## Basic principle.

Basic principle
The centre of gravity in an aircraft can be seen, if you use the “balance” calculation as the point where the “nose heavy” (-) and “tail heavy” (+) forces are equal. If the aircraft was to be hung up then the aircraft would be balanced.
Or, in other words, when the upward force “lift” is applied at this point then the aircraft will not have the compulsion to fly nose-up of nose-down.

Aircraft hung up in centre of gravity.

The centre of gravity of an aircraft is dependent upon the weight and the situation of everything onboard. Any extra baggage added relatively near to the rear of the centre of gravity will automatically move the centre of gravity further to the rear and vice versa.
If an aircraft were to have no trim then the aircraft would have to be so loaded that the centre of gravity would fall at the same point as the lift. If not, it would be constantly necessary for the pilot or auto-pilot to fly the aircraft at the correct altitude.
This movement has to take place within a certain “range”. This range is called the centre of gravity range.
This range has both a forward and backward limit.
The centre of gravity range give the forward and backward centre of gravity where the aircraft fulfils the necessary performance and inspection requirements on the basis of how it has been certified

These limits are accurately laid down during the design and test-phase of the aircraft and are recorded in the Type Specification, certification documentation and the Flight Manual (limitations). The CG range is normally located around a point a 1/3 of a wing chord behind the wing nose section.
At all times during the flight of the aircraft should the centre of gravity be found to be within these limits.
In some aircraft types different limits will be used during different phases e.g. start, cruising and landing.
These limits are expressed in % MAC (Mean Aerodynamic Chord). The MAC is the mean average chord of the wing which is also used for aerodynamic calculations during the aircraft design phase.