Balance control refers to the location of the 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 ch must be located within specific limits for safe flight. 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.
An airplane is designed to have stability that allows it to be trimmed so it will maintain straight and level flight with hand off the controls. Longitudinal stability is maintained by ensuring the cg is slightly ahead of the center of lift this produces a fixed nose-down force independent of the airspeed. This is balanced by a variable nose-up force, which is produces by a downward aerodynamic force on the horizontal tail surfaces that varies directly with the airspeed.
If a rising air current should cause the nose to pitch up, the airplane will slow down and the downward force on the tail will decrease. The weight concentrated at the cg will pull the nose back down. It the nose should drop in flight, the airspeed will increase and the increased downward tail load will bring the nose back up to level flight.
As long as the cg is maintained within the allowable limits for its weight, the airplane will have adequate longitudinal stability and control. It the cg is too far aft, it will be too near the center of lift and the airplane will be unstable, and difficult to recover from a stall. It the unstable airplane should ever enter a spin. The spin could become flat and recovery would be difficult or impossible.
It the cg is too far forward, the downward tail load will have to be increased to maintain level flight.
This increased tail load has the same effect as carrying additional weight the aircraft will have to fly at a hither angle of attack, and drag will increase.