Principles of Flight

Chapter 9: Aircraft Stability

Technical General for Aviators — Capt. Pankaj Pahil

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.

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.

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.

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.

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.

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.