Performance based on which CofG?

[Flap Sup]

Hi,

I have this very silly question: Where can i find documentation on which C of G performance is based on?

My assumption is that it is based on worst case scenario, max fwd C of G. T/O and LDG are affected by longer rwy requirements, cruise by higher fuel flow, so it ought to be worst case scenario, right?

I have tried googling and search func here on pprune, but i am probably searching for the wrong keywords. I assume that the answers can be found in FAR/CS23 and CS/FAR25, but cant find it :(

A different question: On an average aircraft, how much does C of G affect the fuel flow and max weights? AFAIR, the 737-500 had a higher Perf based max landing mass when using aft C of G with the short field kit (never flew 737, that was back in my red cap days).

[Skyjob]

There is no such thing as an average aircraft...

CG limits can be found in performance manuals.
Little common knowledge about CG vs fuel flow etc available though.
Usually only specialised manufacturer data and on request

[Denti]

Both on our 737s as well as on our A320 family we have a CG option on our EFB performance programs. So i guess by making sure that the CG lies within a certain range you can take advantage of the better performance in that range.

[Skyjob]

Denti, true.
As the range of CG is aircraft dependent, to find out what the range is and what performance or fuel penalty there is when not in such range...

Manufacturers hold this data close to their chest, but it would be VERY nice to know what the best CG would be for each airframe so to try and achieve this when loading.

[Flap Sup]

Skyjob,

...nice to know what the best CG would be for each airframe so to try and achieve this when loading.
That one is easy - it is the very aft limit at any given aircraft at any given mass. Some large aircraft even have trimtanks aft to ensure exactly that.

[Skyjob]

Flap Sup: not the case, explanation as provided on page 35 in this Boeing presentation Fuel Conservation 2004 (http://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=5&ved=0CD0QFjAE&url=http%3A%2F%2Fwww.smartcockpit.com%2Fdownload.php%3Fpath% 3Ddocs%2F%26file%3DFuel_Conservation.pdf&ei=UwfyVK3vLNKM7AbNnYDwDw&usg=AFQjCNH1iZNVRkx_b7dK6VFpsa4evvR3cg&sig2=bEaC_ZN633DSGux5pBmPRw&bvm=bv.87269000,d.ZGU)

[violator]

That chart clearly shows the lowest trim drag occurs with the most aft CG (of course).

[john_tullamarine]

We need to keep in mind that there exist a variety of certification and operational requirements.

In respect of CG, you would need to do some research to establish what is critical for which consideration on which aircraft.

For instance

(a) stall speed usually is predicated on max forward CG and may dictate the shape of the forward limit in the higher weight regions.

(b) min trim drag generally occurs for aft CG so that is the region desired for cruise. I vaguely recall that even the DC3 OEM paperwork gave some guidance along these lines .. too far in the past now for the memory to home in accurately.

[renard]

On the 100 seat aircraft I fly, we always try for an "aft CofG" and it can make a big difference on our max take off weight.

On our most limiting runway on an ISA day we can carry about 6 more pax with alt CofG compared with a more forward one.

[john_tullamarine]

Presumably, the AFM prescribes dual envelopes and the more aft forward limit provides a lower Vs and takeoff speeds ?

[galaxy flyer]

I believe there are also cases where the limit the envelope forward to reduce the Vmcg as a consideration.

Were you at Avalon, John?

[john_tullamarine]

Thursday/Friday for a seminar and yesterday to pick up some kit.

Didn't think to check with you to see if you were over this year ... I shall give myself a kick in the tail if you were.

[galaxy flyer]

No, just one of our planes

[john_tullamarine]

Whew .. don't have to stand at my desk for a week, then. Perhaps next show ?

[Linktrained]

Boeing Airliner of January 1974 (!) under "Fuel Conservation at a Glance" states :-
"Aft c. g. shift ( per 4% max shift) 0.5 % for 707, 727 and 737
0.2 % for 747 "


I never had any information on the only DC3 ( one with a Starboard door that I flew as "F/O on freight" in 1951, and so beyond my pay grade.)


With a normally full load of charter passengers, the baggage and flight spares would be distributed " 2/3 forward, 1/3 aft " and on several later types of larger aircraft.
Perhaps the Chief in Charge of these matters had read your reason, in #8 in (a) to keep the load in the "Tested for C of A." position, rather than (b) "for minimal cruising drag".


Load sheets were graphical and could have had an extra line printed, between the limits, to indicate the " Optimum Loading"

[john_tullamarine]

information on the only DC3

In the very dim past, I ran a number of pilot engineering schools for DC3 endorsements. One of the OEM manuals had some words regarding a trim target in the aft region of the envelope for fuel considerations .. would have the manual in the archives somewhere but a major problem to find it ..

keep the load in the "Tested for C of A." position, rather than (b) "for minimal cruising drag".

Not a case of keeping to the forward limit .. that only is relevant for the certification determinations .. most folks would aim for the mid to aft region of the approved range.

Load sheets were graphical and could have had an extra line printed, between the limits, to indicate the " Optimum Loading"

Absolutely, as such a line is just a CG location. As I recall I did just that for one DC3 operator years ago .. again, based on the OEM guidance material. Very flexible and useful loading systems are trimsheets.