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CrimsonNL
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Fuel savings when flying with "optimal trim"

Fri Jan 13, 2017 7:13 pm

Hi guys,

I've been working as a load-controller for several years now, making loadsheets for a variety of airlines and aircraft types. Usually me and my colleagues will try to make a load plan resulting in a good stabilizer/trim setting for the aircraft, which is often designated as optimal or ideal trim in electronic weight&balance systems. Some airlines will supply us with their desired trim values, while others don't seem to care. Airlines from Turkey are especially fond of a good trim. We sometimes get to deal with angry flight crew / station managers when this can not be realized.

The question I've had for a long time now, is how does a good trim setting influence the fuel burn? Say you have an A320 on a 3 hour flight with a MACZFW of 24%, as compared to a more "ideal" value of MACZFW 30%? Can this be translated into a certain weight of fuel saved?

Looking forward to any answers,

Martijn
Always comparing your flown types list with mine
 
benbeny
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Re: Fuel savings when flying with "optimal trim"

Fri Jan 13, 2017 7:57 pm

Well, I believe someone more competent will come, but this is my $0.02.
With optimal aft CG, that means less downforce required from horizontal stabilizer, meaning less AOA from horizontal stabilizer.
Less AOA means less lift (negative lift in this context, but lift nonetheless). Cause lift creates lift-induced drag, reducing lift means reducing drags. Reduced drags results in less fuel. I don't know how much fuel savings realized, but I believe that's significant enough to warrant for optimal CG.
 
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77west
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Re: Fuel savings when flying with "optimal trim"

Fri Jan 13, 2017 8:27 pm

benbeny wrote:
Well, I believe someone more competent will come, but this is my $0.02.
With optimal aft CG, that means less downforce required from horizontal stabilizer, meaning less AOA from horizontal stabilizer.
Less AOA means less lift (negative lift in this context, but lift nonetheless). Cause lift creates lift-induced drag, reducing lift means reducing drags. Reduced drags results in less fuel. I don't know how much fuel savings realized, but I believe that's significant enough to warrant for optimal CG.


Correct!
77West - AW109S - BE90 - JS31 - B1900 - Q300 - ATR72 - DC9-30 - MD80 - B733 - A320 - B738 - A300-B4 - B773 - B77W
 
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Starlionblue
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Re: Fuel savings when flying with "optimal trim"

Fri Jan 13, 2017 11:00 pm

The 330 transfers fuel to the trim tank in the stabilizer during cruise. This gives about 1% fuel savings. I deduce this figure from the MEL since if the trim tank is inop calculated fuel burn is increased by 1%. The CG goes from somewhere around 24-25% MAC on take-off to around 30% in cruise. The 350 achieves the same effect with partial flap extension in cruise.


benbeny wrote:
Well, I believe someone more competent will come, but this is my $0.02.
With optimal aft CG, that means less downforce required from horizontal stabilizer, meaning less AOA from horizontal stabilizer.
Less AOA means less lift (negative lift in this context, but lift nonetheless). Cause lift creates lift-induced drag, reducing lift means reducing drags. Reduced drags results in less fuel. I don't know how much fuel savings realized, but I believe that's significant enough to warrant for optimal CG.


Your understanding is correct. However I think it would be better to say that with aft CG, less downforce is required from the stabilizer, meaning lift is required from the wing to compensate for stabilizer downforce. Less lift gives lower induced drag. The mechanism whereby lower lift happens is lower AoA.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
benbeny
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Re: Fuel savings when flying with "optimal trim"

Sat Jan 14, 2017 1:37 pm

My bad, it's hard enough when you're not an English native speaker :(
 
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Starlionblue
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Re: Fuel savings when flying with "optimal trim"

Sat Jan 14, 2017 1:42 pm

benbeny wrote:
My bad, it's hard enough when you're not an English native speaker :(


No worries, mate.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
Bambel
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Re: Fuel savings when flying with "optimal trim"

Sat Jan 14, 2017 2:15 pm

Starlionblue wrote:
Your understanding is correct. However I think it would be better to say that with aft CG, less downforce is required from the stabilizer, meaning less lift is required from the wing to compensate for stabilizer downforce. Less lift gives lower induced drag. The mechanism whereby lower lift happens is lower AoA.


Thanks for that. But it is possible, that you missed a "less" here? At least from my understanding..

And not to drag this thread OT, but what i allways wondered about fuel tanks in stabilizers: do they need extra fuel lines? Because: there must allready be fuel lines to the rear of the plane for the APU. Can those be used biderectional? That would make a tank there allmost for free?

b.
 
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Starlionblue
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Re: Fuel savings when flying with "optimal trim"

Sat Jan 14, 2017 3:03 pm

Bambel wrote:
Starlionblue wrote:
Your understanding is correct. However I think it would be better to say that with aft CG, less downforce is required from the stabilizer, meaning less lift is required from the wing to compensate for stabilizer downforce. Less lift gives lower induced drag. The mechanism whereby lower lift happens is lower AoA.


Thanks for that. But it is possible, that you missed a "less" here? At least from my understanding..

And not to drag this thread OT, but what i allways wondered about fuel tanks in stabilizers: do they need extra fuel lines? Because: there must allready be fuel lines to the rear of the plane for the APU. Can those be used biderectional? That would make a tank there allmost for free?

b.


It is possible. ;) I noticed it after it was too late to edit.

Fuel tanks in the stabilizers do need extra fuel lines. And extra pumps. The 330 fuel system is way complicated. Way more stuff than you need just for the APU.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
opticalilyushin
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Re: Fuel savings when flying with "optimal trim"

Sat Jan 14, 2017 4:25 pm

Some of the airlines I do load control work for aim for an optimal trim in the rear 1/4 of the flight envelope on the loadsheet (we mostly use manual sheets), whereas some other airlines don't seem to care. I'm told that the financial saving from having an optimal trim is relatively little..loose change found down the back of the chair was quoted to me by a captain..

The explanation given above is spot on though, especially as most aircraft cruise in a slightly nose-up attitude, but I didn't know about the A350 flap thing.
 
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tb727
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Re: Fuel savings when flying with "optimal trim"

Sat Jan 14, 2017 5:52 pm

When I flew the 727 on cargo charter we would try and get a takeoff trim setting of between 4-5 units. It was easy if you only had a small amount of freight, I would have it loaded around the over-wing exits on the after portion of the wing. In cruise you would be in more level flight and we would burn a good amount less fuel, sometimes we could save 2000 pounds on a flight compared to the flightplan. If it was a full heavy load and you were near the limit of 8 we would burn a little more fuel than flight plan. The airplane hand flew very nice if it was around 7-8 units and it was very stable on approach. With the trim closer to 4 it took a little more finesse to get her steady down the glideslope, it wasn't terrible but it was noticeable.

Now on the Airbus, I don't think my company cares much about the setting but we are almost always full so there isn't much you can do anyways.
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zanl188
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Re: Fuel savings when flying with "optimal trim"

Sat Jan 14, 2017 6:36 pm

In my C-5 load planning days it was easy to plan for the optimum trim point with dense cargo like tanks or APCs. Routine trash hauling mission with pallets & pax... not so much...
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thepinkmachine
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Re: Fuel savings when flying with "optimal trim"

Sun Jan 15, 2017 2:02 pm

CrimsonNL wrote:
Hi guys,
The question I've had for a long time now, is how does a good trim setting influence the fuel burn? Say you have an A320 on a 3 hour flight with a MACZFW of 24%, as compared to a more "ideal" value of MACZFW 30%? Can this be translated into a certain weight of fuel saved?

Looking forward to any answers,

Martijn


As others said before, aft CG loading generally gives 1-2% savings compared to forward CG.

However, according to Airbus publications, A320 family is a bit of an outlier - due to some unspecified aerodynamic phenomena, the the effect of CG on fuel consumption is negligible.

Quote from http://ansperformance.eu/references/library/airbus-fuel-economy.pdf

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.


There also might be some benefit of aft CG on takeoff performance, but I'd say it's negligible too...
"Tell my wife I am trawling Atlantis - and I still have my hands on the wheel…"
 
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CrimsonNL
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Re: Fuel savings when flying with "optimal trim"

Mon Jan 16, 2017 5:28 pm

Thanks for the replies so far everyone, keep them coming!

thepinkmachine wrote:

However, according to Airbus publications, A320 family is a bit of an outlier - due to some unspecified aerodynamic phenomena, the the effect of CG on fuel consumption is negligible.

Quote from http://ansperformance.eu/references/library/airbus-fuel-economy.pdf


Thanks for sharing, that link is very informative! It's very interesting to read that these saving effects on the 32S are negligible, as most of the airlines I work with that operate the 320 have published optimal trim values.

Martijn
Always comparing your flown types list with mine
 
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9MMPQ
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Re: Fuel savings when flying with "optimal trim"

Mon Jan 16, 2017 10:17 pm

The target for maximum fuel economy on our B744F was a MAC ZFW of 28% but i can't say i've noticed a very significant saving on fuel used on the best trimmed flights. Crews certainly were not looking at it from a fuel saving perspective & if it were significant it probably would have some consideration in flight planning.

On the grand scale of things a couple of 100kgs here & there might add up a little bit over a year but in the bigger picture it does feel like the loose change found down the back of a chair. Certainly not something to get very upset about. I know it's an apples & oranges comparison with the A320 but i doubt the end result is very different.
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thepinkmachine
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Re: Fuel savings when flying with "optimal trim"

Tue Jan 17, 2017 5:16 am

CrimsonNL wrote:

Thanks for sharing, that link is very informative! It's very interesting to read that these saving effects on the 32S are negligible, as most of the airlines I work with that operate the 320 have published optimal trim values.

Martijn


Don't know why, perhaps they didn't read this booklet :mrgreen: On a serious note, the LCC A320 operator I worked for, used to load all the luggage into the aft cargo compartment, for CG optimization. However, at some point they have changed their policy and started loading all the luggage to the forward hold, which was faster and enabled them to save some time on turnarounds.

This would confirm the theory that fuel savings ar negligible for the 320...
"Tell my wife I am trawling Atlantis - and I still have my hands on the wheel…"
 
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Horstroad
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Re: Fuel savings when flying with "optimal trim"

Tue Jan 17, 2017 9:55 am

Starlionblue wrote:
The 330 fuel system is way complicated.

I assume you have never seen an MD11 fuel system schematic :D
6 separate tanks, 18 fuel pumps, two shrouded fuel lines to the tail (One tail transfer line to the tail tank and one engine feed line. The APU actually shares the fuel feed line with the engine but has two separate fire shut-off valves).

Here is a brief description of tail fuel management:
The fuel in the tail is controlled as follows:
The CG is kept to an initially set aft limit. This function is permitted only if the total fuel quantity is more than 60,000 lbs (27,216 kg). When the quantity of the fuel is less than 51,000 lbs (23,134 kg), no more fuel is moved aft for CG control. If the No. 2 engine is stopped, the tail fuel level is kept to a maximum of 5,000 lbs (2,268 kg). If the CG moves aft of the aft control limit, the FSC goes back to the manual mode. The SEL FUEL SYS MAN alert is then shown on the Engine and Alert Display and the System Display (SD).

When it becomes necessary to move fuel to the tail tank, fuel is moved aft from a non-empty forward tank in the following sequence: Lower Aux, Upper Aux tank 2 (when tank 2 quantity is greater than the tank 1 and tank 3 quantities), tanks 1, 2 and 3 (when all main tanks have equal quantities). If any main tanks dump shutoff float indicates down (below approximately 11,500 lbs (5,216 kg) and the tail tank contains fuel, the FSC will transfer fuel to that main tank from the tail tank. The FSC will stop this transfer when the float indicates up.

If fuel remains in the tail tank during aircraft descent below 19,750 feet, that fuel will be moved forward.

Flow through or pressurization of the tail tank manifold during takeoff and landing
is prevented.

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