The MD-80 family has a 2-3 seat layout as seen in this image:
Does the layout have any influence on the balance of the aircraft and if so what is used to balance out the weight?
Image from seatguru.com
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asked Feb 13, 2016 at 1:40
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2 Answers
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Based on this diagram—found in this Boeing MD-80 series document—we can estimate the average lateral center of gravity along the buttock line for each passenger seat:
Using the seating diagram that you provided in your question, I estimated the average center of weight along the buttock line for A, C, D, E, and F to be -50, -30, +10, +31, and +51 respectively.
We can use the following formula to calculate the lateral CG for the full load of passengers.
Where:
- $A_w$ is the sum of all weights for passengers in
Aseats (and so also forC, etc) - $A_a$ is the lateral moment arm location along the buttock line for passengers in
Aseats (and so also forC, etc) - $C_G$ is the solution to the lateral center of gravity for all seat loads.
$$ C_G=\frac{(A_w\cdot A_a)+(C_w\cdot C_a)+(D_w\cdot D_a)+(E_w\cdot E_a)+(F_w\cdot F_a)}{A_w+C_w+D_w+E_w+F_w} $$
If we assume that every seat depicted in the question diagram is occupied by a 170 lb person, the solution $C_G$ is $3.4$—that is, 3.4 inches right of the center of the cabin, under the left side of seat D.
3.4 inches is very little variation in lateral CG, especially for so large an aircraft.
Other load considerations notwithstanding, I don't think the assymetrical seating arrangement is cause for concern.
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$\begingroup$ Isn't it simpler to ignore the two outer files on each side since they cancel out? You're left with the middle one being half the aisle width off centre. 3 tons of people sounds like a lot but the lever arm is only 30cm so the torque isn't so much. I suppose if it started rolling too much they could announce that people in the odd numbered seats must stand up in the aisle, but I've been on them a few times and it's not happened yet. $\endgroup$ Nov 22, 2019 at 22:10
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2$\begingroup$ @BlokeDownThePub: They don't cancel out for the purpose of computing the centre of gravity. If you removed all the window seats, you would end up with a CG that is further from the midline. $\endgroup$– TonyKNov 23, 2019 at 0:00
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1$\begingroup$ @BlokeDownThePub As TonyK said, they will not cancel out. First, if we were to assume that they had the same absolute value buttock line position (which I do not believe they do) the sums would not work. To illustrate this point, lets remove both outer seat pairs from the CG calculus. In this case, the CG would be centered on remaining the D aisle seat. Secondly, the seats do not seem to fall symmetrically about the aircraft. Consider that the diagram depicts the aisle at 19 inches and the seats at 22 inches of width. $\endgroup$– J WNov 24, 2019 at 12:27
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1$\begingroup$ As you wrote, symmetrical rows play a role in minimizing the unbalance, this is the same for the mass in the hold as well as the structure, assuming they are laterally balanced. Taking them into account should lead to an even smaller moment. $\endgroup$– minsOct 15, 2021 at 9:28
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The "D" seat is very near the aircraft centerline, and thus the roll axis. Since roll moment is the distance from the roll axis times weight, if the distance is small, the roll moment is very small. What roll moment is created can easily be canceled by a little bit of aileron trim if needed.
The water tank on MD-80s is also placed on the LH side of the cargo. Full this is about 350 lbs, further from the centerline than the "D" seats. Also the LH side has an extra service door in the aft and the front LH door is larger than the R1 door, although the RH side also has the cargo doors.
