Fuel Burn and C of G effects
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Fuel Burn and C of G effects
It seems cruise performance can be quite affected by C of G position. Does Boeing, or Airbus, or any other manufacturer offer any correction for a full forward CG?
Any old salts with a ROT, example fuel flow % increase, or TAS decrease, based on CG postion?
Hawk37
Any old salts with a ROT, example fuel flow % increase, or TAS decrease, based on CG postion?
Hawk37
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Hawk
The A300-600R maintains a rather tail heavy configuration (around 37% MAC I belive) in the cruise by transferring fuel to the tailtank. The MD-11 can burn fuel off the tail-tank at a ration of 1:7.5 lbs with the centre (aux?) tank, which maintains a tailheavy configuration without going out of trim. The MD11 will pump around the fuel during descent to arrive at a somewhat neutral (25%ish MAC) in time for landing. Or some sorts black magic 
Forgot to add; yes tailheavy is beautiful ;-) Less trimforce = less drag = reduced fuelburn.
Forgot to add; yes tailheavy is beautiful ;-) Less trimforce = less drag = reduced fuelburn.
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Certainly makes sense. Does anyone know if Boeing ever supplied C of G fuel figures for their older aircraft, before the capability of pumping fuel around to keep an optimum C of G? Older 737’s, or DC8/9 perhaps?
Can newer aircraft extract fuel figures for different C of G from their FMS’s
The fuel or range penalty with a full forward CG must have been considered by manufacturers.
I’ll throw out some wild guesses
Aircraft near max weight, high altitude, long range cruise, 3 to 4 % better specific range (ie nm per lb of fuel burned) at full aft C of G vice full forward C of G
Aircraft light, low altitude, fast, 1 to 2 % better specific range at full aft vice full forward C of G.
Sound reasonable?
Hawk 37
Can newer aircraft extract fuel figures for different C of G from their FMS’s
The fuel or range penalty with a full forward CG must have been considered by manufacturers.
I’ll throw out some wild guesses
Aircraft near max weight, high altitude, long range cruise, 3 to 4 % better specific range (ie nm per lb of fuel burned) at full aft C of G vice full forward C of G
Aircraft light, low altitude, fast, 1 to 2 % better specific range at full aft vice full forward C of G.
Sound reasonable?
Hawk 37
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Yes, sorry, perhaps my wording was not very clear. By “C of G fuel figures” I was meaning data to show different fuel burns based on C of G position. To put it a different way, a different specific range (nautical miles per lb of fuel burned) based on whether the C of G is as far aft as possible, or whether the C of G is as far forward as possible. I’ll use the term “fuel burn” in the future.
If one of the old DC8 freighters, which I’m presuming did not have much or any capability to use fuel to shift C of G around, had it’s heavier, denser cargo loaded forward, resulting in a C of G at the full forward limit, I can only expect the fuel burn to be greater than if the C of G were at the aft limit. It’s this fuel burn difference, or a general Rule-of-thumb for it, that I’m trying to get an idea of.
I’m trying to consider the flight conditions that would make this fuel burn difference a maximum, and I’m surmising that there are 2 main reasons that flying near minimum drag type speeds would have the most to gain from a far aft C of G condition. I’ve included a third reason, but it seems less clear to me.:
1. With a full forward C of G, there is obviously an extra amount of trim drag. However with the full aft C of G, there would no longer be this extra drag, which would mean the aircraft speed would be greater. Now, if the aircraft was near its min drag speed, this decrease in trim drag would provide a greater speed increase than if the aircraft was well above its minimum drag speed.
2. With less down force produced by the tail, the total amount of lift required from the main wing is less, further reducing drag.
3. This one I’m not so sure about. An aircraft at the slower long range cruise speeds vice a normal or hi speed cruise would be developing a higher lift coefficient. Accordingly, the position of the center of pressure (C of P) would be further forward. This further forward C of P would be closer to the C of G. Hence the force couple produced between the C of P and the aircraft C of G would be less, requiring a smaller stabilizing couple from the tail. Hence less trim drag. However, my “Aerodynamics for Naval Aviators” says and shows that moment coefficient, Cmac is constant throughout the range of Cl up to just before the stall, so perhaps this third reason is false.
Of course, I could be all wrong. Please feel free to chime in.
Has Boeing ever provided such data? MacDonald Douglas?
Anyone with an FMS that can input maximum and minimum C of G and get different fuel burns?
Hawk
If one of the old DC8 freighters, which I’m presuming did not have much or any capability to use fuel to shift C of G around, had it’s heavier, denser cargo loaded forward, resulting in a C of G at the full forward limit, I can only expect the fuel burn to be greater than if the C of G were at the aft limit. It’s this fuel burn difference, or a general Rule-of-thumb for it, that I’m trying to get an idea of.
I’m trying to consider the flight conditions that would make this fuel burn difference a maximum, and I’m surmising that there are 2 main reasons that flying near minimum drag type speeds would have the most to gain from a far aft C of G condition. I’ve included a third reason, but it seems less clear to me.:
1. With a full forward C of G, there is obviously an extra amount of trim drag. However with the full aft C of G, there would no longer be this extra drag, which would mean the aircraft speed would be greater. Now, if the aircraft was near its min drag speed, this decrease in trim drag would provide a greater speed increase than if the aircraft was well above its minimum drag speed.
2. With less down force produced by the tail, the total amount of lift required from the main wing is less, further reducing drag.
3. This one I’m not so sure about. An aircraft at the slower long range cruise speeds vice a normal or hi speed cruise would be developing a higher lift coefficient. Accordingly, the position of the center of pressure (C of P) would be further forward. This further forward C of P would be closer to the C of G. Hence the force couple produced between the C of P and the aircraft C of G would be less, requiring a smaller stabilizing couple from the tail. Hence less trim drag. However, my “Aerodynamics for Naval Aviators” says and shows that moment coefficient, Cmac is constant throughout the range of Cl up to just before the stall, so perhaps this third reason is false.
Of course, I could be all wrong. Please feel free to chime in.
Has Boeing ever provided such data? MacDonald Douglas?
Anyone with an FMS that can input maximum and minimum C of G and get different fuel burns?
Hawk
