Aft c of g = improved TOP

[Frosty Hoar]

Just achieved a 500kg increase in allowable take off weight for a company B737-800 through using an MAC setting of 25% in the take off performance calculation- i.e a more rearward c of g setting than "optimum".

I assume that this allows less stab trim to be used, therefore less drag and quicker acceleration to rotation, can any one confirm this to be correct and advise if this creates any handling problems during the take off roll, i.e will the aircraft be light on the nose gear etc? Any feedback appreciated...:confused: :(

[Eckhard]

My understanding is that an aft CofG means that there will be less aerodynamic download required to be produced by the tail as a trim force.

This download increases the effective weight (because it is a force in a downward direction) and therefore any reduction in this aerodynamic download will improve performance, just as if weight was reduced.

I don't think the drag produced by the trim would be a significant factor during the take-off, although it certainly would be in the cruise.

On the 744, we only use CofG enhanced take-off performance at one critical airport at the moment (MEX) and we have to ensure that we use the chart that is appropriate to the actual CofG pertaining for that take-off. Your post suggested that you are using 25%MAC 'as required', or did I miss something?

There will not be any difference in handling, provided that you are used to an MAC of 25% normally. You are simply identifying and then using the 'extra' 500kg performance benefit that was always there anyway!

[Mad (Flt) Scientist]

It's very unlikely that the OEM is accounting for any change in aerodynamic drag on the runway due to a change in stab trim setting; to be brutally honest, there's no way they could identify the effect in any typical test variation.

The possible mechanisms are:

1. Reduced trim drag will improve your performance and give you a higher max allowable TOW if you were WAT limited . Won't do much for you otherwise.

2. Reduced stall speed (due to reduced trim downforce) will give you a lower minimum V2. That may reduce V1 also, depending on the balance of the various factors, and that may allow a higher TOW if you are field length constrained.

If you are light enough that there's no WAT concerns and you are VMC or VMU limited chances are you'll not see much if any difference (although VMU certainly does depend on cg, it would require a bunch of extra tests to take credit for moving the c.g. back if you were limited, and I very much doubt the OEM did that)

[john_tullamarine]

Interesting ... can you give us some more detail as to the documents and lookup procedures you are using ?

[quid]

.......and were there any changes to the V speeds, and by how much?

[SuperRanger]

when the C of G goes aft of the center of lift (for lack of better term), the elevator will need to produce an upward balancing force. therefore, part of the weight being supported by the wing is reduced. in effect, you have slightly 'lighter' airplane.

for the same reason, the stall speed also reduces with an aft C of G.

however, the flight stability of the airplane degrades as the C of G moves aft.

SR

[Frosty Hoar]

Thanks for the feedback,

Eckhard,

We only use MAC 25% when we are looking for a small increase in take off performance after all other means have been tried, as my standpoint is addmitedly that of an operations officer not a pilot I was curious to know exactly what parameter I was playing with, from what I understand 25% is not an optimum setting in terms of aircraft stability and handling for this particular aircraft (737-800) where the normal setting used is between 15 and 20% mac.

These calculations are being carried out in the ops room on a dedicated take off performance tool where we know the airfield to be marginal on the daý.

I just re calculated the figures to confirm the effect on v speeds when changing the mac settings, the only change in all three v speeds was a 1 knot reduction on v2 safety speed from 154knts to 153 knts,(a lower stalling speed) and allowable TOW increased from 77925kgs to 78700 kgs which can make the difference between a tech stop (in this case operating to the Canaries from Scandinavia) or not.

rgds FH

[Eckhard]

Frosty Hoar,

If you are predicating the TOPL on an MAC of 25% to flight plan without a tech stop, you have to be sure that the aircraft will be loaded in such a way that the CofG really is 25% (or within a certain parameter) on the day. This may be possible by loading the bags in the rear of the aft hold and 'filling up' forwards, stowing catering in the aft galley, passenger seating plans, etc.

The fundamental point is that the pilots will be presented with a loadsheet that shows them where the CofG is, and if it isn't 25% or thereabouts they won't be able to use the extra performance.

[Frosty Hoar]

Couldnt agree more, and interestingly the load control computer will normally aim for an optimum mac setting of say 20% in this case which would negate the performance calculation setting of 25%.

Moreover it is not uncommon for the crew to receive a higher TOW on the loadsheet than on the FPL, and to therefore ask for a revised TOP calculation after the aircraft has been loaded and prior to pushback, changing the MAC setting at this point should be done with caution especially as stated in my last post a "magic" 700kgs or so of extra weight can be carried.....

[Eckhard]

FH

'changing the MAC setting at this point should be done with caution'

I'm afraid I can't follow your banter, Frosty old bean. I don't understand how you can change the MAC setting after loading and prior to pushback. If that's the way the bird is loaded - that's it, and the MAC on the final loadsheet isn't going to change unless you rearrange the load distribution somehow. If the crew have calculated a TOPL based on an assumed MAC that turns out not to be correct then they either have to redistribute the load or work out a lower TOPL for the actual MAC.

I get the impression that we both agree on the fundamentals but I think I'm misinterpreting/misunderstanding bits of your posts. If that's the case then I apologise for restating the obvious.

E

[Frosty Hoar]

No problem Eckhard, and yes I understand that the loadsheet trim setting will take precedence over everything else.

I can imagine that reloading the a/c to achieve desired MAC would probably cause chaos (some of our airport agents have enough trouble loading and trimming in the first place) especially at short notice etc.

[john_tullamarine]

Without knowing the particular envelope, if a specific MAC range gives you an RTOW advantage .. for whatever reason ... why doesn't the operator target that MAC, subject to available load and where it can reasonably be put ?

This discussion seems a little strange to me ? Can someone in the know indicate the reason for the variations quoted ? ...

[Bellerophon]

john_tullamarine

A bit off-thread, but as an aside, consider the following, although I accept it is not typical, and I have simplified for the sake of brevity.

On my aircraft type, on every take-off, we actually do target the Aft Limit of the CoG.

We avoid much of the chaos that would arise on other aircraft types during loading because we are able, within limits, to move the CoG into the correct position - after pushback from the ramp and before take-off - by using the VAPOUR system (Voice Activated Pilots Onboard Universal Resource), or Flight Engineer as he is otherwise known, to move fuel around the aircraft.

At heavy weights we are glad of the RTOW improvement that this brings, but we have to be careful at lighter weights, particularly with lower fuel loads, for the following reason. Once the desired take-off CoG has been achieved, the question then is "Will it stay there during take-off?"

Well the crew and passengers are strapped in, hopefully so is all the freight, so what else is left that can move about during the take-off roll? The answer, of course, is the fuel in those tanks which are not completely full.

The relatively rapid acceleration encountered during take-off will tend to make this fuel surge rearwards in the tanks, and the high pitch attitudes encountered after take-off will tend to keep it there.

Both these effects would combine to move the CoG a bit further rearward than we could accept, given that we were planned at the Aft Limit to start with, and so, on those lighter weight take-offs, where fuel tanks are not full and this surge effect may be encountered, the AFM imposes reduced (further forward) Aft CoG Limits as the aircraft weight decreases.

On very short sectors, or with low fuel loads, we can end up with an RTOW decrement, because of the reduced Aft CoG Limit.

The aircraft Zero Fuel Weight CoG position is also very important to us, and leads to an interesting little dilemma.

A Forward ZFWCG position means we can load more fuel (high fuel loads move the aircraft CoG rearwards) and still stay within take-off CoG limits, but it will also require a greater amount of fuel to be transferred aft, as ballast, during SuperCruise at M2.0, to move the CoG to the optimum cruise position.

This ballast fuel is unusable whilst at M2.0, and the more that is required at the rear of the aircraft, obviously the less there is available in the wings to feed the engines. A point will arrive when this ballast fuel is required back in the wing tanks, to feed the engines, and this will force the aircraft to decelerate to subsonic flight, as if we can't keep the CoG aft, then we must make the CoP move forward.

With a Forward ZFWCG position, this point can be reached before the ideal TOD point, which can prove a trifle awkward, given that the subsonic range is considerably less than the supersonic range!

Sometimes asking for the aircraft to be loaded to a slightly more rearward ZFWCG, reducing the maximum fuel load somewhat, but allowing a longer SuperCruise, may actually be an improvement on a long sector!

Anyway, I've just noticed it's the 25 December where you are, so enough of this, and a very Happy Christmas to all "Down Under".

Seasons Greetings

Bellerophon

[john_tullamarine]

B,

Greetings generally .... and, likewise, Seasons of the same for when you awake in an hour or two ... haven't seen you around for some time ... the relaunch of the pointy bird presumably is keeping you busy ? .. and you will be pleased to hear that the Antipodean weather is a tad warmer than you are probably experiencing just now ....

I'm with you .. your story is eminently logical and understandable considering the Industry atypical needs of your particular machine ... and I suspect that many of us would be only too pleased to have to endure such rigours ..... but having no data on the 738 I am a bit mystified by this thread.

Normally one would expect a target for subsonic aircraft to be somewhere near 25 percent (and considerably further aft for most of the time on your steed, of course). Indeed, as on Concorde, the later Boeing and Airbus machines employ tail tanks to facilitate this in order to stretch the range a bit. One 744 operator, for instance, did a study some years ago and found that they could push the Boeing standard fuel program even further and pick up a commercially significant benefit on payload-limited sectors .. which was a consideration for their network.

I am interested in finding out the reason for this apparently strange circumstance. Then again the 800 has the odd idiosyncrasy or two ...for instance, I understand that the stab schedule is skewed forward compared to the earlier models to increase rotation stick load in turn to reduce the likelihood of tailstrike ... I wonder if this has something to do with the preference for a somewhat forward CG ?

I find it unlikely that Boeing would have done the work to vary performance weights with CG for this aircraft .. apart from an envelope forward limit spike (or some other interesting forward limit restriction) providing the opportunity to increase RTOW with an aft movement of CG, I am, I must confess, just a tad bemused by it all .....

FH, you couldn't list the AFM envelope limits on your machine for us and indicate the typical wt/CG position where the problem arises by any chance ? If you are targetting somewhere near the forward limit, then I would be looking for a spike in the envelope ......

[john_tullamarine]

FH,

Thanks .. but could you list all the CG intersections (so I can plot the envelope) and the specific loading (GW/CG) where the original problem occurs.

[Frosty Hoar]

JT Sorry, could you clarify what the "problem" is that you are trying to solve.....and what exactly is a "SPIKE" in the envelope?

[northwing]

Remember to check out the loading case on the ground. If you are busy putting all the bags in the back, the self loading cargo is filing into the back of the cabin, fuel is part loaded from the rear and the FO has gone down the back for a pee it may be only the captain's wallet that is holding the nose down. It has been done.

[john_tullamarine]

FH,

Speculating here but I think that the first port of call is to see if the problem is one of just another loading restriction.

Consider if the forward limit has a comparatively shallow slope (this may occur toward the max weight or if the various limitations result in a visor-shaped spike in the graph .. not seen all that often .. depends on how the OEM worked up the limitations). If the loading puts the aircraft at the forward limit in this case, then an aft movement of CG will permit an increase in GW as one moves along the limiting line.

Hence my interest in the actual envelope (the TCDS doesn't list the details .. merely refers one to the AFM) and the GW/CG intersection where the problem has been noted.

[Frosty Hoar]

Ok, some co-ordinates from the specimen loadsheet for 737-800
according to our ops part b......


Therefore the following data can be used (crudely) to plt fwd limit, with index units scale across top of chart and gw
down the side....


gw 60t = 6 units
70t = 10
75t = 18
79t = 31

The forward limit is a steep gradient until near MTOW where it flattens off, hence the jump from 18 to 31 units.

and aft limit....

60t = 80 units
65t = 84 units
70t = 94 units
75t = 88 units
79t = 62 units

and again with the aft limit at MTOW 79t the aft trim range is considerably reduced, and at mtow 79T the mac range on the trim graph runs from 16% to 25%, but between 79t and 72t the mac runs from 25% to 34% mac.

I tried to incorporate said chart onto this post but my computer skills are somewhat remedial....!

rgds FH

[Motormouse]

hmmmm,

As I understand it ,the problem with an aft c of g is more 'critical'
during landing phase, when as flaps are deployed,the centre of pressure (center of lift,aerodynamic neutral point etc) can move forwards of the c of g.

This will cause a rapid pitch up at low airspeed,which can not
always be countered by available elevator movement.

[Frosty Hoar]

MMouse,

Interesting point, and I can admit to being partly responsible for giving a loadsheet to a crew that indicated an aft cg position when the ac was loaded from the front; and had in reality a cg near the fwd limit.

The crew consequently trimmed the ac nose down (with a fwd cg) and only just hauled it off the deck according to the F/O.

[john_tullamarine]

FH,

Thanks for that .. and what would be a typical CG/IU co-ordinate for the loading when you would consider rearranging the load to move the cg aftwards ?

I don't have any 800 background .. but on the earlier models .. it gets interesting if the cg is aft and the trim set for forward ... the aircraft wants to leap off the ground at an alarmingly low speed .. gets the chaps' (or ladies' or chap's and lady's) attention very quickly ..

[Frosty Hoar]

At close to MTOW of 79t the approximate position would be MAC 25 with 64 index units,interestingly at 78t the aft limit extends to MAC 32 with 82 index units, hope this makes sense.

As an aside to all this we did suffer a 738 tailstrike this week, no damage, and nothing to do with the cg but nasty anyway...

[john_tullamarine]

FH,

Just two quick questions of clarification ..

(a) in the earlier post re envelope intersections, could I get you to recheck the weight and MAC information, please ?

(b) in your last post, the 79T/25MAC is before or after you have changed the loading ? .. if, as I suspect, after, what was the original loading IU ?

[Frosty Hoar]

OK;

(a) Data is correct but bear in mind that I am taking it from the manual loadsheet specimen and it is not the best basis for manually plotting the c.g envelope.

(b) The target MAC for MTOW 79t will normally be around about mac 18%/38 index units,( I guess it varies according to loading on the day) my original query arose for this post because at certain airports the take off perf. computer would limit me to lets say 78t
due to runway length, assuming MAC of around 18%,I would then input MAC25% (theoretically move c.g. aft) and recalculate the data, and would normally find an extra 500 to 1000kgs.

RGDS FH

[john_tullamarine]

I think I would need to look through the AFM and the WB manuals to form a definite opinion. The trimsheet data should be fine as the only envelope changes will be truncations etc. in the envelope to accommodate the error analysis.

My best guess at this stage is that the answer to the first question would end up being just a matter of following the forward limit up to the higher weight .. by moving aft, the limit permits a higher weight ....

regards....