- Example of the A330-200 -
The important points to notice are:
. the position and dimension of the MAC
. the position of the point located at 25% of the MAC‘s length. This point is the
reference when it comes to measuring moments.
Conversion from %MAC to H-arm
. H.arm . H.arm .
CG Leading Edge of MAC
%MAC = .. ..×100
. Length of MAC .
H.armCG . 31.3380
In the above case, the formula would be: %MAC = .
0.072700 Length of MAC × %MACH.arm = H.arm +
CG Leading Edge of MAC
100
7.27 × MAC
In the above case, the formula would be: H.armCG = 31.3380 + .
100 In order to facilitate the operator‘s work, conversion tables from H-arm to %RC and conversion tables from %RC to H-arm are available in the Weight and Balance Manual in chapter 1.00.06.
WEIGHT AND
BALANCE

A. GENERALITIES

2. FORCES APPLIED ON FLYING AIRCRAFT
a) Diagram of forces
Let us first consider main forces applied on the aircraft. . The weight, applied on the aircraft CG. . The lift, applied on the center of pressure (CP). . The thrust, due to the engines power. . The drag.

In order to keep the airplane level, a downward force is created by the Trimmable Horizontal Stabilizer (THS) which has to be trimmed accordingly. This additional force creates both lift degradation and important pitch-up moment that counters the pitch-down moment. The pitch up moment is due to important balance arm between the aircraft CG position and the THS where the additional force is applied.

WEIGHT AND
BALANCE
Note : for aircraft stability reasons the CP is always located behind the CG.

A. GENERALITIES

3. INFLUENCE OF THE CG POSITION ON PERFORMANCE
The impact of CG position on performance varies depending on the flight phase. All influences are linked to the impact of the CG position on the stall speed.
3.1. Impact on the stall speed
The stall speed (Vs) is speed at which the aircraft will stall. At that speed the upward force (mainly lift) is equal to the downward force (mainly weight + THS counter force) applied on the aircraft.
The lift is directly related to the aircraft speed: the faster the aircraft flies the greater the lift.
CG forward CG aft
.
High pitch down moment .Small pitch down moment

.
Need for high THS counter moment .Need for small THS counter moment

.
lift = weight + THS counter force .lift = weight + THS counter force

.
Stall speed is high .Stall speed is smaller than for forward CG

The more forward the CG position, the greater the stall speed (Vs) value.
WEIGHT AND
BALANCE
WEIGHT AND
BALANCE
A. GENERALITIES

. Example : A340 (High Gross Weight A340-313 with CFM65-5C4 engines) at take off CONF 1+F, Gear up
The following graph gives the stall speed value for different aircraft weights, and for two different CG positions (Most forward CG and CG at 26%MAC).
　
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