Stability: Static and Dynamic
Definitions
Static stability is the initial tendency to return to equilibrium after a disturbance; dynamic stability describes the subsequent motion over time. An aircraft may be statically stable but dynamically unstable, producing an increasing oscillation. Candidates must be able to classify positive, neutral and negative static stability and relate them to the modes of dynamic motion: short-period pitch oscillation, phugoid, Dutch roll, spiral divergence and short-period yaw.
Longitudinal, lateral and directional
Longitudinal stability concerns pitch, lateral stability concerns roll, directional stability concerns yaw. Longitudinal static stability requires a negative gradient of pitching moment with angle of attack, normally achieved by placing the centre of gravity ahead of the aerodynamic centre and by the tailplane. Directional stability comes mainly from the vertical fin and aft fuselage side area. Lateral stability comes from dihedral, sweep, keel area and the fin's contribution above the centre of gravity.
Traps and a worked example
Traps: confusing stability with control (a stable aircraft may be hard to manoeuvre); assuming more dihedral is always better (it causes Dutch roll and sideslip coupling); and forgetting that aft centre of gravity reduces longitudinal stability while forward centre of gravity increases it but raises pitch control forces and reduces range.
Rule of thumb: moving the centre of gravity aft reduces static margin and therefore reduces longitudinal stability.
Worked example: if the aerodynamic centre is at 25% mean chord and the centre of gravity at 15%, static margin is 10% chord. Moving the centre of gravity to 22% leaves only 3% margin, so pitch stability is markedly reduced and elevator authority in the flare becomes critical.