Flight Stability and Dynamics CASA 8.4

Aircraft Stability

• Aircraft stability is essential for safe and controlled flight across different flight conditions
• Stability refers to an aircraft's ability to return to its original flight condition after being disturbed.
• Stability is not synonymous with balance or trim.
• An aircraft can be stable in one flight condition (straight-and-level) but unstable in another (inverted).
• Inherent stability describes an aircraft's natural stabilizing tendencies.

Static Stability

• Static stability is the initial tendency of an aircraft after a disruption to equilibrium.
• Three types of static stability exist, described by the aircraft's response to disturbance:
  • Positive: The aircraft returns to its original attitude.
  • Negative: The aircraft moves further away from its original attitude.
  • Neutral: The aircraft remains in its displaced attitude
• Static stability can also be called passive stability as it doesn't require active control.

Dynamic Stability

• Dynamic stability describes how quickly an aircraft responds to its static stability.
• It is determined by the aircraft's tendency to oscillate after initial displacement.
• An aircraft can have positive static stability but positive, negative or neutral dynamic stability.
• Positive dynamic stability: The aircraft returns to its original attitude directly or through decreasing oscillations
• Negative dynamic stability: Oscillations increase in magnitude over time.
• Neutral dynamic stability: The aircraft oscillates neither increasing nor decreasing in magnitude.

Aeroplane Axes (Stability)

• Aircraft movements in flight are described in relation to three axes:
  • Longitudinal: Runs from nose to tail.
  • Lateral: Runs from wingtip to wingtip.
  • Vertical: Runs perpendicular to the other two axes.
• The centre of gravity (CG) is a point where the aircraft's total weight is concentrated. The three axes intersect at the CG.
• Lateral stability refers to the aircraft's ability to return to its original attitude after rolling motion.
  • The wing's dihedral angle, vertical stabiliser, and fuselage contribute to lateral stability.
• Longitudinal stability refers to the aircraft's ability to maintain a constant angle of attack in pitch.
  • The horizontal stabiliser is the primary surface controlling this stability.
  • Elevator trim is adjusted to balance the aircraft for a desired flight path.
• Directional stability refers to the aircraft's ability to maintain a straight course.
  • The vertical stabiliser is the primary control surface, aided by the wing sweep and dorsal fin.
• The pendulum effect is caused by the centre of gravity being below the centre of lift. This effect helps restore straight-and-level flight in high-wing aircraft.
• The torque effect occurs due to the propeller's rotation, causing a yawing moment.
  • Washout on one wing and wash in on the other help counter this effect.
• Ground effect refers to the "floating" sensation an aircraft experiences near the ground due to the downwash and air trapped between the wing and the ground.
  • This effect reduces stalling speed and is influenced by factors such as wing area, height, and wing loading.

Oscillatory Instability

• Oscillatory instability involves a combined rolling and yawing motion.
• It can be categorized into:
  • Dutch Roll: When rolling motion predominates.
  • Snaking: When yawing motion predominates.
• The amount of dihedral and the vertical fin size affect oscillatory instability.
• Dutch Roll occurs when dihedral effect is stronger than the vertical fin's effect.
  • This causes an oscillation in the direction of the slip, leading to a Dutch roll.
• Spiral instability occurs when the dihedral effect is not strong enough to counteract the yawing moment caused by the vertical fin.
  • This can lead to a continuous rolling and yawing, resulting in a spiral dive.
• A spin is distinct from spiral instability and involves a stalled condition followed by yawing.

Active Stability

• Modern high-performance aircraft are designed to be aerodynamically unstable but rely on active stability control systems.
  • These systems maintain stability through digital flight control, directly influencing the aircraft's motion around the three axes.