Flight Stability and Dynamics Overview

Flight Stability and Dynamics

• Flight stability is an aircraft's ability to return to its original flight condition after a disturbance, without pilot intervention.
• Stability can be static or dynamic.
• Static stability describes the initial tendency of the aircraft to return to equilibrium.
• Dynamic stability describes the time it takes for the aircraft to respond to the initial displacement, as well as the oscillation and damping of the aircraft as it returns to equilibrium.
• Three types of static stability exist: positive, negative, and neutral.
• Positive static stability means the aircraft tends to return to its original attitude after a disturbance.
• Negative static stability means the aircraft moves further away from its original attitude after a disturbance.
• Neutral static stability means the aircraft neither returns to nor moves further from its original attitude after a disturbance.
• An aircraft can have positive static stability but negative or neutral dynamic stability.
• Three axes are used to describe flight movements: longitudinal, lateral and vertical.
• Longitudinal axis runs from nose to tail, and rotational movement around this axis is known as rolling.
• Lateral axis is perpendicular to the longitudinal axis, and rotational movement in this axis refers to pitching.
• Vertical axis is perpendicular to the other two, and rotation along this axis is called yawing.
• The centre of gravity (CG) is the common reference point for these axes.
• Stability about the longitudinal axis is called longitudinal stability.
• The lateral stability is determined by the dihedral angle of the wings.
• The dihedral angle is the acute angle between the wing and the lateral axis.
• Anhedral is the opposite of dihedral, and involves a downward angle between wing and axis.
• Anhedral is commonly used in high mounted swept wings.
• Torque effect dictates the aircraft will tend to rotate in the opposite plane to the propeller, and this is often countered using washout and washin. Washout refers to decreased angles of incidence on one wing, and wash-in is the opposite.
• Ground effect occurs when an aircraft is close to the ground, reducing stalling speed through airflow interactions. Several factors impact ground effect, including wing area, height above the ground, and wing loading.
• Dutch roll defines the combined effects of rolling and yawing that result in an oscillatory movement. This movement can become unstable.
• Aircraft can be statically stable but have no active measures for dynamic stability.
• Modern high-speed, high-altitude aircraft are typically aerodynamically unstable and rely on active control systems for stability. These systems use digital controls to maintain stability. Yaw dampers are sometimes used to regulate dutch roll.
• Spiral instability happens when the vertical fin's effect is greater than dihedral, causing the plane to spiral.
• Dihedral is the acute angle between a wing and the lateral axis, used for lateral stability.
• Anhedral is when the wings are angled downward to counteract dutch roll.
• Longitudinal dihedral is the angle between the horizontal stabilizer and the wings.
• Static and dynamic stability have different characteristics, with dynamic stability dealing with the response and oscillations of an aircraft after an initial displacement from equilibrium.

Additional notes

• Aircraft stability is often described in relation to specific axes of rotation (longitudinal, lateral, and vertical), and the characteristics of each axis.
• Various factors contribute to stability, such as wing design (dihedral, anhedral), the location of the center of gravity (CG), and the design of control surfaces.
• The design of an aircraft can include features to counteract torque effects.
• The presence of ground effect and its resulting effects on stalling speed is a factor in aircraft design and operation.
• There are different types of stability, including positive, negative, and neutral for both static and dynamic situations.
• The response of an aircraft to disturbances following an initial displacement is an aspect of dynamic stability.