Stability and control
Overview
Stability and control tests static and dynamic stability, the three axes, CG effects and the control surfaces. Questions often ask about static margin, dihedral and Dutch roll.
Core rules
- Static stability is the initial tendency to return to equilibrium: positive, neutral or negative.
- Dynamic stability is the long-term motion: phugoid, short-period pitch, Dutch roll and spiral divergence.
- Longitudinal stability requires the CG ahead of the neutral point; static margin = (NP − CG)/MAC. A more forward CG means more stability but heavier stick forces.
- Lateral stability comes from dihedral, sweep and high wing; directional stability from the fin (weathercock effect).
- Controls: elevator pitch, ailerons roll, rudder yaw. Adverse yaw is countered with rudder or differential ailerons.
- Trim balances control forces; it does not change stability.
Worked example
Neutral point at 25 % MAC, CG at 18 % MAC. Static margin = (25 − 18)/100 = 7 % MAC, positive and comfortably stable. Moving the CG to 27 % MAC gives −2 %, i.e. instability.
Traps
- Positive static stability does not guarantee positive dynamic stability.
- Too much dihedral can worsen Dutch roll; too little can cause spiral divergence.
- An aft CG improves cruise economy but reduces stall and spin recovery margins.