Stability and Control: Static and Dynamic Stability, Longitudinal, Lateral and Directional
This domain tests the aeroplane's response to a disturbance and to a control input: whether the initial tendency is restoring, and whether the resulting motion dies away. Items ask you to classify stability, place the centre of gravity relative to the neutral point, and identify the dynamic modes.
Static and dynamic stability
- Static stability is the initial tendency after a disturbance: positive, neutral or negative.
- Dynamic stability describes the whole motion over time. An aircraft can be statically positive but dynamically unstable, as in the phugoid or spiral divergence, so static stability is necessary but not sufficient.
Longitudinal stability
- The CG must be forward of the neutral point for positive static stability; the distance between them is the static margin.
- Moving the CG aft reduces the margin and the stick force per g, and eventually gives neutral or divergent pitch behaviour.
- The stick-fixed neutral point lies forward of the stick-free neutral point, because a free elevator floats and reduces stability.
- Phugoid: a long-period exchange of height and speed at nearly constant alpha, lightly damped. Short-period: a rapid alpha oscillation, usually heavily damped.
Lateral and directional stability
- Dihedral effect: sideslip produces a rolling moment that rolls the aircraft away from the sideslip.
- Weathercock stability: the fin produces a yawing moment that aligns the aircraft with the relative wind.
- Dutch roll is a coupled yaw/roll oscillation, often underdamped, which is why transports carry a yaw damper. Spiral divergence is a slow, weakly damped increase in bank and is a dynamic instability.
Worked example
CG at 25% MAC and neutral point at 40% MAC gives a static margin of 15% MAC. Moving the CG aft by 5% MAC leaves 10% MAC, so stick force per g falls by about one third and the aircraft sits closer to neutral. The aeroplane is still stable, but it is lighter in pitch and more sensitive to elevator input.
Traps
- Positive static stability does not imply positive dynamic stability.
- Dutch roll and spiral divergence are different modes with different fixes; do not mix them.
- The CG must stay inside the certified forward and aft limits, not merely ahead of the neutral point.
- An aft CG can increase range and reduce trim drag but degrades stability and stall recovery.