Aircraft Design - asymmetric seat configurations

[mnemosyne]

Last night my husband and I were discussing the Bombardier C-Series plane (and where it will fit/compete in the aviation market) and were looking at some of the mock-up images online. We noticed that the proposed seat configuration is 2-3 in the economy section, and started wondering about aircraft balance and centre of gravity.

It seems unusual to not have the passenger weight evenly distributed around the CG, although Googling has shown us that the SAAB 340 has done this before with a 1-2 configuration (with all the overhead storage above the 2 seats). I’m sure there are other examples, but I’m not so much interested in that.

How does an asymmetric configuration like that affect the plane’s design - how is the extra weight of the seats and passengers on one side compensated for to avoid having the plane roll?

Can the cargo area and placement of luggage be reliable enough to offset the extra weight on one side? Is the fuel burned from one wing first (this doesn’t seem probable)? Clearly the lighter side isn’t weighted down with more hydraulic lines/electical wiring, because then you’d lose a certain amouint of redundancy. We just can’t come up with a satisfactory idea for how to deal with a heavier airplane half!

[Johnny_L.A]

There’s not a lot of moment. Let’s say a seat and a passenger weigh 250 pounds. Assume the default position for a single row of seats is next to the windows. If the ‘extra’ seats are on the centerline, then the moment is 250 pounds times zero inches = 0 inch/lb. But let’s say the seat is offset 12" to the right. That’s only 3,000 in/lb per seat; pretty small compared to the moment created by the large wing extending a great distance from the longitudinal datum.

FWIW, I’ve never seen a lateral weight-and-balance chart for a fixed-wing aircraft; but helicopters (not having a rigid connection to the ‘wings’) do have such charts.

[Huerta88]

Embraer regional jets have asymmetrical configurations, too:

http://www.continental.com/web/en-US/content/travel/inflight/aircraft/erj145.aspx

[Gorsnak]

The DC-9 had 3-2 seating. It doesn’t seem to be very uncommon.

[gaffa]

@mnemosyne

Last night my husband and I were discussing the Bombardier C-Series plane (and where it will fit/compete in the aviation market)…

Nothing to add to the discussion other than thank you…for demonstrating to my wife and myself that we aren’t really as big a pair of geeks as the mainstream media would have us believe. This is the sort of thing you never see on sitcoms or dramas, that happily married couples can, and do, discuss subjects as esoteric as the seating arrangements of new planes. In movies, one rarely sees women with any sort of geeky interests, other than as the larval stage of a beautiful woman, when she lets her hair down and takes off those Buddy Holly glasses.

(Definition of geek: Text chatting while in the same room on different computers)

[mnemosyne]

Hmm…I’m not satisfied that it just isn’t significant enough to worry about. You’re right that the moments of the wings are so much bigger, but there’s two of those, in opposite directions to balance things out. That isn’t the case with these seats.

I did some rough calculations, although there’s a damn good chance that I either 1) screwed them up or 2) made some glaringly bad assumptions. Anyways, this is what I came up with.

Assuming your suggested 250lb seats offset by 12" with a 3000lb-in/seat moment, I then assumed an aircraft with 90 seats in economy in a 1-2 configuration for 30 rows. Summing moments along that axis, you get 3000x30 = 90 000 lb-in. This doesn’t even consider the weight of carryon luggage, which in the SAAB is all located on that side of the plane in the overhead bins.

Now, to get something to compare it to in terms of effect, I thought about a 2-engined plane losing an engine…ignoring the yaw from the sudden thrust difference, that plane would certainly roll and need to be compensated.

So I grabbed some numbers off of wikipedia for the 737-500, which is bigger than my hypothetical plane above, but whatever. The dry weight of the CFM56-3B1 engine is 4276lb, and using the very scientific measures of guestimation, the engines appear to be mounted about one fuselage-width away from the plane’s body. The fuselage has a width of 148", and so the engine mass is about 148+74" away from the centreline. So an engine has a moment of 4276x222 = 949 272 lb-in. About 10 times as much as the seats+passengers I’m asking about.

So the roll that could be caused by a magically disappearing engine would have to be compensated for by the other engine(s)/control surfaces, etc. (though I’m not talking about that type of accident, just the effect on the plane’s attitude of that moment being there or not).

I can’t help but feel that 10% of that roll/compensation is still pretty significant in the design of an airplane, but perhaps I am wrong. Clearly, I am far from being an expert in this (or anything at all, really!) It just feels big to me.

So please tell me where my faulty thinking is; I tend to learn best when having things explained with numbers and examples! Thanks!

[mnemosyne]

@gaffa

Nothing to add to the discussion other than thank you…for demonstrating to my wife and myself that we aren’t really as big a pair of geeks as the mainstream media would have us believe. This is the sort of thing you never see on sitcoms or dramas, that happily married couples can, and do, discuss subjects as esoteric as the seating arrangements of new planes. In movies, one rarely sees women with any sort of geeky interests, other than as the larval stage of a beautiful woman, when she lets her hair down and takes off those Buddy Holly glasses.

(Definition of geek: Text chatting while in the same room on different computers)

You’re welcome! He works in the industry and I’m an engineering student that wants to work in aviation safety - we talk about airplanes a LOT! I have a particular fascination with commercial airliners, because of what happens when you put 50 or 100 or 500 passsengers who know nothing about planes into a finely-engineered metal tube with wings, bring them 35 000 feet into the air, and then have something go wrong. How can we make this super complicated thing simple enough for people to deal with if they have to? Human factors and flight safety fascinate me.

[Johnny_L.A]

@mnemosyne

Now, to get something to compare it to in terms of effect, I thought about a 2-engined plane losing an engine…ignoring the yaw from the sudden thrust difference, that plane would certainly roll and need to be compensated.

I’m not multi-engine rated. And again, we’re talking about large aircraft; so I’ll defer to the experts. Paging LSLguy!

But consider something small, like a Cessna 172. They are commonly flown with only the pilot’s seat filled, so they’re flying around all the time with asymmetrical seating. I’ve never noticed any difference in ‘balance’ along the longitudinal access from that. (But it’s there when there’s a large difference in fuel in each wing.)

[gaffa]

@mnemosyne

You’re welcome! He works in the industry and I’m an engineering student that wants to work in aviation safety - we talk about airplanes a LOT! I have a particular fascination with commercial airliners, because of what happens when you put 50 or 100 or 500 passsengers who know nothing about planes into a finely-engineered metal tube with wings, bring them 35 000 feet into the air, and then have something go wrong. How can we make this super complicated thing simple enough for people to deal with if they have to? Human factors and flight safety fascinate me.

Cool. I was watching a show on the Discovery Channel HD (actual Discovery channel content) about particular airplane crashes and how they provided ways to make planes safer. Like TWA 800 showing the need to pressurize fuel tanks with nitrogen. (Australian grain elevator electrical systems have used this technique for decades) and how that produced a cheaper method of producing nitrogen with a system that filters out the larger oxygen molecules, or the design for a new seat that could handle 16 Gs.

[Richard_Pearse]

@Johnny_L.A

FWIW, I’ve never seen a lateral weight-and-balance chart for a fixed-wing aircraft; but helicopters (not having a rigid connection to the ‘wings’) do have such charts.

I’m not aware of any that have a chart for it, but larger aircraft have limitations to the amount of lateral fuel imbalance they can safely cope with. As an example, the Dash 8 200/300 both have a max imbalance of 600 lbs of fuel in the main tanks which are in the wing, outboard of the engines. The aux tanks, if fitted, have a max imbalance of 1500 lbs. These are located in the wing between the engine and the fuselage. Someone with a bit more brain power than me could use those figures to work out a max imbalance 12" from the centreline. For what it’s worth, our Dash 8 300s have cabin fuel tanks in place of normal passenger seats, they hold 2000 lbs of fuel each side and there is no published lateral imbalance limit for them.

I don’t know the answer to the op’s question. It could be that it is too small a difference to worry about (my gut feeling) or they could offset some of the other equipment in the aircraft to balance it out, APU, Galley, Toilet, Avionics etc, or maybe they do something structurally.

[aerodave]

@mnemosyne

Hmm…I’m not satisfied that it just isn’t significant enough to worry about. You’re right that the moments of the wings are so much bigger, but there’s two of those, in opposite directions to balance things out. That isn’t the case with these seats.

I did some rough calculations, although there’s a damn good chance that I either 1) screwed them up or 2) made some glaringly bad assumptions. Anyways, this is what I came up with.

Your 90000 in-lb (7500 ft-lb) sounds like a big number to wrestle with, but it’s still easily overcome by slight adjustments to the aircraft’s trim. Consider that your 90-passenger aircraft is on the large end of regional jets, and would have a wingspan of about 80 feet. That’s going to put the centroid of each aileron about 30 feet outboard.

The force applied by one of those ailerons to counter the torque of the seating imbalance is 250 lbs. That means that a single passenger could sit on the wingtip and balance the 30 “off center” passengers in the plane.

[aerodave]

Just to show how little correction is really necessary, here is the quick and dirty estimate for trim deflection on an aileron at cruise to generate 250 lb of force.

Assume a 20 ft[sup]2[/sup] control surface, at cruise conditions of 500 mph and 30,000 ft. Dynamic pressure is 240 psf, so the lift coefficient (C[sub]L[/sub]) necessary is 250 lb / (240 psf * 20 ft[sup]2[/sup]), or 0.052. Using a flat-plate approximation (C[sub]L[/sub] = 2πα), the angle necessary is 0.0083 radians, or 0.48 degrees.

Even if my back-of-the envelope assessment is off by a factor of 2, it’s a very tiny adjustment.

[Magiver]

@mnemosyne

Hmm…I’m not satisfied that it just isn’t significant enough to worry about. You’re right that the moments of the wings are so much bigger, but there’s two of those, in opposite directions to balance things out. That isn’t the case with these seats.

Think of it this way, the outer 2 seats balance each other out so the odd seat is on one side of the center line and the empty aisle space is on the other side of the center line. There is almost no moment with the weight. Now consider the fuel out in the wings and how much affect it has with weight and balance. It is a much greater weight with much more leverage. In airplanes, it is the weight forward and aft of the center of gravity that is of concern.

[Chief_Pedant]

I know nothing about almost everything, including why planes fly at all, so thanks for giving me a new worry.
I do fly a couple or three segments a week for business. Assymmetric seating is pretty common.

Many of the smaller regional jets I fly have one-two configurations, and since I fly AA out of ORD I am on MD80s :mad: --pieces of junk, but reasonably safe junk. MD 80s are two-three seating: SeatGuru Seat Map American Airlines

The only time we ever get reseated for weight distribution is on the little ERJ series (Embraer) regional jets. These are all one-two but the only time we ever get asked to re-seat is forward/aft.

[Chief_Pedant]

On MD 80s the three-seat side aisle seat actually crosses the center line, so the weight distribution seems pretty even to me.

[LSLGuy]

Pilot type …

As others have said, the lateral moment from assymetric seating is negligible. Other interior equipment, such as galleys and lavs, are also assymetrical. The vast bulk of the permanent structure is symmetrical, but there are a lot of system components (hydraulic pumps, eletronics boxes, etc) which are mounted asymetrically.

Somebody in the manufacturer’s engineering department keeps tabs on all that so it doesn’t all end up causing a list to starboard.

Even a small lateral imbalance in fuel out in the wings would be a larger to much larger factor. And there’s always some lateral fuel imbalance due to slop & calibration differences in the guages.

[Johnny_L.A]

Ah. You got my page.

[mnemosyne]

@1920s Style "Death Ray"

I don’t know the answer to the op’s question. It could be that it is too small a difference to worry about (my gut feeling) or they could offset some of the other equipment in the aircraft to balance it out, APU, Galley, Toilet, Avionics etc, or maybe they do something structurally.

I guess that’s what I’m asking about. Even the lavs and galleys are often starboard, simply because the entrance doors and aisles are usually port side… nevermind flying the plane - even empty and on the ground on a three-point set of wheels, the starboard MLG wheel would be experiencing more weight. I recall seeing planes at Bombardier being balanced and leveled with a bob on three scales - but I don’t know what the criteria are and how that balance was acheived.

@aerodave

Assume a 20 ft[sup]2[/sup] control surface, at cruise conditions of 500 mph and 30,000 ft. Dynamic pressure is 240 psf, so the lift coefficient (C[sub]L[/sub]) necessary is 250 lb / (240 psf * 20 ft[sup]2[/sup]), or 0.052. Using a flat-plate approximation (C[sub]L[/sub] = 2πα), the angle necessary is 0.0083 radians, or 0.48 degrees.

I’m not far enough along in my education at all to do these sorts of calculations, but thank you for them. How much extra drag would a 0.48 degree trim cause, and would that have enough of an affect on fuel efficiency to merit balancing out the plane in it’s design (empty, on ground).

@LSLGuy

Somebody in the manufacturer’s engineering department keeps tabs on all that so it doesn’t all end up causing a list to starboard.

I guess that’sn the person I want to talk to! As is, before putting any fuel or passengers into the tank, it seems that a plane with completed interior in an asymmetric configuration would lean to the right unless someone did something about it. Trim wouldn’t help while on the ground.

I’m beginning to think that this is too specific a question, and it’s possible that any given manufacturer will have a different approach to it. I don’t know. Maybe I’m just overthinking it. I just keep imagining tilting planes, and since I know they don’t tilt, I’m trying to reconcile that. Did I mention we were drinking when we started talking about this? Maybe we are just the ones that were tilting…!

[Magiver]

@mnemosyne

I guess that’sn the person I want to talk to! As is, before putting any fuel or passengers into the tank, it seems that a plane with completed interior in an asymmetric configuration would lean to the right unless someone did something about it. Trim wouldn’t help while on the ground.

The interior is located between the wings. As you move outward from dead center the wings represent a HUGE moment of leverage. 10 gallons of fuel in the outboard tank that is 50 feet away will have many times the leverage of a seat that is inches away from dead center.