Looking for insights into airliner CG management system priciples

kontiki

kontiki

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

The system I've been looking at 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 used to provide indication of aircraft CG.

I asume, based on the aircraft design and test data, the designers know there are certain relationships between stab trim and elevator that hold true for their design.

Real time calculation of CG isn't a problem. I'd like to understand how optimum CG is derived, interaction with flight control systems, stability, and how performance is measured.

Starting out with a good overview instead of hunches would help. There are times when the graphic novel explanation would really save a lot of time.
 
It's a numbers game. Law of averages. Everyone weighs 184 in the summer. Everyone weighs 189 in the winter. ;)
 
More trim means more downforce on the elevator. Move some fuel rearward, then you need less trim, which results in less downforce on the elevator. More weight to the rear, within the CG limits of course, means more speed and/or less fuel burn, which means more dollars profit for the airline.
 
Yes there is a whole science to this. My work plane, MD11 differs from the DC10 in that the horizontal stab/elevator is 36% smaller. This was done to "modernize" the plane, get rid of the flight engineer and to increase the range via fuel transfer. It was Douglas's plan to compete with the older 747 but only 195 were built but most (five lost in landing accidents and one to inflight fire) are still hauling the freight. I will attempt to explain how it works but forgive me if I don't get too technical as it is usually automatic and magic. Ideally the plane is loaded to approximately 25% MAC ( Mean Aerodynamic Chord) and flown when the fuel load is high enough for long distances fuel is transferred to maintain 32% MAC. This "unloads" the tail down trim force required and the pitch the wings need for lift, all in an effort to achieve maximum fuel efficiency and range. The engineers designed it to operate automatically and life is really really good when it does but when it is operated in the manual mode we must add an extra 2.7% fuel for the loss of efficiency. How much fuel can be transferred to the tail? 14100 pounds. What about fuel freezing? Temp is monitored and fuel is transferred forward to the wing tanks or aux tanks and fuel from those tanks are transferred back to the tail to "warm" it up. At a certain fuel level or at a certain altitude on descent (26750 feet) all fuel is transferred forward.
During ground refueling, the fueler controls the fuel from the wing fuel attachment but as soon as the fuel access door is closed the computer takes over and moves the fuel where it wants and it might open valves and pump 100 lbs to a different tank...it is that good. Like I said, works great when it works and it increase my work load tremendously when it doesn't as do all of the systems on the plane. We can operate with only one system in manual i.e. fuel, air, hydraulic, electrical due to the increased work and monitoring.

As far as loading the aircraft. The company uses a weight and balance computer and can move around containers of freight to achieve the best loading possible within the constraints of when the load arrives and the need to depart on time. Overall they do a good job. As stated by someone else, the FAA allows an allowance per passenger for W&B control (I love it because I finally weigh 190 lbs for sure).
 
Thanks MD. I'm roughly familiar with the basic operation for a different type aircraft (a bus). I'm interested in how changes in CG can actually be detected (sensed) in flight vs continuous calculation, and maybe the interaction with other automation like pitch augmentation and auto flight systems. It's really a bit beyond GA. In as much as fuel efficiency is a real competitive selling point for for an air frame OEM, I'm starting to accept that a simple overview of the working principles is going to be hard to find (IP).
 
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Kontiki
We have a constant readout of the CG but beyond that it is all engineering magic.
 
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