves about 2.7% of total burn. VMCG and VMCA are always figured for max aft CG.
CG position is constantly displayed in the cockpit and the fuel system uses the tail tank to control the CG to the best aft position in flight.

[Mike773]

Quote kijangnim

Sorry badly written :ouch:, 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. :ok:

That's what I thought you might mean. :ok:

My company are currently pushing us for 23-28%MAC of the 747 for fuel efficiency. They don't give us any numbers to back up their request though. Maybe that should be a project for me! :)

[kijangnim]

Greetings MIKE 773

What I did was to run different FRWD C of G settings using the takeoff performance software provided by Boeing, cause at that time the airline wanted to certify the B767 at a higher take off weight ( which is a paper certification, nothing to change in the airplane structure:})
the delta takeoff weight they wanted to obtain was far beyong what we needed, and the 3 tonnes was sufficient :)
get in touch we flight ops engineers (in your airline) I am sure they will be delighted to help you.:ok:

[Mike773]

Thanks for that, kijangnim. I'm sure our Flight Ops Engineering guys have the data. I might see what I can find out. Much better than blindly following "the rule", although that's what they expect sometimes! :}

[GlueBall]

"...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..."

Our runway analysis [B74] is runway specific; it determines the max allowable takeoff weight, based on temp, QNH, etc . . . .

There is no "takeoff weight" adjustment based on C of G variations. :confused:

[kijangnim]

Greetings GlueBall

The software generating your runways specific takeoff tables, has some inputs, among these features the C of G....:)

[crjlover]

Hi all,
I have a question linked to this topic.
Let's say that with aft CG I have an Higher RTOW, does this means that I can flex more??

[BoeingOnFinal]

Let's see if I get this right:

A more aft CG gives an increase in Vmcg (minimum control ground), which increases the lower V1 limit (Decision speed, go or no-go). And if the ASDR (accelerate-stop distance required) exceeds the ASDA (available), TOM (actual take-off mass) would have to be reduced as a consequence of a more aft CG.

If you meant a more forward CG, this would result in a reduction the Vmcg and a possible reduction in V1. This does not necessarily mean an increase in TOM is available, but if the ASDA was limiting (unbalanced take-off), you would be able to do so (a reduction in V1 would decrease ASDR, but increase TODR).
And as far as my understanding goes, an increase in TOM can be "substituted" with a FLX take-off.

But I must say my "knowledge" are pure theoretical, so a confirmation from someone with practical experience would be excellent. :)

[john_tullamarine]

I've been watching this thread develop and, I must confess, I am getting a little confused along the way ... time to fill some holes in my knowledge bank, I guess ....

(a) conventionally, certification stalls are done at forward CG as this gives the highest stall values.

(b) takeoff speed schedules then are based on this conservative stall paradigm.

(c) Fwd C of G will increase your take off weight (gives better control in case of engine failure)

this seems a bit avant garde .. conventionally, the (forward CG determined) stall determines the speed schedule independent of CG variation in operations ? Perhaps someone can throw some light on Kijangnim's comment ?

(d) we tried it on the B767, and the difference can be up to 3 tonnes

I'm not a 767 man but have played in the AFM in the past with no recollection of such a certification-driven variation. Perhaps Kijangnim can amplify the comment as I'm missing something along the way here ?

(e) If you meant a more forward CG, this would result in a reduction the Vmcg and a possible reduction in V1

while there is nothing to prevent multiple Vmcg demonstrations, it is not routinely addressed .. ie, the usual deal is Vmcg is fixed and determines min V1

(f) but if the ASDA was limiting (unbalanced take-off), you would be able to do so

in general, no ... min V1 usually is fixed for the conditions

(g) an increase in TOM can be "substituted" with a FLX take-off

I think you might be confusing flex with derate .. two generally similar considerations for the line pilot ... but very different animals when it comes to Vmcg/min V1 considerations.


Would be nice if someone could give us a bit of hard data on where the variation of RTOW with CG originates ? Certainly could be arranged .. but not generally seen.


yours in confusion ....

[Magnm]

In a normal (not ALT 1 or ALT2) dispatch, T/O performance charts assume the CG to be at the worst position (FWD limit). Every time your actual CG is behind the FWD limit, you will have excess performance both for accelerate stop as well as for accel Go, even if your TOW=MTOW. If you want to use this excess performance to improve your TOW limitations, you must use certified procedures ALT 1 or ALT 2, which no longer assume the CG is in the FWD limit, and the loading has to be done is such a way so not to have a CG forward of the new limits for ALT 1 or ALT 2 as applicable. Actually, taking your CG AFT of the FWD limit improves your TOW capacity. This is applicable to the 767.

[kijangnim]

Greetings

Thanks Magnm :ok:

[BoeingOnFinal]

"while there is nothing to prevent multiple Vmcg demonstrations, it is not routinely addressed .. ie, the usual deal is Vmcg is fixed and determines min V1"

Ah, thank you for that info. While a fixed Vmcg assuming worst CG position might be used in diagrams in the POH, does anyone know if any FMS uses this kind of calculations?

"in general, no ... min V1 usually is fixed for the conditions"

Would you care to elaborate?

I understand that my example is pure theoretical, because in many cases Vmcg might be fixed, as you mentioned.

What about using fixed derate to decrease Vmcg, thus increasing TOM if takeoff is Vmcg limited? Wouldn't that be a similar scenario, except we are reducing Vmcg in different ways?

"I think you might be confusing flex with derate .. two generally similar considerations for the line pilot ... but very different animals when it comes to Vmcg/min V1 considerations."

No, I am thinking of the Assumed Temperature method, ASST or FLX takeoff. If you have a gross mass margin for the actual runway conditions, you can use this method. Is this not correct?

With fixed derate I understand derating the engine in the FMC N1 limit page, which is also allowed in contaminated runway conditions.

Thank you for your input, I appreciate learning from the pilots with practical experience. Please do correct me if I'm way off.

[john_tullamarine]

First up the references to ALT-1 and -2 give me the answer to my concerns .. presumably the restricted forward limits are associated with lower speed schedules which, in turn, give an improved RTOW for a runway limited case. Now I can go back to sleep in comfort ...

"in general, no ... min V1 usually is fixed for the conditions" - Would you care to elaborate?

With a fixed certification basis then, given the ambient conditions, min V1 generally is a fixed number limited by Vmcg

What about using fixed derate to decrease Vmcg

.. that changes the certification basis (derate functionally is a "recertification" of the engine .. as opposed to flex which is just a reduced thrust setting) .... so all bets are off and we start anew ..

No, I am thinking of the Assumed Temperature method

If I am reading your earlier post correctly, it appears that you see flexing as providing an opportunity to increase RTOW .. it is this point which suggests to me that you are confusing flex and derate. On shorter runways, where a takeoff may well be min V1 limited, use of derate may give you a reduced min V1 (due to the reduction in certification Vmcg). As the weights will be low in any case, this reduction in speed schedule may then permit an increase in RTOW when compared to the higher thrust operation.

Important thing is to be aware that flex and derate are quite different .. although, functionally, the pilot will see much the same results for most operations. Both, of course, can be combined to give you a flex reduced derated takeoff.

If I have misunderstood your earlier post (and that may be the case) please do post some clarification and we will continue the discussion ...

[BoeingOnFinal]

Thank you John, for that elaboration.

You may have misunderstood my initial statement. What I meant by "substituting" increase in TOM for FLX, is that if you have a gross takeoff mass margin, meaning you have enough runway to increase TOM, you could instead use flex to save engine life. And that is of course used in cases where you have no need to increase TOM.

Have I understood it correctly?

[john_tullamarine]

Ah hah !! .. that's fine ... that's the whole basis for using flex.

Pass, friend ....:ok: