BALANCE THEORY

Balance control refers to the location of the center of gravity (CG) of an aircraft. This is of primary importance to aircraft stability, which determines safety in flight.

The CG is the point at which the total weight of the aircraft is assumed to be concentrated, and the CG must be located within specific limits for safe flight. This is the main responsibility of load control agent.

Both lateral and longitudinal balance are important, but the prime concern is longitudinal balance; that is, the location of the CG along the longitudinal or lengthwise axis.

General principle:

To illustrate the Theory of Balance we will look at the following see­ saws and how they are affected by placing weights on them.

MOMENT = WEIGHT X ARM

Example 1:

Left MOMENT tends to make the see-saw move anticlockwise, and via versa.
As a result:

Left MOMENT = right MOMENT : see - saw leveling
Left MOMENT > right MOMENT : see-saw moving anticlockwise
Left MOMENT < right MOMENT : see-saw moving clockwise

Example 2:

Basing on the Theory of Balance, the manufacturer designs the air plane to have wings with the ability of balancing.

If loads are forward of the balance datum line their moment arms are usually considered negative (-). Loads behind the balance datum line are considered positive (+)

Example 3:

Left MOMENT (Forward hold of the aircraft) : - 3000 kgm
Right MOMENT (After hold of the aircraft)    : + 3000 kgm

The aircraft is optimally trimmed when loads are sufficiently distributed in forward and aft hold.  

Example 4:

For safe aircraft operation, there must be calculated limits to the forward (nose heavy) and the aft (tail heavy) cg movement measured from a datum point and specified by the manufacturer (Safe loading limits).

Within the Safe loading limits, nose heavy or tail heavy is corrected by using the Horizontal Stabilizer located at aircraft tail.

The distance from the Horizontal Stabilizer to the aircraft CG determines the range of correction, as shown below.

Example 5: