Weight & Balance
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Weight & Balance
Just curious, what are the centre of gravity limits on the big jets ?
Given that a 27 foot Cessna has around a 10" limit, would a 180 foot long 767 have a c.o.g. range around 6-7 times larger ? ... 5-6 feet ?
Mike
Given that a 27 foot Cessna has around a 10" limit, would a 180 foot long 767 have a c.o.g. range around 6-7 times larger ? ... 5-6 feet ?
Mike
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Mike, don't know what the range is on the big birds but it isn't solely a function of length.
The absolute aft limit is the centre of pressure- a CG behind the CP will make the plane want to fly backwards. Even keeping it at the CP can be dicey, because ideally you want the CG to be ahead of it so that the plane will right itself if something knocks it around. The aft CG limit published in the POH is well ahead of the CP for this reason, which is one of the reasons the Cessna 172 is such a docile airplane.
The forward CG limit depends on the amount of elevator authority available and the load that can be placed on the airframe- the further forward it is, the more downforce needed on the tailplane to hold the nose up. This extra downforce will increase drag and increase the stall speed as more lift is needed, and will also put more stress on the fuselage between the wings & tail. The published forward limit is what the engineers figure the aircraft can safely fly at with a reasonable stall speed and plenty of elevator for pitch control (which is why the limit for the C172 moves forward as weight decreases).
Therefore, making the plane longer isn't the only way to increase the CG range. One can also increase the range by making the elevator bigger and/or further aft, and by reducing the payload. I'm sure the pros here know a few other tricks for extending the range as well.
The absolute aft limit is the centre of pressure- a CG behind the CP will make the plane want to fly backwards. Even keeping it at the CP can be dicey, because ideally you want the CG to be ahead of it so that the plane will right itself if something knocks it around. The aft CG limit published in the POH is well ahead of the CP for this reason, which is one of the reasons the Cessna 172 is such a docile airplane.
The forward CG limit depends on the amount of elevator authority available and the load that can be placed on the airframe- the further forward it is, the more downforce needed on the tailplane to hold the nose up. This extra downforce will increase drag and increase the stall speed as more lift is needed, and will also put more stress on the fuselage between the wings & tail. The published forward limit is what the engineers figure the aircraft can safely fly at with a reasonable stall speed and plenty of elevator for pitch control (which is why the limit for the C172 moves forward as weight decreases).
Therefore, making the plane longer isn't the only way to increase the CG range. One can also increase the range by making the elevator bigger and/or further aft, and by reducing the payload. I'm sure the pros here know a few other tricks for extending the range as well.
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Here are some examples for big jets for you:
1. B767
From 7%-37% MAC safe range is 71.25 Inches
2. A320
From 10.5%-43% MAC safe range is 1.36 metres
3. A330
From 18%-40% MAC safe range is 1.59 metres
4. A340
From 18%-42% MAC safe range is 1.60 metres
So not much difference heh!
1. B767
From 7%-37% MAC safe range is 71.25 Inches
2. A320
From 10.5%-43% MAC safe range is 1.36 metres
3. A330
From 18%-40% MAC safe range is 1.59 metres
4. A340
From 18%-42% MAC safe range is 1.60 metres
So not much difference heh!
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Icarus,
Close to my guesstimate.
When loading heavy pallets, etc I would imagine it might be very easy to exceed the limits.
I assume that some kind of computer program is used to calculate the loading configuration of the cargo bay ??
And would that even extend to calculating what is loaded onto each pallet ? ... I.e. one pallet being XXX lbs lighter / heavier than the next ?
Mike
Close to my guesstimate.
When loading heavy pallets, etc I would imagine it might be very easy to exceed the limits.
I assume that some kind of computer program is used to calculate the loading configuration of the cargo bay ??
And would that even extend to calculating what is loaded onto each pallet ? ... I.e. one pallet being XXX lbs lighter / heavier than the next ?
Mike
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In our company we only let people use computerized weight & balance once they have proved they can work it out by hand.
B747 11% - 33% MAC. Can be narrower at low weight.
B747 11% - 33% MAC. Can be narrower at low weight.
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Mstram,
Like cargo rat, we ensure people are competent at manual w&b before allowing them to produce EDP loadsheets etc.
You would be surprised how difficult it is to exceed limits (cargo aircraft excepted) actually.
Passengers have an influence that must be considered first and then you control the CofG with your deadload.
Simply, wt x distance = a moment.
That moment is usually converted to a much smaller number by using a constant denominator (10,000 30,000 100,000 etc) to give a index value for ease of use.
Don't forget that on large aircraft you will load both ends, so the 'net' effect is not that large.
Some aircaft are 'tricky' with low passenger lows (MD80 B727 etc) where they become extremely tail heavy. Combi aircraft present unique problems with compartment weight restrictions dependant upon other compartment loads, lateral restrictions etc.
However, W&B is not that difficult if you give it the time and thought necessary. In fact weight problems occur more than trim problems. Sounds difficult to control the CofG within such a small linear dimension; but in real life it is not such a task.
Like cargo rat, we ensure people are competent at manual w&b before allowing them to produce EDP loadsheets etc.
You would be surprised how difficult it is to exceed limits (cargo aircraft excepted) actually.
Passengers have an influence that must be considered first and then you control the CofG with your deadload.
Simply, wt x distance = a moment.
That moment is usually converted to a much smaller number by using a constant denominator (10,000 30,000 100,000 etc) to give a index value for ease of use.
Don't forget that on large aircraft you will load both ends, so the 'net' effect is not that large.
Some aircaft are 'tricky' with low passenger lows (MD80 B727 etc) where they become extremely tail heavy. Combi aircraft present unique problems with compartment weight restrictions dependant upon other compartment loads, lateral restrictions etc.
However, W&B is not that difficult if you give it the time and thought necessary. In fact weight problems occur more than trim problems. Sounds difficult to control the CofG within such a small linear dimension; but in real life it is not such a task.
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Icarus,
on some AC it is easy to get out of trim, especially on a full or empty DC9/MD80. Worse is a combi AC (727, 737 or 747) with lots of cargo and few pax (or the other way around). All these problems are known though, and easy to get around, but only by taking off payload or adding ballast.
FS
on some AC it is easy to get out of trim, especially on a full or empty DC9/MD80. Worse is a combi AC (727, 737 or 747) with lots of cargo and few pax (or the other way around). All these problems are known though, and easy to get around, but only by taking off payload or adding ballast.
FS
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Hi Icarus. Freighters work in absolutle weights, not "guesstimates" like in the pax biz.
Calculating a "trim" by hand is a must in terms of competence (or you won't pass our w&b course - MINIMUM required is 90%)
...
Calculating a "trim" by hand is a must in terms of competence (or you won't pass our w&b course - MINIMUM required is 90%)
...
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Icarus, you reminded me of an amusing anecdote involving a certain Canadian carrier that shall rename nameless (the one with the shiny red engines). They had an A320 picking up 76 pax at YYZ, then the rest of the planeload at YUL before proceeding to someplace sunny. Some genius in passenger service figured that boarding would go a lot quicker if all the YYZ pax were seated in the back of the cabin . Never did make it to YUL, as they left the APU exhaust pipe on the runway at YYZ.
. Never did make it to YUL, as they left the APU exhaust pipe on the runway at YYZ.
