Aircraft Stability

9.1 Definitions of Stability

• Stability is the tendency of an aircraft to return to a steady state of flight without pilot
• intervention after being disturbed.

•  Static Stability: The initial reaction of the aircraft after a disturbance. 195195
• Positive Static Stability: The aircraft initially tends to return to its original position.
• Neutral Static Stability: The aircraft tends to remain in the new, disturbed position.
• Negative Static Stability (Instability): The aircraft tends to diverge further from its original
• position.
•  Dynamic Stability: The motion of the aircraft over time following the initial reaction.
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• Positive Dynamic Stability (Damped Oscillation): The oscillations get smaller and
• eventually cease. 200200200200
• Neutral Dynamic Stability (Undamped Oscillation): The oscillations continue with the
• same amplitude.
• Negative Dynamic Stability (Divergent Oscillation): The oscillations get progressively
• larger.
• For an aircraft to have positive dynamic stability, it must first have positive static stability.

9.2 The Three Axes of Motion

• An aircraft is free to rotate around three axes, all of which pass through its Center of Gravity
• (CG).
• 1. Longitudinal Axis (Roll): Runs from nose to tail.
• 2. Lateral Axis (Pitch): Runs from wingtip to wingtip.
• 3. Normal (Vertical) Axis (Yaw): Runs vertically through the fuselage.

9.3 Longitudinal Stability (Pitch Stability)

• This is the stability of the aircraft around its lateral axis. It is crucial for maintaining a
• constant angle of attack.
•  Main Contributor: The horizontal stabilizer (tailplane) is the primary component
• providing longitudinal stability.
•  How it Works: The relationship between the aircraft's Center of Gravity (CG) and the
• Aerodynamic Center (AC) of the wing and tailplane is key.
• The wing's AC is typically ahead of the CG, creating an unstable nose-up pitching moment
• when AOA increases.
• The tailplane is positioned far behind the CG and produces a downward force in normal
• flight. If a gust increases the aircraft's AOA, the lift on both the wing and the tailplane
• increases. The increased lift on the tailplane creates a powerful nose-down moment that
• counteracts the wing's unstable moment and restores the aircraft to its original AOA.
•  Static Margin: The distance between the aircraft's CG and its Neutral Point (the point
• where the aircraft would have neutral stability). A larger static margin (CG further forward of
• the neutral point) provides greater longitudinal stability. 209209
•  Factors Affecting Longitudinal Stability:
• CG Position: Moving the CG aft decreases stability. The aft CG limit is set to maintain a
• minimum static margin. 210210210
• Power: Propeller slipstream increases dynamic pressure over the tail, which can increase
• stability.

• High-Lift Devices: Flaps increase downwash over the tail, which is destabilizing.

9.4 Directional Stability (Yaw Stability)

• This is the stability of the aircraft around its normal (vertical) axis, often described as
• "weathercock stability."
•  Main Contributor: The vertical stabilizer (fin) is the primary source of directional
• stability.
•  How it Works: If the aircraft yaws, creating a sideslip, the relative airflow strikes the side of
• the vertical fin. This creates a lift force on the fin that pushes the tail back into alignment with
• the airflow, correcting the yaw. The large moment arm of the fin behind the CG makes this
• effect powerful.
•  Other Factors:
• Fuselage: The fuselage ahead of the CG is generally destabilizing.
• Sweepback: A swept-back wing provides a positive contribution to directional stability.

9.5 Lateral Stability (Roll Stability)

• This is the stability of the aircraft around its longitudinal axis. It's the tendency to correct for
• a wing drop.
•  Main Contributor: Dihedral is the primary design feature for lateral stability. Dihedral is
• the upward angle of the wings relative to the horizontal.
•  How Dihedral Works: When a wing drops, the aircraft sideslips towards the low wing. Due
• to the dihedral angle, the low wing meets the relative airflow at a higher angle of attack than
• the high wing. This generates more lift on the low wing, causing it to rise and return the
• aircraft to a wings-level attitude.
•  Other Factors:
• Sweepback: A swept wing also provides a strong positive "dihedral effect," contributing to
• lateral stability.
• Wing Position: A high-wing configuration (like on many trainers and cargo planes) provides
• additional lateral stability due to a pendulum effect.
• Fin: A large vertical fin above the CG also contributes positively to lateral stability.

9.6 Coupled Oscillations

• Because lateral and directional stability are interconnected, disturbances can lead to coupled
• scillations.
•  Dutch Roll: This occurs when lateral stability (dihedral effect) is strong compared to
• directional stability. It is a continuous, coupled rolling and yawing oscillation. Modern
• transport aircraft are fitted with a
• Yaw Damper to counteract this tendency. 226226226226

•  Spiral Divergence: This occurs when directional stability is strong compared to lateral
• stability. A small disturbance can cause the aircraft to slowly enter a tightening, descending
• spiral. This is an unstable condition but is usually slow enough for the pilot to correct easily.