Balance is a calculation of the distribution of the weights in the aircraft. Conventional light aircraft have the effective mass of the aircraft, called the centre of gravity, located ahead of the effective lift point called the centre of pressure. This allows gravity to pull the aircraft ahead and keep the wings below the stalling angle relative to the air flowing over them. Should something disturb this configuration the tendency will be for the aircraft to return to an unstalled condition. While having the aircraft balance forward is generally good, the ability to change the pitch of the aircraft works on the opposite side of the balance point. If the weight is distributed too far forward, the aircraft may have a tendency to pitch down excessively and be difficult to land in a low energy configuration. Balanced too far aft you have increased tendency to stall and difficulty in recovering from the stall. This is the reason why the balance of the weight is as important as the load factors.
The Centre of Gravity is the point from which an airplane would balance if suspended. The position of this point is determined by the weight and balance computations. Its distance from a specific reference datum is found by dividing the total moment by the total weight. The Centre of Gravity limits (C of G) are the extremes of the location of the centre of gravity within which an airplane must be operated at a given gross weight. We describe this balance by multiplying weights by distances (called ‘arms’) from a known reference point on the aircraft (Reference Datum). Any point along the longitudinal axis of the aircraft as measured from the Reference Datum is called a Station. A moment is the product of the weight of an item multiplied by its arm. When you divide the total all the moments of various weights of pilots and the aircraft, by the total weight, you are left with a distance from the reference point. This distance marks the loaded centre of gravity of the aircraft. The aircraft designer has calculated and tested that if this distance is within a certain range that the aircraft will exhibit normal control characteristics.
An example of this calculation for a 2-33 with the Reference Datum, Station 0 at nose and a all-up weight of 1040 lbs. If the weight properly distributed it will have a loaded centre of gravity between station 78.20" forward and station 86.10" rear(aft) limit. Given:
- Front pilot weight of 170 lbs at sta. 43.80"
- Rear pilot weight of 150 lbs at sta. 74.70"
The calculation would be as follows:
Weight X ARM = MOMENT
Sailplane Empty 612 96.12 58,825
Front pilot 170 43.80 7,446
Rear pilot 150 74.70 11,205
Total Weight 932
Total Moment 77,476
Total Moment/Total Weight = 77,476/932 = 83.13
The total weight of 932 pounds is less than the maximum all-up weight of 1040
pounds and the loaded centre of gravity is 83.13 inches which is within the
design limits. The aircraft is properly loaded.