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euroflyer
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Longitidunal axis CoG on B717/A220

Wed Jan 09, 2019 8:32 am

Hello,
Maybe it's a silly question and has been discussed before, but I always wondered how the center of gravity is balanced on single aisle A/C with 5 seats abreast (such as B717 or A220). My question is primarily focused on the longitudinal axis since one side of the aircraft is having approx 20% more seats than the other side (however I assume the aisle is slightly displaced on the side with less seats).
I guess weight distribution can be arranged as far as the plane is not fully occupied, but with a 100% LF isn't it problematic ? Are the fuel tanks designed to absorb this imbalance ?

Thanks
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fr8mech
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Re: Longitidunal axis CoG on B717/A220

Wed Jan 09, 2019 8:42 am

Without having any real numbers available, the easy, and most intuitive answer, is that the moment-arm created isn't that large because the "third-seat" is almost on the center line of the aircraft.

I'm very confident in saying that fuel management does not factor into it at all.
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euroflyer
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Re: Longitidunal axis CoG on B717/A220

Wed Jan 09, 2019 8:54 am

From this cabin cross section of B717, looks like the 3rd seat is pretty much out of the centerline though...

Image
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fr8mech
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Re: Longitidunal axis CoG on B717/A220

Wed Jan 09, 2019 9:04 am

I would suspect the inboard arm rest is on the center-line. Regardless, the moment isn't that large. Anecdotally, I don't think it's a big deal.
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zanl188
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Re: Longitidunal axis CoG on B717/A220

Wed Jan 09, 2019 10:32 am

I suspect aileron trim takes care of it quite readily.
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fr8mech
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Re: Longitidunal axis CoG on B717/A220

Wed Jan 09, 2019 10:43 am

Aileron trim would take care of it, but costs money.

I don't know, maybe a pilot of the type can chime in.
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fr8mech
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Re: Longitidunal axis CoG on B717/A220

Wed Jan 09, 2019 12:16 pm

You know, looking at the drawing, along with other drawings, the Proximity of the third seat to the center line wouldn’t matter. The force is still being exerted on the seat track. So, there is a little more moment than I initially thought.

Still, I’m not sure it’s enough to matter. The options to counteract such force are unsavory:

-trim is aerodynamically inefficient and costs money
-ballast is weight, and that costs money in fuel and payload reduction
-active fuel management? I seriously doubt that
-a built in bias? That’s about the only option I would think likely, but still costs money
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planewasted
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Re: Longitidunal axis CoG on B717/A220

Wed Jan 09, 2019 12:18 pm

Some quick calculations to add to the discussion (correct if totally wrong).

Lets say a passenger and seat weighs 100 kg in average. That weight is on average 0.3 meters from the center line. plane has 30 rows.
That gives an extra moment of: 0.3*100*30 = 900 kgm.

An A220 has a wingspan of 35 meters. So to counter the moment you need to put a weight of:
900/(35/2) ~= 51 kg on the wing tip.

Lets say the fuel tank is in the middle of the wing's span:
You need fuel with a with a weight of: 900/(35/4) ~= 102 kg extra in one wing to compensate for the moment.
 
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euroflyer
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Re: Longitidunal axis CoG on B717/A220

Wed Jan 09, 2019 2:07 pm

planewasted wrote:
Some quick calculations to add to the discussion (correct if totally wrong).

Lets say a passenger and seat weighs 100 kg in average. That weight is on average 0.3 meters from the center line. plane has 30 rows.
That gives an extra moment of: 0.3*100*30 = 900 kgm.

An A220 has a wingspan of 35 meters. So to counter the moment you need to put a weight of:
900/(35/2) ~= 51 kg on the wing tip.

Lets say the fuel tank is in the middle of the wing's span:
You need fuel with a with a weight of: 900/(35/4) ~= 102 kg extra in one wing to compensate for the moment.


Thanks for these calculations, seems that the impact is lower than what I was thinking, I guess such moment has no or very little impact on the stability given the overall weight of the A/C

fr8mech wrote:
You know, looking at the drawing, along with other drawings, the Proximity of the third seat to the center line wouldn’t matter. The force is still being exerted on the seat track. So, there is a little more moment than I initially thought.

Still, I’m not sure it’s enough to matter. The options to counteract such force are unsavory:

-trim is aerodynamically inefficient and costs money
-ballast is weight, and that costs money in fuel and payload reduction
-active fuel management? I seriously doubt that
-a built in bias? That’s about the only option I would think likely, but still costs money


Option 4 : you mean a definitive bias or active bias ? Wouldn't a fixed built in bias just create another imbalance given the variability of the payload ?
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GalaxyFlyer
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Re: Longitidunal axis CoG on B717/A220

Wed Jan 09, 2019 2:31 pm

The C-5 had two lateral rows of 18 pallet positions each. Put 18 pallets on one side and you’d never know it flying the plane. Just not that much arm.

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fr8mech
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Re: Longitidunal axis CoG on B717/A220

Wed Jan 09, 2019 6:37 pm

euroflyer wrote:

Option 4 : you mean a definitive bias or active bias ? Wouldn't a fixed built in bias just create another imbalance given the variability of the payload ?


Yes, but it would take into account the empty aircraft's configuration which would get you closer to a mathematical balance when loaded. This is the easiest, permanent solution to a problem I don't think really exists in operation.

Again, I'm pretty sure there is nothing proactively, or actively done to account for an imbalance that close to the centerline.
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trijetsonly
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Re: Longitidunal axis CoG on B717/A220

Wed Jan 09, 2019 7:48 pm

The only aircraft where lateral imbalance really matters (apart from freighters) is the 767.
Due to the narrow cargo holds LD-3 containers can only be loaded on one site and operators must have a procedure to check the effect. Depending on the resulting moment, maximum weight might be restricted.
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brindabella
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Re: Longitidunal axis CoG on B717/A220

Sun Jan 13, 2019 12:57 am

trijetsonly wrote:
The only aircraft where lateral imbalance really matters (apart from freighters) is the 767.
Due to the narrow cargo holds LD-3 containers can only be loaded on one site and operators must have a procedure to check the effect. Depending on the resulting moment, maximum weight might be restricted.


Thanks! Flew 'em for years, was cognisant of the requirement but never knew why! :D

To the OP - summary seems to be that if you are fully loaded with pax you have to accept a bit of drag from the aileron trim ... if so, the T/O performance data calculations presumably have the S/E case allowing for same(?).

Interesting.

cheers
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