I have found the document I was talking about few pages ago about the CG.
Airbus A330 Instructor Support - Normal Operations (DEC 2000)
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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.
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 3,165 m to 3,241 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%.
In cruise, an AFT CG minimizes the THS induced drag, thus improves fuel consumption. For example, the fuel increase on a 1,000 nm cruise segment is as follows, considering a heavy aircraft in high altitude and CG 20% or 35%:
- Handling Characteristics considerations
On Fly By Wire aircraft, in Direct Law, the handling characteristics of the aircraft are affected by the location of the CG as a mechanically controlled aircraft:
Stability Issue
- Aerodynamic Centre or Neutral Point
The aircraft is considered as stable, if in case of a perturbation or gust, the aircraft tends to react back towards its previous status. The aerodynamic centre, also called neutral point, is the location where an increase (or decrease) of lift is applied when the aircraft angle of attack varies.
Maneuvering criteria – Maneuver point.
Depending upon the CG location, a given deflection of the elevator causes a more or less sharp aircraft maneuver. In other words, the CG has a direct influence on the maneuverability of the aircraft. If a very small deflection of the elevator causes “a lot of g”, the efficiency of the elevator is very high; the aircraft is considered as very touchy to maneuver.
The maneuver point is the location of the CG for which the efficiency of the elevator is infinite. The CG must obviously be forward of the maneuver point by a lot. This lot is defined by a maneuverability criteria which states that “at least 1° of elevator deflection is required to pull 1g load factor”. This condition defines the AFT CG limit maneuverwise.
But the CG must not be too far forward: indeed, the maximum elevator deflection must allow to pull at least the maximum acceptable load factor (e.g. 2.5 g). This condition defines a FWD CG limit maneuverwise.
Ground handling characteristics
Essentially at high GW (thus at Take-Off), the CG is limited AFT so as to ensure enough Nose Gear adherence to allow an efficient aircraft steering on the ground.
Take-Off rotation characteristics
The CG must be limited so as to allow:
- enough maneuverability during rotation -> FWD CG limit.
- enough margin versus potential tailstrike -> AFT CG limit.
Obviously the the THS is preset nose up in case of forward CG or nose down in case of AFT CG, in order to get homogeneous aircraft rotation. But certification maneuvers require to demonstrate “abuse cases” such as taking-off with FWD CG limit while THS is set nose down.
THS stall potential
- in approach with flaps extended, there is a nose down moment counteracted by a THS nose up setting. The more CG is forward, the more THS nose up setting is required. This may lead to a THS stall, more particularly in cases of push over where the pilot pushes hard on the stick when he notices a significant speed decrease. This limits the FWD CG in approach.
- in Go-Around or Alpha Floor, the thrust increase to TOGA, more particularly at low speeds, induces a significant pitch up moment which increases when CG is more AFT. The elevator efficiency must allow to counteract this pitch up moment, even at very low speed. This limits the AFT CG in Go-Around and Alpha Floor.
Structural Considerations
The CG cannot be too much forward due to Nose Gear structural limits; it cannot be too much AFT due to wing and main landing gear strut limit.
Loading Considerations
All the previous criterias allow to determine limits which, for example, would favor AFT CG configurations for obvious performance efficiency. However, the CG envelope must also take into account loading flexibility constraints.
Passenger movement
The CG envelope must also allow passenger to move in the cabin. This is the reason why once the Take-Off CG envelope has been determined, as well as the landing one (which is less constraining), then the inflight envelope is defined usually providing at least a 2% margin with the previous envelopes.
1) Performance / loading compromise at Take-Off and Landing
2) Nose gear strength structural limit
3) Main gear strength structural limit
4) Alpha Floor limit
5) Nose gear adherence limit
6) Alpha Floor limit (landing)
The inflight limit is deduced from the Take-Off / Landing envelope by adding a typical 2% margin, provided all handling characteristics criteria are fullfilled.
