Question

For a conventional fixed-wing aircraft, which of the following statements are true?

• A. Making $C_{m_\alpha}$ more negative leads to an increase in the frequency of its short-period mode.
• B. Making $C_{m_q}$ more negative leads to a decreased damping of the short-period mode.
• C. The primary contribution towards $C_{l_p}$ is from the aircraft wing.
• D. Increasing the size of the vertical fin leads to a higher yaw damping.

Hint:

Stability derivatives follow intuition: bigger tail → more damping; more negative $C_{m_\alpha}$ → stronger restoring pitch tendency.

Answer 1

The Correct Option is A, C, D

Step 1: Understanding the coefficients.
$C_{m_\alpha}$ is the static longitudinal stability derivative. More negative $C_{m_\alpha}$ → stronger restoring moment → higher natural frequency → **(A) is true**.
$C_{m_q}$ contributes to pitch damping. More negative $C_{m_q}$ → stronger pitch damping → **(B) is false**.

Step 2: Roll damping ($C_{l_p}$).
The dominant contribution to roll damping comes from the wing because rolling motion changes local angle of attack along the span — **(C) is true**.

Step 3: Yaw damping.
Increasing fin size → larger side area → increased directional damping → **(D) is true**.
Hence correct answers are **A, C, D**.