CG formula anyone ?
Would you mind if I joined this discussion? I've done what QAYS is trying to do for several aircraft but only Boeing and McDonnell Douglas, never Airbus.
However the principle is the same.
You need certain basic information before you start (as has already been mentioned), most of it available in the manufacturer's manual.
LEMAC (leading edge of MAC)
Length of MAC
Reference arm
Constant (used to convert index into manageable figure)
When you have the operating empty weight and index, it becomes
relatively easy then to calculate %MAC.
%MAC = (Constant x (index/weight) + reference point-LEMAC)/length of MAC *100
Think I'm just saying the same thing in different words?
However the principle is the same.
You need certain basic information before you start (as has already been mentioned), most of it available in the manufacturer's manual.
LEMAC (leading edge of MAC)
Length of MAC
Reference arm
Constant (used to convert index into manageable figure)
When you have the operating empty weight and index, it becomes
relatively easy then to calculate %MAC.
%MAC = (Constant x (index/weight) + reference point-LEMAC)/length of MAC *100
Think I'm just saying the same thing in different words?
Moderator
Would you mind if I joined this discussion?
The more the merrier as with all threads ..
I've done what QAYS is trying to do for several aircraft but only Boeing and McDonnell Douglas, never Airbus. However the principle is the same.
This stuff is all the same whether we are talking a glider, C172, or B747. Once you get the head around the techniques it is all a bit of a doddle. Getting the head around it in the first place is the problem.
Think I'm just saying the same thing in different words?
Absolutely ... but why make something simple so complex ? Doesn't appear to achieve anything to a simple Simon like me ...
Explanation -
If we ignore any IU shift for, say, a trimsheet, the general formula becomes
IU = [(FS-D)*WT]/C
which, on rearrangement, becomes
FS = [(IU*C)/WT] + D
and, if you plug that expression for FS into the stock standard %MAC equation
%MAC = {[(FS-LEMAC)]/MAC}*100
you end up with
%MAC = {{[(IU*C)/WT] + D - LEMAC}/MAC} * 100
where D = reference arm .. which is more generally referred to as the trim datum.
The more the merrier as with all threads ..
I've done what QAYS is trying to do for several aircraft but only Boeing and McDonnell Douglas, never Airbus. However the principle is the same.
This stuff is all the same whether we are talking a glider, C172, or B747. Once you get the head around the techniques it is all a bit of a doddle. Getting the head around it in the first place is the problem.
Think I'm just saying the same thing in different words?
Absolutely ... but why make something simple so complex ? Doesn't appear to achieve anything to a simple Simon like me ...
Explanation -
If we ignore any IU shift for, say, a trimsheet, the general formula becomes
IU = [(FS-D)*WT]/C
which, on rearrangement, becomes
FS = [(IU*C)/WT] + D
and, if you plug that expression for FS into the stock standard %MAC equation
%MAC = {[(FS-LEMAC)]/MAC}*100
you end up with
%MAC = {{[(IU*C)/WT] + D - LEMAC}/MAC} * 100
where D = reference arm .. which is more generally referred to as the trim datum.
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LEMAC, TMAC...Basic PPL stuff guys, alluded to by John in the beginning of this thread...applies to all A/C...
Weight x Arm = Moment...
CG = Total Moment divided by total weight...
Very simple, really...
Weight x Arm = Moment...
CG = Total Moment divided by total weight...
Very simple, really...
Moderator
Very simple, really...
True .. but only once one gets one's head around the techniques.
When playing with trimsheets one sees a range of designs ranging from very bad through to very good (and the latter, generally, are presented fairly simply).
I can recall a task to design a GII trimsheet many years ago now (about 30 years ago, as I recall). That little exercise took me some weeks to figure out a way to present a tidy double envelope to account for trimming at ZFW but still needing to determine CG at takeoff (as I recall that was the main problem). Once I figured out a way to do it, it was all very simple .. but I had a few worrying moments along the way trying to make something work. Another very experienced engineer had given up and suggested that it just couldn't be done ..
True .. but only once one gets one's head around the techniques.
When playing with trimsheets one sees a range of designs ranging from very bad through to very good (and the latter, generally, are presented fairly simply).
I can recall a task to design a GII trimsheet many years ago now (about 30 years ago, as I recall). That little exercise took me some weeks to figure out a way to present a tidy double envelope to account for trimming at ZFW but still needing to determine CG at takeoff (as I recall that was the main problem). Once I figured out a way to do it, it was all very simple .. but I had a few worrying moments along the way trying to make something work. Another very experienced engineer had given up and suggested that it just couldn't be done ..
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John... agree with you, however, my point was, if you have the basics down, you don't need all the "newfangled" automated cr@p to work this stuff out...
For example...B-727F...12 pallet positions...find out how much each one weighs and have them loaded accordingly...too heavy?...take less fuel and make a stop or leave a pallet or two behind and go N/S...
Oops, leaving a pallet or two behind makes the remainder of the load out of limits...NO PROBLEM!!!
Rework the numbers, rearrange the pallets and go on about your business...makes life hell for the handlers I know, but in the final analysis, it is "simple, really"...Yes Really...
For example...B-727F...12 pallet positions...find out how much each one weighs and have them loaded accordingly...too heavy?...take less fuel and make a stop or leave a pallet or two behind and go N/S...
Oops, leaving a pallet or two behind makes the remainder of the load out of limits...NO PROBLEM!!!
Rework the numbers, rearrange the pallets and go on about your business...makes life hell for the handlers I know, but in the final analysis, it is "simple, really"...Yes Really...
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John, not "enthuisast", however, got stuck with it and adapted...As for my prior (to freight) 18 yrs plus of instructing/FAR135/121 Pax experience, I still feel the same, with the exception that when the load sheet is presented, you really do have to check and double check, because the agent is just trying to get you out of there...
For example....on contract to a UK hiring company, flying a S. African (SAFAIR) B-727-200 for Cameroon Airlines (filling in for their B-737-200, which was in Joburg undergoing a "C" Check with , you guessed it SAFAIR), I had to constantly examine and re-examine the loadsheet...
It was printed in French, fuel was in Kilos, and 90% of the time it was done on the numbers for the 737, not the 727 we were in...it almost bit my backside once, but never again...that's called experience...fool me once, shame on you....
Still, after all was said and done, we ended up doing our own W&B and life was great...even in Douala...Great people and wonderful hosts....First Rate, as far as that area of the world....
For example....on contract to a UK hiring company, flying a S. African (SAFAIR) B-727-200 for Cameroon Airlines (filling in for their B-737-200, which was in Joburg undergoing a "C" Check with , you guessed it SAFAIR), I had to constantly examine and re-examine the loadsheet...
It was printed in French, fuel was in Kilos, and 90% of the time it was done on the numbers for the 737, not the 727 we were in...it almost bit my backside once, but never again...that's called experience...fool me once, shame on you....
Still, after all was said and done, we ended up doing our own W&B and life was great...even in Douala...Great people and wonderful hosts....First Rate, as far as that area of the world....
Moderator
when the load sheet is presented, you really do have to check and double check, because the agent is just trying to get you out of there...
Never a truer statement uttered.
Likewise, I have fond memories of a 737-200 training contract in RSA some years ago. Lovely people, in the main, and a most interesting country.
I guess the enthusiasm for doing things at 0-dark-30 wore off a bit for me after some years and, if I were to be completely honest, I was just a little bit glad to get back onto normal passenger operations ..
Never a truer statement uttered.
Likewise, I have fond memories of a 737-200 training contract in RSA some years ago. Lovely people, in the main, and a most interesting country.
I guess the enthusiasm for doing things at 0-dark-30 wore off a bit for me after some years and, if I were to be completely honest, I was just a little bit glad to get back onto normal passenger operations ..
Moderator
(a) if you wanted to, sure. But why bother ?
(b) main effect would be to mirror the trimsheet image for no purpose other than confusing the great majority beyond reasonable comprehension.
(b) main effect would be to mirror the trimsheet image for no purpose other than confusing the great majority beyond reasonable comprehension.
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Just want to ask a basic question, when looking at the FCOM on how to do a manual load sheet it says:
DATA
Dry Operating Weight = 42500 kg and CG = 27 % (H-arm = 18.93 m)
Deviation or adjustment = + 100 kg in zone F
Cargo = 5500 kg with the following distribution :
cargo 1 = 2000 kg ; cargo 3 = 1500 kg ; cargo 4 = 1500 kg ; cargo 5 = 500 kg
Passengers = 145 pax with the following distribution :
cabin OA = 50 ; cabin OB = 55 ; cabin OC = 40
Ramp Fuel = 13200 kg ; Taxi Fuel = 200 kg ; Fuel Density = 0.785 kg/l
DESCRIPTION
Enter Master data in (1).
Compute Dry Operating Weight Index using the formula indicated in (2) and report in (3).
Dry Operating Index = 53.4.
My question is about the H-arm. I looked at the Getting to Grips document and there is no definition of H-arm. I understand the H-arm is a reference point but would it stay the same on every aircraft on an identical fleet? And if not, where would you find the specific aircraft's H-arm to enter the Airbus manual loadsheet in the top left box, page 121 in the Getting to Grips document? In this example above, is H-arm = 18.93 a generic figure or is that the one to use on the manual loadsheet?
Thanks for the replies,
Screwballs
DATA
Dry Operating Weight = 42500 kg and CG = 27 % (H-arm = 18.93 m)
Deviation or adjustment = + 100 kg in zone F
Cargo = 5500 kg with the following distribution :
cargo 1 = 2000 kg ; cargo 3 = 1500 kg ; cargo 4 = 1500 kg ; cargo 5 = 500 kg
Passengers = 145 pax with the following distribution :
cabin OA = 50 ; cabin OB = 55 ; cabin OC = 40
Ramp Fuel = 13200 kg ; Taxi Fuel = 200 kg ; Fuel Density = 0.785 kg/l
DESCRIPTION
Enter Master data in (1).
Compute Dry Operating Weight Index using the formula indicated in (2) and report in (3).
Dry Operating Index = 53.4.
My question is about the H-arm. I looked at the Getting to Grips document and there is no definition of H-arm. I understand the H-arm is a reference point but would it stay the same on every aircraft on an identical fleet? And if not, where would you find the specific aircraft's H-arm to enter the Airbus manual loadsheet in the top left box, page 121 in the Getting to Grips document? In this example above, is H-arm = 18.93 a generic figure or is that the one to use on the manual loadsheet?
Thanks for the replies,
Screwballs
Moderator
when looking at the FCOM
One should keep in mind that this stuff tends to have all sorts of terminology differences across the Industry. As a result, it is necessary to maintain good housekeeping and cite which documents are being referred to ?
there is no definition of H-arm.
Generally one would expect H-arm to be the CG position of interest .. either of the whole aircraft or a given loading item ie a variable quantity. Numerous references in the AI document are consistent with this suggestion.
where would you find the specific aircraft's H-arm
.. in the descriptive material relating to the weight and balance manual (or similar document) for the Type or the loading system for the particular tail number.
in the top left box, page 121 in the Getting to Grips document? In this example above, is H-arm = 18.93 a generic figure or is that the one to use on the manual loadsheet?
For the location cited, the H-arm of interest is the particular tail number CG for the aircraft in the dry operating weight configuration (which can vary a bit so one needs to check the definition for the particular use).
The value 18.93m, as cited, is this DOW configuration value.
Note that you must read the term in context. It might be referring to the whole aircraft CG or it might be referring to the local loading CG position for a particular load item going into the aircraft.
One should keep in mind that this stuff tends to have all sorts of terminology differences across the Industry. As a result, it is necessary to maintain good housekeeping and cite which documents are being referred to ?
there is no definition of H-arm.
Generally one would expect H-arm to be the CG position of interest .. either of the whole aircraft or a given loading item ie a variable quantity. Numerous references in the AI document are consistent with this suggestion.
where would you find the specific aircraft's H-arm
.. in the descriptive material relating to the weight and balance manual (or similar document) for the Type or the loading system for the particular tail number.
in the top left box, page 121 in the Getting to Grips document? In this example above, is H-arm = 18.93 a generic figure or is that the one to use on the manual loadsheet?
For the location cited, the H-arm of interest is the particular tail number CG for the aircraft in the dry operating weight configuration (which can vary a bit so one needs to check the definition for the particular use).
The value 18.93m, as cited, is this DOW configuration value.
Note that you must read the term in context. It might be referring to the whole aircraft CG or it might be referring to the local loading CG position for a particular load item going into the aircraft.
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HI
I found an interesting discussion on calculation formulate in progress. I would like to contribute on the same.
Before we know what is C , K or other terms are. We need to know basics of CG shift and law of movement.
Each aircraft when manufactured needs to specify various dimension i.e. distances. So, a arbitrary point is fixed by the manufacturer from where all calculations will be written. This reference point is called Datum Point.
Thereafter so we are suspending a load of 1000 kg at a longitudnal distance from the datum point. The impact of this load needs to be studied. So, arm x wt = movement. This was all types of loads are computed for movement. Now the movement put together becomes so big a figure which is difficult to handle. As such, a figure is selected which will reduce the big size. This is called C constant for which we need to rely on the engineer who has freezed the formula. The index movement is poistive K constant say 50 or 100 is selected.
Trust it gives some information to your field of discussion.
Ajay Saxena
I found an interesting discussion on calculation formulate in progress. I would like to contribute on the same.
Before we know what is C , K or other terms are. We need to know basics of CG shift and law of movement.
Each aircraft when manufactured needs to specify various dimension i.e. distances. So, a arbitrary point is fixed by the manufacturer from where all calculations will be written. This reference point is called Datum Point.
Thereafter so we are suspending a load of 1000 kg at a longitudnal distance from the datum point. The impact of this load needs to be studied. So, arm x wt = movement. This was all types of loads are computed for movement. Now the movement put together becomes so big a figure which is difficult to handle. As such, a figure is selected which will reduce the big size. This is called C constant for which we need to rely on the engineer who has freezed the formula. The index movement is poistive K constant say 50 or 100 is selected.
Trust it gives some information to your field of discussion.
Ajay Saxena
