Stability is the tendency of the helicopter to return to its original condition (equilibrium), after it has been disturbed. The Pilot establishes equilibrium by trimming the flight controls, until all forces are in balance and the helicopter is maintaining the desired attitude.

Static stability describes the immediate response of an object, when it is displaced from equilibrium and is measured by the force or moment per unit of displacement that acts to restore the object to equilibrium.

Static stability has three categories:

• Positive: A force applied to the object will result in some initial movement, but within a very short time it will return to its initial position.
• Neutral: A force is applied to the object, it moves for a while and then comes to a rest, without returning to its initial position.
• Negative: A force is applied to the object will result in a movement of increasing rate, without returning to its initial position.

Similarly, a helicopter:

• With positive longitudinal static stability will try to return to its original pitch attitude, if displaced by a gust of wind.
• If it were neutrally stable the gust of wind would result in the aircraft setting at a new pitch attitude.
• If it were unstable, the same gust would cause the helicopter to pitch either up or down and then continue pitching at an ever increasing rate.