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WhiteKnight
19th Jun 2007, 21:41
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

Mad (Flt) Scientist
19th Jun 2007, 21:58
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?

blackmail
19th Jun 2007, 22:27
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:rolleyes:

wiggy
19th Jun 2007, 22:57
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.

blackmail
19th Jun 2007, 23:11
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:hmm:

mutt
20th Jun 2007, 05:59
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

Rainboe
20th Jun 2007, 07:00
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.

Wizofoz
20th Jun 2007, 08:00
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.

WhiteKnight
20th Jun 2007, 08:39
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?

Capt Pit Bull
20th Jun 2007, 09:32
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

wiggy
20th Jun 2007, 10:31
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.

zon3
20th Jun 2007, 10:42
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...
:O

john_tullamarine
20th Jun 2007, 15:05
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 ..

mutt
20th Jun 2007, 15:46
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.

Actually it is a paperwork exercise as it isnt based on the CG of the day but rather the smaller envelope.

Mutt

Wizofoz
20th Jun 2007, 15:59
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?

wiggy
20th Jun 2007, 16:10
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.

mj23
26th Jun 2008, 10:51
Hi,

I was wondering whether aft CG loading consumes less fuel, and why is that so? Anyone can comment? Thanks!

EnzoC
26th Jun 2008, 13:16
@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.

cwatters
26th Jun 2008, 14:41
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.

CR2
26th Jun 2008, 16:33
On the 744F, alternate fwd cg will give you a few more tons performance. T/O CG has to be aft of 20% MAC.

Todders
26th Jun 2008, 19:36
In my company we often have to use either 20% or 22% mac to get out.

We have figures published for both of these for the different flap settings on all our limiting rnw's. 737-800/900 by the way. Standard loading nearly always has us around 22-23% mac. Makes all the diff some days.

MTOW is a figure calculated to ensure the aircraft can safely operate out of a given rnw on a given day with give conditions ie wet/dry and temp. Normally it is calculated for the worst case situation ie. the actual CofG being on the forward limit.

Now if you have your load sheet and on that day it tells you that you have an actual CofG on that day of say 24%mac you now know that you are safe useing figures for MTOW calculated for 24%mac. Airlines are unlikely to have figures derived and tabluated for you for every given CofG possiton. For this reason in my company we have tables/(BLT) giveing you MTOW figures for 20%mac and 22%mac.

If you know you have a CofG aft of the CofG on the tables you know you are safe.


In simple it is a method of degrading the safety margin in order to operate the aircraft from limiting airports safely.------

:ugh:Granted not a great statement the same safety margin exists just the amount the actual aircraft performance on the day exceeds the required performance is reduced.

john_tullamarine
27th Jun 2008, 00:03
A lot of folks getting too involved in the weeds level, here ... considerations are all to do with certification ..

(a) takeoff performance starts with the AFM stall speed for the weight.

(b) AFM stall speed usually is determined at the forward limit for reasons tied up with the tail download consideration mentioned in several posts above .. ie the forward limit normally gives the highest flight test stall speed within the envelope CG range at a given weight.

(c) the consideration is a bit like derate in that the OEM is perfectly entitled to do several mini-certifications for the aircraft. In the same way derate lets the operator take advantage of lower Vmcg/Vmca, restricting the AFM forward cg limit permits the operator to take advantage of a lower stall speed for a given weight . .this lower stall speed then flows into the other speeds pertinent to takeoff speed schedule and comes up with a few extra kilos RTOW especially for runway limited circumstances .. this may not apply at lower weights if the basic performance is minV1-limited.

(d) in all cases, to take advantage of the permission, the operator has to have the supplement in the AFM (generally at humungous cost .. it has always ever been .. and will always ever be ... the same)

(e) In simple it is a method of degrading the safety margin in order to operate the aircraft from limiting airports safely.

Not quite .. the minimum "safety margins" remain the same for certification .. what the OEM is doing is a multiple certification .. a bit like having two slightly different aircraft models within the same Type

(f) technically speaking, you should have the tool to calculate takeoff-data for each CG position within the env

Those of us with the tech background could come up with some plausible data .. however, at the end of the day, the operator can only operate within the boundaries of the AFM .. so, if secondary CG limits are not published then this argument remains academic.

Anp
27th Jun 2008, 12:07
From the Airbus Instructor Support A-320 Family:

Some considerations about the CG

The location of the CG has significant influence on Performance, on Loading flexibility, on structure and on handling characteristics when in Direct Law.
All those factors contribute to define the CG envelope.

- Performance considerations
The weight and lift forces do create a pitching moment which is counteracted by the THS setting.
When the CG is located forward, the resulting pitching down moment is counteracted by a large THS nose down
setting which induces a lift decrease and a drag increase.

As a general rule, FWD CG penalizes the Performance.

• At Take-off and landing, it affects:
* The Stall speeds: Typically on A330/A340, the stall speed increases by 1.5 kts when CG varies from 26% to full forward CG. This affects take-off and landing speeds thus associated distances.

* The rotation maneuver: It is "heavier", thus longer at forward CG.
This affects the take-off distance. For example, on an A340 at 250 t, the TOD increases from 3165 m to
3241 m, when CG varies from 26% to full forward CG, which represents a 2.42% TOD increase (T/O, FLAP3, PACK: OFF, ISA, ALT 0).

* The climb performance itself: For example, if a climb gradient of 5% is required (e.g. due to obstacles) in the previous take-off conditions, the MTOW is reduced from 257.6 t down to 256.2 t when CG varies from 26% to
full forward CG.
This is why on A320 and A340 take-off performance charts are provided at forward CG (which in most cases is penalizing) and at 26%; these last charts may be used provided the actual aircraft CG is at least 28%

ANP