Rotational Motion

Stability and Control

An aircraft is stable if it returns to its initial equilibrium flight conditions when it is perturbed. There are two main types of aircraft instability:

Stability and Control

An aircraft with static instability uniformly departs froman equilibrium condition

An aircraft with dynamic instability oscillates about the equilibrium condition with increasing amplitude.

There are two modes of aircraft control: one moves the aircraft between equilibrium states, the other takes the aircraft into a non-equilibrium (accelerating) state. Control is directly opposed to stability.

Time = 0.0

Time = 1.0

Time = 2.0

Aircraft encounters gust Nose pitches up

cg

Tail Force

Airflow Direction

Longitudinal Static Stability

Tail Force

Tail Force

Resulting Motion

Lateral Static Stability

Resulting Motion

Weight

Dihedral Angle

Lift Vectors

Weight

Aircraft rolls slightly to the right

View Downstream

Time = 0.0

Time = 1.0

Time = 2.0 Dihedral Angle

Lift Vectors

Weight