Static and Dynamic Stability
Stability describes the behaviour of an aircraft after a disturbance (e.g. from turbulence or a control input). Two concepts are distinguished, both of which must hold for good flying behaviour.
Static stability
Definition: the initial tendency of an aircraft to return to its original state after a disturbance.
Three classes
| Class | Initial reaction |
|---|---|
| Positive static stability | aircraft tries to return to original state |
| Neutral static stability | aircraft stays in the new state |
| Negative static stability (unstable) | aircraft amplifies the deviation |
Dynamic stability
Definition: the time history of the response.
Three classes
| Class | Time behaviour |
|---|---|
| Positive dynamic stability (damped) | oscillations decay, aircraft returns |
| Neutral dynamic (undamped) | constant amplitude — oscillation runs forever |
| Negative dynamic (divergent) | amplitude grows → unstable |
Combinations
Statically stable + dynamically stable = ideal: aircraft swings damped back to the original state.
Statically stable + dynamically neutral = aircraft returns but oscillation runs constant (rare in practice).
Statically stable + dynamically unstable (divergent) = aircraft tries to return, but oscillation grows → pilot must intervene (e.g. in spiral-unstable aircraft).
Statically unstable = aircraft does not stay trimmed; must be actively controlled by pilot or fly-by-wire (fighters like F-16 are designed statically unstable for manoeuvrability).
Example: pendulum analogy
- Pendulum at rest: statically stable (swings back after deflection), dynamically stable (oscillation damped by friction).
- Inverted pendulum: statically unstable (tips over at the slightest push).
- Frictionless pendulum: dynamically neutral (swings forever).
Pilotage consequences
Stable aircraft (typical GA):
- Easy to fly — almost flies straight by itself.
- Hold trim setting — no constant correction needed.
- Trainers are all stable.
Less stable:
- More attention required.
- Aerobatic aircraft often only neutrally stable — more agile, more demanding.
Unstable:
- Fighters (F-16, Eurofighter): designed aerodynamically unstable → fly-by-wire controlled.
- Pilot would not cope without computer.
Three axes, six stabilities
An aircraft has 3 axes; each has static AND dynamic stability:
| Axis | Motion | Stability name |
|---|---|---|
| Longitudinal axis (roll) | roll | Lateral stability |
| Lateral axis (pitch) | pitch | Longitudinal stability |
| Vertical axis (yaw) | yaw | Directional stability |
The following lessons treat each axis separately.
Oscillation modes of typical aircraft
Longitudinal (pitch):
- Phugoid: long pitch-altitude oscillation (period 30-90 s). Usually damped.
- Short-period pitch: short, fast pitch oscillation (period 2-5 s). Strongly damped.
Lateral-directional coupled:
- Dutch roll: yaw + roll out of phase. Period 2-10 s.
- Spiral: yaw + roll in phase. Period 30+ s.
- Roll subsidence: pure roll damping. Short period.
See lesson "Coupled instabilities" for details.
Stability vs controllability
Trade-off:
- High stability → harder to manoeuvre (high yoke effort, slow reaction).
- Low stability → easy to manoeuvre but demanding in straight flight.
PPL trainers sit on the stable side of the spectrum.