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Flight Stability & Dynamics
Static vs dynamic stability and the longitudinal, directional and lateral axes, including Dutch roll and spiral modes.
Flight stability & dynamics — summary notes
FreeMain ideas
- STATIC stability is the aircraft's INITIAL tendency after a disturbance: positive = it starts to return to trim, neutral = it stays put, negative = it diverges. DYNAMIC stability describes the motion OVER TIME — a positively dynamically stable aircraft's oscillations die away (damped).
- An aircraft can be statically stable yet dynamically unstable — it starts back toward trim but overshoots into oscillations that grow rather than damp (e.g. an undamped Dutch roll).
- LONGITUDINAL (pitch, about the lateral axis) stability comes from the tailplane and the CG position relative to the neutral point: a forward CG is more stable but gives heavier stick forces; an aft CG reduces stability. Its dynamic modes are the slow phugoid and the fast short-period.
- DIRECTIONAL (yaw, about the normal axis) stability is 'weathercock' stability provided by the fin/vertical stabiliser, which swings the nose back into the relative airflow after a yaw.
- LATERAL (roll, about the longitudinal axis) stability comes from wing DIHEDRAL, sweepback, keel effect and a high wing: after a roll the resulting sideslip generates more lift on the lower wing to roll it back level. Lateral and directional motion couple into Dutch roll (damped by a yaw damper) and spiral modes.
- ⚠ Exam trap: don't confuse the axes — DIHEDRAL gives lateral (roll) stability, the FIN gives directional (yaw) stability, the TAILPLANE gives longitudinal (pitch) stability. And 'static' = the initial response, 'dynamic' = the response over time.
Solved examples
A gust rolls an aircraft slightly right wing down. Explain how wing dihedral restores it to level.
The roll makes the aircraft sideslip toward the lower (right) wing. Because of the dihedral angle, the lower wing meets the sideslipping airflow at a greater effective angle of attack than the upper wing, so it develops more lift. That extra lift on the lower wing rolls the aircraft back toward wings-level — positive lateral static stability.
Stability concept map
FreeFlight Stability & Dynamics
Flight stability & dynamics quiz
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