Centre of gravity movement

In many aircraft types quite large changes in centre of gravity position can occur during the flight, and it is important that the stability should not be adversely affected. We would not wish an airliner to become unstable every time there was a rush for the toilet. Note, that on most airliners, the small changes in trim caused by such activity can be sensed, and corrected automat­ically by auto-trimmers.

As the centre of gravity moves forward, the aircraft becomes more stable, as explained earlier. The maximum forward movement of centre of gravity is limited by the amount of trimming and control moment that can be produced by the tail. This moment depends on the tailplane lift force, and on the prod­uct of the tailplane area and its moment arm; the distance from the centre of gravity to the centre of tail lift. Because this product has the units of a volume, it is referred to as the tail volume.

Although a forward centre of gravity position makes the aircraft very stable, the tailplane and main-plane are pulling in opposite directions creating un­wanted extra trailing vortex drag; trim drag. The aircraft also becomes difficult to manoeuvre. Large elevator control movements have to be made, the con­trol forces are high, and the response is sluggish. Eventually the point is reached where the nose can hardly be raised at all, and the aircraft becomes unflyable.

The rearward movement of the centre of gravity is limited by the fact that the aircraft eventually becomes unstable as in Fig. 11.6. Here the elevator has had to be pushed down in order to trim, and the effective longitudinal dihedral has disappeared.

If the centre of gravity is moved rearwards, a condition is reached where the aircraft is just on the point of becoming unstable. Here it is said to be neu­trally stable, and the centre of gravity location at which this occurs is known as the neutral point. Note that the trim drag reduces as the centre of gravity moves aft.