Stability and Control
Static and dynamic stability
Understand the three axes: longitudinal (pitch), lateral (roll) and directional (yaw). Static stability describes the initial tendency to return to equilibrium; dynamic stability describes the subsequent motion over time. Know that positive longitudinal static stability requires the centre of gravity to be forward of the neutral point, and that moving the CG aft reduces stability. Understand the roles of the horizontal stabiliser, vertical fin and dihedral, and the effect of the CG on control forces and trim.
Control effectiveness and adverse effects
Know how control surfaces produce moments, and the effects of speed on control effectiveness. Recognise adverse yaw from ailerons, and the need for rudder coordination. Understand the difference between static and dynamic stability and the three dynamic modes: phugoid, short-period pitch oscillation and Dutch roll.
Traps
Common traps include confusing lateral and directional stability, assuming dihedral provides directional stability, and forgetting that an aft CG reduces both static margin and stall recovery capability. Another trap is treating static stability as sufficient for dynamic stability.
Worked example
If the CG is at 25% MAC and the neutral point at 40% MAC, static margin = 15% MAC. Rule of thumb: a positive static margin means pitch stiffness is positive; a larger margin increases stability but reduces manoeuvrability.