% Static Stability %
When an aircraft undergoes some turbulence (or some form of static imbalance) when in equilibrium flight, the nose tilts slightly up or down (an increase or decrease in the angle of attack), or there will be a slight change in flight attitude. There are additional forces acting on the aircraft, and it is no longer in the equilibrium condition. If the aircraft continues to increase the orientation after disturbance, the aircraft is said to be statically unstable. If there are no further changes in flight attitude and if the aircraft retains the position, which means there are no net forces or moments acting on the aircraft in the new orientation too, then the aircraft is said to be statically neutral. If forces are generated on the aircraft in a way such that forces causing the disturbance are countered, and the aircraft attains its original position, then the aircraft is said to be statically stable.
In aircrafts, three types of dimensional stabilities are considered. Those are the longitudinal stability that concerns the pitching motion, the directional stability that concerns the yawing motion, and the lateral stability that concerns the rolling motion. Often the longitudinal stability and directional stability are closely interrelated.
% Dynamic stability %
If an aircraft is statically stable, it may undergo three types of oscillatory motion during flight. When imbalance occurs the airplane attempts to retain its position, and it reaches the equilibrium position through a series of decaying oscillations, and the aircraft is said to be dynamically stable. If the aircraft continues the oscillatory motion without decay in the magnitude, then the aircraft is said to be on dynamically neutral. If the magnitude oscillatory motion increases and the aircraft orientation start to change rapidly, then the aircraft is said to be dynamically unstable.
