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