8.4 Flight Stability and Dynamics

Stability

An aircraft's ability to return to a particular condition of flight without pilot effort after being slightly disturbed.

Inherent Stability

Stability of an aircraft in a nosedive where it resists returning to straight-and-level flight.

Trim

The balance of an aircraft

Static Stability

Initial tendency of an object to return to its original attitude after being disturbed.

Positive Static Stability

Aeroplane's tendency to return to its original attitude after displacement.

Negative Static Stability

Aircraft's tendency to move farther away from the original attitude following a disturbance.

Neutral Static Stability

Aeroplane remains in its displaced attitude without returning to the original.

Dynamic Stability

Time required for an aircraft to respond to its static stability

Positive Dynamic Stability

Aeroplane returns to original attitude directly or through decreasing oscillations.

Negative Dynamic Stability

Aircraft exhibits increasing oscillations over time after a disturbance.

Neutral Dynamic Stability

Aeroplane attempts to return to equilibrium state with oscillations neither increasing nor decreasing.

Longitudinal Axis

Axis an aircraft rotates about from nose to tail

Lateral Axis

Axis an aircraft rotates about from wingtip to wingtip

Vertical Axis

Axis an aircraft rotates about from top to bottom

Lateral Stability

Tendency of an aircraft to return to its original attitude from lateral or rolling motion.

Longitudinal Stability

Aircraft's tendency to maintain a constant angle of attack with reference to airflow

Directional Stability

Aircraft's stability about the vertical axis

Horizontal Stabiliser

Primary surface controlling longitudinal stability

Centre of Gravity (CG)

Theoretical point where the entire weight of the aircraft is concentrated.

Pitching Moment

Result of the wing's center of lift being behind the center of gravity

Elevator Trim

Adjustment by the pilot to balance the aircraft by producing a required download at any speed or attitude.

Stabiliser

Produces a force to counteract disturbances in pitch

Yawing Moment

Result of side force on vertical tail

Dihedral

Positive acute angle between wing and lateral axis

Longitudinal Dihedral

Angle between horizontal stabilizer and wing chord

Anhedral

Negative dihedral angle

Oscillatory Instability

Combined rolling and yawing movement

Dutch Roll

Rolling motion predominates in oscillatory instability

Snaking

Yawing motion predominates in oscillatory instability

Pendulum Effect

Centre of gravity below centre of lift on high-wing aircraft

Wash Out

Decreasing angle of incidence on one wing to counteract rotation tendency

Wash In

Increasing angle of incidence on the other wing to counteract rotation tendency

Cushion Effect

Reduces stalling speed during landing

Spiral Instability

Result of negative static stability in roll and yaw

Spin

Aircraft stalls and yaws while airflow separates

Aerodynamically Unstable

Modern high-speed aircraft designed for active stability control systems

Active Control

Utilized through digital flight control systems on three axes