Stability

Question 1

What is a statically stable aircraft defined as?

Answer 1

A statically stable aircraft has the ability to return to some particular condition of flight after being disturbed without the aid of the pilot.

Question 2

What is the difference between static and dynamic stability of an aircraft?

Answer 2

Static stability is the immediate response of the aircraft to the disturbing force being removed; dynamic stability is the long-term response of the aircraft to the disturbing force being removed.

Question 3

Name each axis, the movement about each axis, and the stability about each axis.

Answer 3

X-X (Longitudinal): Roll, Lateral Stability
Y-Y (Lateral): Pitch, Longitudinal Stability
Z-Z (Normal/Vertical): Yaw, Directional Stability

Question 4

How does moving the aircraft’s center of gravity backward affect directional, lateral, and longitudinal stability?

Answer 4

Directional stability: Reduced
Lateral stability: Reduced
Longitudinal stability: No effect

Question 5

Which parts of the aircraft contribute to the three types of static stability?

Answer 5

Lateral: Wings/mainplanes
Longitudinal: Tailplane
Directional: Fin

Question 6

What is the difference between passive and active stability?

Answer 6

Active stability is achieved by the automatic operation of the flying controls. Passive stability is achieved by the interaction between the airflow and the aircraft.

Question 7

Explain what statically stable and statically unstable means in terms of an aircraft.

Answer 7

Statically stable: The aircraft returns to its original position after a disturbance.
Statically unstable: The aircraft moves further away from its original position after a disturbance.

Question 8

Explain the difference between dynamically stable and dynamically unstable.

Answer 8

Dynamically stable: The aircraft oscillates and the oscillations reduce to zero over time.
Dynamically unstable: The oscillations increase in amplitude over time.

Question 9

Name three design features that affect passive lateral stability.

Answer 9

1. Swept wings
2. Dihedral wings
3. High wing position (or fin and fuselage design)

Question 10

Name two design features that affect passive longitudinal stability.

Answer 10

1. Position of the center of gravity compared to the center of pressure
2. Design of the tailplane (shape, position, and size)

Question 11

Explain how a symmetrical fin or a larger side surface area behind the center of gravity gives an aircraft passive directional stability.

Answer 11

A symmetrical fin creates a side force when disturbed in yaw, pushing it back to its original position. A large surface area behind the center of gravity causes airflow to produce a restoring force, helping the aircraft return to its original yaw position.

Question 12

Explain the difference between dynamic stability modes in pitch: Short-Term Pitch Oscillation and Phugoid.

Answer 12

Short-term pitch oscillation: A short-period oscillation (e.g., 0.5 seconds) around the center of gravity with no change in altitude.
Phugoid: A long-period oscillation (e.g., 30 seconds) where the aircraft trades altitude for speed in a continuous cycle.