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It is well established (How Centre of Gravity (CG) is adjusted? and elsewhere) that adjustments can and need to be made to insure that airplane loading maintains a suitable center-of-gravity for the aircraft fore/aft. However, these questions and answers appear to deal only with fore/aft adjustments. Does a plane such as the MD-88 which has unequal seating (3 seats on the port (left) side of the aisle and 2 on the starboard (right)) have much of an issue with port/starboard balance, and if so, is there standard way of dealing with more passenger weight on the port side of center-line?

I would assume this could be handled with unequal fuel tank loading in each wing, or air surface adjustments, but am curious if in practice this is an issue or not. Or is there anything built into the design of the aircraft to handle this expected unequal passenger loading?

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    $\begingroup$ It's not a problem for pax aircraft. However, large cargo aircraft typically have a max lateral imbalance moment. As I remember, it's 10,000,000 in-lb for a 747-100/200. if you want to see an example, go to 747.terryliittschwager.com, dismiss the opening modal window (click ok), scroll to the bottom of the page and click the button to the right of item 6. The resulting test load, obtained by loading only one side, will come up and will violate that limit. You can scroll down the page to the LATERAL LOADING IMBALANCE envelope to see the violation. $\endgroup$
    – Terry
    Mar 2, 2020 at 22:30
  • $\begingroup$ Interesting. We never calculated lateral CG in the C-130. $\endgroup$
    – Michael Hall
    Mar 2, 2020 at 23:07
  • $\begingroup$ You'd be amazed how little the flying characteristics of a radio-controlled model airplane (in at least once case) were altered by adding a significant weight to one wingtip only. Of course, that might have been due to the fact that the model in question had lots of dihedral, so increasing the wing loading of one tip to make it "want" to fly faster created a yawed/slipping condition that contributed a roll torque component AWAY from the weight, partially offsetting the "direct" roll torque component caused by the weight. $\endgroup$
    – quiet flyer
    Mar 3, 2020 at 18:13

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Asymmetrical lateral loads within the fuselage are not far enough away from the longitudinal axis to generate any significant moment arm. It is an issue, but not one that is planned around or proactively compensated for. Any imbalance in flight can easily be overcome with a little bit of aileron trim.

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Passenger airliners do have lateral C of G limits in addition to longitudinal ones. Nobody ever actually calculates the limits but it's usually what's behind the lateral fuel imbalance limits, taking into account normal left/right passenger distribution. Airplanes with an offset center aisle will have only a minor lateral imbalance compared to symmetrical seating, although it would be interesting to see if the MD-88 has slightly asymmetrical fuel imbalance limits to compensate. In any case with, normal passenger lateral distribution and balanced fuel, there isn't normally an issue on any airliner, offset aisle or not.

Lateral CG limits have an influence on required trim authority. For FAR 25 certification, the airplane is only required to be able to be trimmed hands off in roll with lateral CG at the maximum offset throughout the normal speed range (approach thru Vmo). Most airliners have way more than enough trim authority to meet that requirement.

On the Regional Jet, pilots would get way too fussy and frequently snag cruise lateral trim requirements (caused by aerodynamic asymmetries of an otherwise balanced a/c) of more than about 13% (basically if the trim input in flight was enough to be outside the takeoff range after landing, they would snag it) although per FAR 25, way more trim could be needed and still meet cert requirements in the RJ's (or most other a/c's case).

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  • $\begingroup$ The difference would only be one seat slightly to the left (or right) and the moment (distance) is pretty insignificant. $\endgroup$
    – Anilv
    Mar 3, 2020 at 9:37
  • $\begingroup$ Yes almost certainly small enough to be "lost in the noise" of other factors. $\endgroup$
    – John K
    Mar 3, 2020 at 17:29
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On the MD11F we needed to calculate the lateral imbalance and there would be a Max Take-off weight penalty but the penalty was capped at 15,000kgs IIRC (around 17years ago!).

Reason we were told was to reduce side stress on the landing gears.

Anilv

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