Alternate Forward CG
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Alternate Forward CG
Hello,
I heard about an "alternate forward cg " option you can purchase from Boeing to improve your allowed TOW in some circumstances. Can anybody explain to me the idea behind it. I have no clue, why you can increase your TOW?
Thanks for any help.
WK
I heard about an "alternate forward cg " option you can purchase from Boeing to improve your allowed TOW in some circumstances. Can anybody explain to me the idea behind it. I have no clue, why you can increase your TOW?
Thanks for any help.
WK
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Not knowing, just speculating.
If I choose to specify a further aft number for my max forward CG (and accept the loss in flexibility in loading) such that my CofA says "max fwd 15%cg" instead of "max fwd 10% cg" then I may be able to take advantage of:
* slightly better stall speeds due to less trim downforce at the stall
* slightly better Vmu speeds
* slightly better rotation characteristics (if my mistrim case drove minimum Vr, perhaps)
Alternatively, if your nosewheel is load limited, by having a forward cg cutback, maybe the structural MTOW can increase?
If I choose to specify a further aft number for my max forward CG (and accept the loss in flexibility in loading) such that my CofA says "max fwd 15%cg" instead of "max fwd 10% cg" then I may be able to take advantage of:
* slightly better stall speeds due to less trim downforce at the stall
* slightly better Vmu speeds
* slightly better rotation characteristics (if my mistrim case drove minimum Vr, perhaps)
Alternatively, if your nosewheel is load limited, by having a forward cg cutback, maybe the structural MTOW can increase?
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alternate fwd cg for take off
hello wk,
find here under quote of mr. boeing flight crew training manual, 3.11 fct 737ng(tm) dd. october 31, 2005.
" operation with alternate fwd cg limit for take off.
take off performance is based on the fwd cg limitations as defined in the AFM. however, t/o perfo can be improved by taking credit for an alternate (further aft) fwd cg limit if shown in the AFM. use of this data provides higher perfo-limited t/o weights than the basic AFM perfo data.
typically alternate fwd cg is used to increase perfo-limited t/o weight for field length, climb or obstacle limited departures. another potential benefit of alternate fwd cg is to allow greater thrust reduction which increases engine reliability & reduces engine maintenance costs. However, this improved perfo capability is only available if the airline has the certified data in their AFM & has approval from their regulatory agency to operate the airplane at an alternate fwd cg limit.
a more aft cg increases the lift available at a given angle of attack due to the reduction in nose up trim required from the horizontal stabilizer. this allows Vr & V2 to be reduced, which in turn reduces the field length required for t/off. reduction in field length required can also permit an increased field length limited weight. in most instances this reduction in nose up trim also results in a decrease in drag which improves the airplane's climb capability.
NOTE : the FMC calculated t/off speeds & QRH t/off speeds are not valid for operations using alternate fwd cg. t/off speeds must be calculated using alternate fwd cg perfo data normally provided by dispatch or flight operations."
hope this helps,
kind regards,
bm
find here under quote of mr. boeing flight crew training manual, 3.11 fct 737ng(tm) dd. october 31, 2005.
" operation with alternate fwd cg limit for take off.
take off performance is based on the fwd cg limitations as defined in the AFM. however, t/o perfo can be improved by taking credit for an alternate (further aft) fwd cg limit if shown in the AFM. use of this data provides higher perfo-limited t/o weights than the basic AFM perfo data.
typically alternate fwd cg is used to increase perfo-limited t/o weight for field length, climb or obstacle limited departures. another potential benefit of alternate fwd cg is to allow greater thrust reduction which increases engine reliability & reduces engine maintenance costs. However, this improved perfo capability is only available if the airline has the certified data in their AFM & has approval from their regulatory agency to operate the airplane at an alternate fwd cg limit.
a more aft cg increases the lift available at a given angle of attack due to the reduction in nose up trim required from the horizontal stabilizer. this allows Vr & V2 to be reduced, which in turn reduces the field length required for t/off. reduction in field length required can also permit an increased field length limited weight. in most instances this reduction in nose up trim also results in a decrease in drag which improves the airplane's climb capability.
NOTE : the FMC calculated t/off speeds & QRH t/off speeds are not valid for operations using alternate fwd cg. t/off speeds must be calculated using alternate fwd cg perfo data normally provided by dispatch or flight operations."
hope this helps,
kind regards,
bm
Are you sure it's "forward"?
As far as I understand it ( and in our Company we use an aft CG procedure to improve payload out of Hot and High airports) shifting the C/G aft reduces the tail down force -and when you are trying to lift something a down force is the last thing you need. Now over to Mad Flight Scientist or Mutt for a more reasoned answer.
As far as I understand it ( and in our Company we use an aft CG procedure to improve payload out of Hot and High airports) shifting the C/G aft reduces the tail down force -and when you are trying to lift something a down force is the last thing you need. Now over to Mad Flight Scientist or Mutt for a more reasoned answer.
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alternate fwd cg for take off
wiggy, hello,
it is the fwd cg limit shifted more aft = alternate fwg cg method.
& i also remember a statement: " as a rule, aft cg saves fuel,( due to reduced trim drag)".
bm
it is the fwd cg limit shifted more aft = alternate fwg cg method.
& i also remember a statement: " as a rule, aft cg saves fuel,( due to reduced trim drag)".
bm
Wiggy,
I presume that you are talking about how you load the aircraft. In this case they are talking about ALTERNATE FORWARD CG, this means that the forward CG limit is moved aft and the performance is based on this position for the reasons mentioned by MFS and Blackmail.
In your case, i dont see how you can benefit from moving cargo aft unless you have a seperate CG envelope and associated performance.
On the E170, moving the Forward CG limit from 7%-16% can increase limiting takeoff weights by around 3-400 kgs.
Mutt
I presume that you are talking about how you load the aircraft. In this case they are talking about ALTERNATE FORWARD CG, this means that the forward CG limit is moved aft and the performance is based on this position for the reasons mentioned by MFS and Blackmail.
In your case, i dont see how you can benefit from moving cargo aft unless you have a seperate CG envelope and associated performance.
On the E170, moving the Forward CG limit from 7%-16% can increase limiting takeoff weights by around 3-400 kgs.
Mutt
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At performance limited airfields, it is a valid option. In BA at MEX, the TOW is usually limited as the place is hot and very high. I recall it gave a few tons more payload.
777-300s with EFBs for performance calculations have an "Alternate CG" selection. If the actual CG is 26%MAC or more, selecting it will increase Performance limited MTOW, as well as increasing the allowed de-rate.
As previously mentioned, it's because it allows for the fact that with a more rearward CG, the downforce produceded by the stab is less, therefore increasing the available lift at a particular speed.
As previously mentioned, it's because it allows for the fact that with a more rearward CG, the downforce produceded by the stab is less, therefore increasing the available lift at a particular speed.
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Thanks for all the replies so far. But I still have a problem with this concept.
Let's say I calculate a CG which is actually near the aft limit of the cg envelope. How does a shift of the forward cg limit change anything if you´re not operating in this area in the first place?
Let's say I calculate a CG which is actually near the aft limit of the cg envelope. How does a shift of the forward cg limit change anything if you´re not operating in this area in the first place?
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WK,
It doesn't. But if you are working near the rear anyway you are getting better than worst case performance and this procedure allows you to increase mass as a result; as long as you 'agree' not to then move the CofG forewards again.
I think. Somebody tell me if I'm talking out of my overboard valve.
pb
It doesn't. But if you are working near the rear anyway you are getting better than worst case performance and this procedure allows you to increase mass as a result; as long as you 'agree' not to then move the CofG forewards again.
I think. Somebody tell me if I'm talking out of my overboard valve.
pb
Perhaps I'm being paranoid but I'm sensing a degree of scepticism here.
Mutt, we might be talking at cross purposes but, yes we do use specific take-off data ( paper or from datalink) for the aft CG case. Our ( Mine and Rainboe's) outfit uses the procedure on it's 747s at MEX, JNB and GRU and it adds a significant amount to the payload we can lift when compared with using "normal figures".
Capt Pit Bull: Agreed. This isn't just a paperwork exercise, the cargo actually has to be loaded properly to get the C of G where we are planning for it to be...otherwise the performance figures are not valid.
Mutt, we might be talking at cross purposes but, yes we do use specific take-off data ( paper or from datalink) for the aft CG case. Our ( Mine and Rainboe's) outfit uses the procedure on it's 747s at MEX, JNB and GRU and it adds a significant amount to the payload we can lift when compared with using "normal figures".
Capt Pit Bull: Agreed. This isn't just a paperwork exercise, the cargo actually has to be loaded properly to get the C of G where we are planning for it to be...otherwise the performance figures are not valid.
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The T/O performance isn't calculated based on your actual c/g, it's calculated using a "worst case scenario" for c/g which is the forwardmost one. This usually gives conservative figures for mtow, v1 etc, as you are rarely operating with c/g this much forward. If you move the forward limit for c/g aft, ie make sure that you will never have an actual c/g that is forward of this new limit, the manufacturer can publish better performance figures (higher rtow) simply because the "worst case scenario" in terms of c/g is improved.
Hope it makes sense...
Hope it makes sense...
Moderator
While I can't speak to the specific circumstance being discussed here, there are a number of examples where the forward CG has been tweaked aft to adjust the stall speed down. The other speeds are predicated on, amongst other things, stall speed. Generally, if one can reduce stall speed then one can reduce the V speeds and, for a given (critical) runway length .. pick up some extra kilos.
The concern is not where you might be operating within the envelope on the day .. rather that the restricted forward limits (generally in the higher weight region) must be observed to take advantage of the benefit. Conceptually a bit like thrust derate ... have your cake and eat it sort of situation ..
The concern is not where you might be operating within the envelope on the day .. rather that the restricted forward limits (generally in the higher weight region) must be observed to take advantage of the benefit. Conceptually a bit like thrust derate ... have your cake and eat it sort of situation ..
Agreed. This isn't just a paperwork exercise, the cargo actually has to be loaded properly to get the C of G where we are planning for it to be...otherwise the performance figures are not valid.
Mutt
A question that might clear things up- As I said, even operating a big aircraft and using a pretty sophisticated performance tool, we still only have two options for CG (Full, which is the normal forward limit, or alternate if the CG is 26%MAC or more.)
Does anyone have the tools to use the actual CG at the time (i.e as per the load sheet) to calculate performance for that takeoff?
Does anyone have the tools to use the actual CG at the time (i.e as per the load sheet) to calculate performance for that takeoff?
Zon3 - thanks - even I understand it now..
Mutt - My comment about it not being a paperwork exercise was because one of our crews used "aft" CG figures to help get the payload on that the station wanted to shift. However the station staff then loaded the aicraft with no regard to the aft CG figures and presented a loadsheet with the CG forward of the new limit....
regards.
Mutt - My comment about it not being a paperwork exercise was because one of our crews used "aft" CG figures to help get the payload on that the station wanted to shift. However the station staff then loaded the aicraft with no regard to the aft CG figures and presented a loadsheet with the CG forward of the new limit....
regards.
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@wizofoz: technically speaking, you should have the tool to calculate takeoff-data for each CG position within the envelope. However, you legally might not be allowed to. On some Boeing aircraft (appendix to the AFM) you must decide on one (or two) distinct Alt FWD CG position(s). Your Boeing database will allow to calculate every position, but alas, legally you have to stick to the selected one(s).
@mj23:
Aft CG positions will lower fuel flow to a considerable amount.
Imagine the airplane to be a balance, hang up at the center of pressure (CoP). The center of gravity (CoG) for a typical airplane design is fwd of the CoP and thus, the weight induces a pitch down momentum. To out-balance this, your stabilizer has to create a downforce (pitch-up momentum). The farther your CoG goes aft (closer to CoP), the smaller is the pitch-down momentum induced by the weight. The smaller the pitch-down momentum is, the smaller the pitch-up momentum, induced by the stabilizer, has to be to keep the airplane balanced. Smaller pitch-up momentum means less downforce on the stabilizer.
The total lift, the wing has to produce, is the sum of the weight plus the downforce. Now, the lower the downforce is, the lower the total lift has to be for the same weight. Lower lift also means lower drag, which in turn means less fuel consumption.
I hope I could help a bit.
@mj23:
Aft CG positions will lower fuel flow to a considerable amount.
Imagine the airplane to be a balance, hang up at the center of pressure (CoP). The center of gravity (CoG) for a typical airplane design is fwd of the CoP and thus, the weight induces a pitch down momentum. To out-balance this, your stabilizer has to create a downforce (pitch-up momentum). The farther your CoG goes aft (closer to CoP), the smaller is the pitch-down momentum induced by the weight. The smaller the pitch-down momentum is, the smaller the pitch-up momentum, induced by the stabilizer, has to be to keep the airplane balanced. Smaller pitch-up momentum means less downforce on the stabilizer.
The total lift, the wing has to produce, is the sum of the weight plus the downforce. Now, the lower the downforce is, the lower the total lift has to be for the same weight. Lower lift also means lower drag, which in turn means less fuel consumption.
I hope I could help a bit.
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Aside: This is one reason why a canard (tail first) is claimed by some to be superior...it's because to counter the pitching moment of the wing the "tail" at the front produces positive lift rather than negative. Thus it adds to rather than subtracts from the lift available drom the wing. I've heard it claimed this is why the Wright brothers choose a canard layout but I've not looked to see of there is evidence for that.