C of G and pitch stability

[ant1]

Imagine a model airplane, in a wind tunel (moderate wind), sitting on a fulcrum (The fulcrum in flight would be at the intersection between the longitudinal axis and the wing's center of lift)

The aircraft is sitting horizontally in equilibrium.

Now, hit the aircraft at the nose or the tail. It will start oscillating around its horizontal equilibrium position due to the weathervaning effect of the elevator surfaces.

Repeat the experiment but now stick some weight on the aicraft nose: you have moved the CG forward.

Now, to maintain horizontal equilibrium, you have to set the elevator surfaces in such a position that they exert a higher downward force.

If you now hit the airplane nose or tail with the same force you used before, the aircraft won't move as far from its equilibrium as it did previously and will return faster to it through a lower amplitude and more dampened oscillation because both moments on each side of the fulcrum are higher. That is why it is more stable.

If you prefer, imagine that in the two previous experiments, instead of weight and elevator downward force, this time, the fuselage is linked to two springs (one forward and one aft of the fulcrum) each one exerting a downward force.

In the second experiment the springs are more tense and exerting a stronger pull, hence a stronger moment, around the fulcrum (that's the case witth a forward CG).

Cheers