Lecture 17 - Stability of an Airplane

Stability of an Airplane

The lecture explains stability using familiar mechanical examples, then applies the same idea to an aircraft’s pitch, roll, and yaw motions. An aircraft is stable in a given direction when a disturbance produces a response that tends to restore it to its original equilibrium.

1. What stability means

A ball illustrates three types of stability:

• Stable: A ball in a cup rolls back to its original position after being displaced.
• Unstable: A ball balanced on the outside of an inverted cup rolls farther away after a small disturbance.
• Neutrally stable: A ball on a level surface stays in its new position after being moved; it neither returns nor moves farther away.

A disturbance, or perturbation, is a temporary displacement from equilibrium. Aircraft stability is considered separately for pitch, roll, and yaw.

2. Pitch stability (longitudinal stability)

Pitch is rotation about the aircraft’s lateral axis. A pitch-up disturbance raises the nose and increases the angle of attack of both the wing and horizontal tail.

• In steady, level flight, the wing and tail produce moments about the center of gravity (CG) that balance one another.
• After a pitch-up disturbance, both surfaces produce additional lift.
• Because the horizontal tail is far behind the CG, its increased lift acts through a relatively long moment arm, producing a restoring nose-down moment.
• The lecture illustrates this with approximate values: the tail moment arm is about three wing chord lengths, the wing lift acts about 0.1 chord from the CG, and the horizontal-tail area is about 15% of the wing area. In the example, these values make the tail’s disturbance-induced restoring moment larger.

Mechanism: The horizontal tail provides pitch stability by generating a restoring moment after a change in pitch.

3. Roll stability (lateral stability)

Roll is rotation about the aircraft’s longitudinal axis. The lecture describes a disturbance in which the starboard wing rises and the port wing drops.

• The descending port wing encounters relative airflow that increases its effective angle of attack, so its lift increases.
• The rising starboard wing encounters relative airflow that decreases its effective angle of attack, so its lift decreases.
• The difference in lift produces a moment that opposes the original roll and tends to level the aircraft.

Mechanism: The wings’ differing lift during a roll provides a restoring effect.

The lecture uses nautical terms to avoid ambiguity: starboard is the pilot’s right side, and port is the pilot’s left. It also notes the conventional green starboard and red port lights.

4. Yaw stability (directional stability)

Yaw is rotation about the aircraft’s vertical axis. In the example, a disturbance turns the nose to the right.

• The vertical tail, which produces no lift in the lecture’s idealized straight-flight example, is now at an angle to the airflow.
• It generates a side force.
• Acting behind the CG, that force produces a moment that tends to turn the nose back toward its original direction.

Mechanism: The vertical tail provides directional stability by opposing yaw disturbances.

Overall takeaway

The lecture assigns the main restoring role for each motion to a different aircraft surface:

• Pitch / longitudinal stability: horizontal tail
• Roll / lateral stability: wings
• Yaw / directional stability: vertical tail

Speakers and source

• Speaker: One unnamed lecturer/instructor.
• Source: Lecture 17 – Stability of an Airplane, from the IITM BS Aerospace channel.
• No guest speakers or other quoted sources are featured.