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 conentrated at the cg will pull the nose back down. It the nose should drop in flight, the airspeed will increase and the increased doward 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 unstble, and difficult to recover from a stall. It the unstable airplane should ever enter a spin. The spin conld 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.