Anyone know why forward CG improves longitudinal stability? Or why aft CG worsens it?
I can visualize that an arrow shot from a bow would be unstable if the CG were at the tail end (if that's an appropriate analogy), I just can't figure out why that would be...
To take us back to the beginning. The arrow is a simple and effective way to explain the problem. How the four forces are applied in a modern aircraft has been explained but force x distance hasn't;
A Newton = 1 Kg over 1 metre.
So a body in motion can only be maintained resisting deviation (wobble) by an external body that is equal but that isn't always enough: Unstable air adds complex additional forces. It was then discovered, from no doubt the observation of nature, air resistance could be used as an additional and variable force to neutralise the erratic wobbling forces. Three feathers provided the required force effect in pitch, roll and yaw.
The aeroplane trimmed stability has to be varied to change vectors using pitch, roll and yaw. This is achieved by adding a flap to the three planes (feathers) in order to vary the force of each by the pilot at will. If the CG is in the middle then to raise the nose weighing 1 kilo 1 metre then the force from the horizontal tailplane downward must also be effective to an additional 1 kilo for 1 metre. Move the CG forward to 25% then to raise the nose 1 metre the tail plane must be lowered 2 metres. If the tailplane force available cannot do that then it will be impossible to raise the nose sufficiently. This will also be the case if restricted by the runway on landing and the nose lands first. So by moving the CG aft the balance will be eventually be found and this will be the most forward limit but on the edge. Keep moving the CG further aft and reach a safe forward limit published by the manufacturer. Moving further aft again to the point the aircraft becomes unstable and now an advanced level of skill is required not to overcontrol. Then bring the CG forwards again to a point where only the average pilot skill is required; this is the published aft CG.