Looking for references on Airliner CG Management 1

[kontiki99]

Been wrestling with issues related to CG Control systems on large commercial aircraft.

I'm very familiar with basic aircraft weight and balance as typically limited to General Aviation, but this is several magnitudes more complicated. Large airliners are typically equipped with systems that move fuel around to shift CG in cruise to optimize fuel performance.

It would be helpful to have a primer that outlined the basic principles. The real time calculation of CG isn't a problem; it's understanding how optimum CG is derived, interaction with flight control systems, stability, and how actual performance is measured.

I've Googled and Amazon-ed, but would only be guessing at what titles had what I need. Any suggestions would be helpful.



My posts reflect my personal views and are not in any way endorsed or approved by any organization I'm professionally affiliated with.

 

[VEBill]

"Large airliners are typically equipped with systems that move fuel around to shift CG in cruise to optimize fuel performance."

I understood that large airliners have fuel tanks located so the C of G doesn't move as the fuel is consumed.

No point using fuel to balance the aircraft, if burning fuel inexorably leads to imbalance and thus to an inevitable crash three hours before scheduled arrival.

 

[MikeHalloran]

The specific knowledge you seek has some commercial value, so it may not be available in spoonable form.

Were I to get interested in the subject, I'd search on 'adaptive controls' and related subjects, not limiting myself to the aircraft world.

A useful algorithm might be as simple as this:
1. Wait for flight controls to say 'cruising'.
2. Ask for and remember the fuel flow.
3. Pump a few lbs of fuel in an arbitrary direction.
4. Ask for the fuel flow again.
5. If the fuel flow went down, go to step 3, pumping more fuel in the same direction.
6. If the fuel flow went up, change the direction in which you're pumping and go back to step 3.


Mike Halloran
Pembroke Pines, FL, USA

 

[VEBill]

"Pump a few lbs of fuel in an arbitrary direction."

A large airliner will have passengers, crew, and food trolleys wandering up and down the aisles.

 

[KENAT]

Simplistically, I'd expect what they are doing is using the fuel to 'trim' the aircraft in lieu of aerodynamic (i.e. drag inducing) trim tabs or elevator pitch.

So whatever the control algorithm is for setting the trim can be extrapolated to the fuel distribution. May end up that the elevators are used for rapid pitch control, trim tabs are used for medium response pitch control and then shifting fuel is used more slowly to try and unload the trim tabs.

(To confuse things some aircraft with an all moving tail

https://en.wikipedia.org/wiki/Stabilator

may not have trim tabs as such.)

Sorry VE1BLL but using fuel distribution in lieu of aerodynamic trimming makes all the sense in the world, and given the amount of reserves you're required to carry for safety (i.e. if you have to divert last minute or join the circuit for an extended period) will probably work well all the way through a standard commercial flight. If you start getting really low on fuel you'll have to use increased aerodynamic trimming instead but that's a secondary concern at that point.

The mean C of G of the fuel storage at max capacity probably needs to be around the center of pressure not the C of G. However there's all kinds of games you can play at less than max capacity. A relatively small tank far from the C of P can probably have a significant impact on trim.

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[VEBill]

Google: FAA C of G

Four PDFs:
Aircraft Weight and Balance Handbook - FAA
Weight and Balance - FAA
Chapter 06: Weight and Balance - FAA
Need and Requirements for Aircraft Weighing - FAA
...

 

[VEBill]

KENAT, Right you are.

A diagram for the 787 shows a "Trim Tank" in the tail. Can't quite make out the volume, maybe 6000 US gallons.

I stand corrected.

PS: Excellent explanation by the way, "If you start getting really low on fuel you'll have to use increased aerodynamic trimming instead but that's a secondary concern at that point."

 

[KENAT]

Cheers VE1BLL, had to dust off some brain cells I haven't used for a few years and try to remember my flight mechanics lectures.

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[kontiki99]

The system we're working with has a CG computer that calculates a cruise target CG slightly aft in CG range. When the aircraft gets to a cruise altitude, fuel is pumped aft to a trim tank to move the CG to some slightly aft target. This unloads the horizontal stab and elevator of the drag associated with holding the rear end of the aircraft down aerodynamically.

There is feedback to a flight warning system from position transducers on the horizontal stab. I don't really understand how position of the trimable horizontal stab is an indication of aircraft CG.

I understand how CG is calculated with EWCG, plus impact of load, plus impact of fuel burned, plus effects of CG control system. There are times when the graphic novel explanation would help a lot.

My posts reflect my personal views and are not in any way endorsed or approved by any organization I'm professionally affiliated with.

 

[KENAT]

kontiki, you understand basic aircraft stability right - how the Center of pressure and Center of mass need to be balanced to counteract the pitching moment from the lift of the lifting surfaces.

The horizontal stabilizers are used to adjust the location of the center of pressure of the entire aircraft by varying the amount of 'lift' they give.

You also understand that lift is proportional to angle of attack of the lifting surface correct?

So angle of incidence of the horizontal stabilizer is proportional to it's effect on the overall center of pressure which in turn is proportional to the the C of G.

So for a given flight speed and angle of attack the angle of the horizontal stabilizer is related to the C of G.

Clear as mud?

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[kontiki99]

KENAT, This is what I suspect, which is why I'm looking for insights into the basic aerodynamic principles. I'm Avionics (primarily EE) but am also a pilot and aircraft owner & A&P). But, it's obviously not my airplane that I'm dealing with.

There are other systems possibly involved. Auto flight augmentation systems may also generate trim inputs. With a federated architecture, it's hard to follow which box is doing what and how the data is being passed around between boxes.

As you say, "So for a given flight speed and angle of attack the angle of the horizontal stabilizer is related to the C of G." So based on the aircraft design and test data, designers know there are certain relationships that hold true for their design.

My hunch is there are autoflight/pitch augmentation system trim inputs (or something) being automatically generated to maintain a steady pitch angle at cruise with no changes in thrust?

After a few days of this, assuming the aircraft is being loaded properly, it probably distills down to asking if it's a tight flight control system vs properly rigged sensor feedback system. Along these lines, it could be stab or elevator, one with obviously greater influence than the other too. I have some flight data recorder data and see the surfaces moving not sure which ones are cause and effect. We have OEM support, but you never really know if you have the wise guru or the new hire responding to your tech support requests. This aircraft model has been around for decades.

I haven't found the time to read through the maintenance data on all the systems and look for clues into what all the actors are. I have no access to the design data. Starting out with a good overview instead of hunches would help. I never know enough.

I believe MikeHalloran is correct too. This is part of the secret sauce that makes one product line profitable to operate over another. Thanks all.









My posts reflect my personal views and are not in any way endorsed or approved by any organization I'm professionally affiliated with.

 

[WKTaylor]

There is a whole engineering discipline on the art/science of 'weight and balance engineering'... which is a critical element for all but the most trivia of engineering tasks.

Obviously the control of weight-and-balance of any aircraft is one of the most essential elements of a safe flight. The recent crash of a cargo B747 at Bagram AB Afghanistan is a classic example of runaway CG:

https://www.youtube.com/watch?v=7sUWC2jfjqI.

Also, the Qantas Airbus A380 that had an inboard engine disintegrate on take off [Singapore?] was almost a casualty of uncontrolled CG migration. The disintegrating engine-fan 'liberated' blade pieces that sprayed the wing and fuselage structure. One piece of debris cut wiring to the horizontal stabilizer fuel tanks, resulting in the inability to transfer fuel from the tanks for CG control. The acft came very close to the 'aft CG limit'... and was forced to land just before controllability was lost.

Perhaps the SAWE [Society of Allied Weight Engineers, INC] can shed some light on this complex subject.

https://www.sawe.org/training/aircraft_weight_balance

Regards, Wil Taylor

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[rb1957]

I think the key question is how does the fuel mgmt system know to pump fuel ?

It might be a simple schedule ... assuming the CG of the cabin "contents" (moving about as they are) is some nominal position, and assuming the pilots have entered the correct W&B for the flight, then the CG position is "known" and as fuel is burnt off (or as other trim are desired) then "knowable" changes will produce the result desired.

another day in paradise, or is paradise one day closer ?

 

[berkshire]

I think what Konti is looking for is airline weight and balance operations.
This is very often the perview of the dispatchers and load operators. They have handbooks for this which are very often proprietary, and closely held.
B.E.

You are judged not by what you know, but by what you can do.

 

[kontiki99]

I'm actually interested in how changes in CG can actually be detected (sensed) in flight vs calculation, and about interaction with other automation like pitch augmentation and auto flight systems.

My posts reflect my personal views and are not in any way endorsed or approved by any organization I'm professionally affiliated with.

 

[berkshire]

Konti,
Some racing sailplanes use a trim tank in the tail to offset the CG changes of a full water ballast system. In that when the water ballast tanks are full the CG is too far forward. This manifests itself as the pilot having to trim the sailplane aft i.e. apply up elevator trim, thereby increasing overall drag. When the trim tank is filled this cancels this out allowing the trim to be restored to neutral. When partial water ballast is dumped as in weak soaring conditions, the CG moves aft requiring the pilot to trim forward again i.e. apply down elevator trim. by releasing small amounts of water from the trim tank the pilot is again able to achieve neutral trim and thus reduce drag.
It strikes me that if your aircraft has a trim tank in the fuel system that can pump fuel in and out, then a feedback loop from the trim tab servo should do what you need. I.E. if elevator is trimming up, pump ballast into tank, if elevator is trimming down, pump ballast out, until neutral state is obtained.
See if this helps .
B.E.

You are judged not by what you know, but by what you can do.

 

[kontiki99]

berkshire,

I think this is what we're seeing. We're trying to understand a lengthy history of problems on 6 similar airplanes. There is a CG computer that manages fuel transfers, based on zero fuel empty weight CG, fuel load, cargo load sheet. It kicks on at cruise and adjusts to a slightly aft CG for efficient cruise. Completely separate, we have horizontal stab angle sensor (a synchro) that can trigger an advisory message on a cockpit warning system. It's completely independent of the CG management system. Were trying to understand why it goes off so often.

When I fly my little bug smasher, I trim until I can't feel control wheel pressure and I'm holding altitude.

On the airliner, is stab trim neutral position predefined so it's always the same angle when the airplane is properly rigged, or could there be some feedback from the flight control system to see how much force it's exerting?

We see horizontal stab and elevator fluctuations in the flight data recorder data, with no yoke inputs. Of course there are other systems that drive the flight controls too. The descriptions in the aircraft maintenance manual sometimes leaves a lot unsaid.

My posts reflect my personal views and are not in any way endorsed or approved by any organization I'm professionally affiliated with.

 

[MikeHalloran]

I wonder if the CG computer has a 'wordy' mode, where it can spit out clues about what it's doing and why, say on a serial port or a bluetooth connection or something like that.



Mike Halloran
Pembroke Pines, FL, USA

 

[dgapilot]

I suspect that the stab angle sensor and the cg computer are on the same data bus, along with lots of other systems. Hence the concept of highly integrated systems. It could be something as simple as one of the sensors is throwing a bit when it's not supposed to, or the sensors aren't properly aligned, or the CG computer is throwing an incorrect bit, or the timing of the fuel transfer is off due to some restriction or any number of other things. I would start at the very beginning, and go back and weigh the aircraft. Could be as simple as they have an incorrect empty Weight and CG, from which all other calcs would follow.

 

[rb1957]

changes in CG would be difficult to separate from changes in airflow (gusts).

But if you know the starting point (with everyone seated) ... which you could from s/g on the LG, maybe tire pressure ?, and some baseline data, like CG of empty airplane (from maintenance, load cells under LG), and you know how fuel is being burnt, then you can determine the CG.

but trying to account for moving pax, etc, I don't see.

another day in paradise, or is paradise one day closer ?