The aircraft and everything on board has a total weight that can be said to act through one point – the centre of gravity. It is the balance point of the aircraft and if you could place this point on a pivot, the aircraft would balance perfectly.
Any items added behind this point will move the centre of gravity backwards, and any items added forward of it will move the centre of gravity forwards.
The lift and weight forces are almost never acting along the same line – typically the centre of gravity is forward of the centre of pressure.
This means the lift force is trying to pitch the aircraft nose down – a pitching moment is created about the lateral axis. If no other forces existed, the aircraft would pitch down sharply and descend.
The pitching moment is counteracted by the tail, which provides a downwards force to keep the aircraft in level flight. We will look at this in further detail in a later topic.
Aircraft weight and centre of gravity position have significant effects on how the aircraft behaves. When aircraft weight is increased (more fuel, another passenger etc.):
We will look at each of these effects of weight in more detail in later lessons.