Question

The yawing moment coefficient per sideslip angle is positive. What does this indicate?

• A. The aircraft is statically unstable in yaw
• B. The aircraft has positive static directional stability
• C. The aircraft will enter a spiral dive immediately
• D. The rudder is ineffective

Hint:

Directional stability criterion: \[ \boxed{ C_{n_\beta}>0 \Longrightarrow \text{Stable in yaw} } \] A positive restoring yawing moment helps the aircraft naturally align with the airflow after a sideslip disturbance.

Answer 1

The Correct Option is B

Step 1: Recall the directional stability criterion. The yawing moment coefficient derivative with respect to sideslip angle is \[ C_{n_\beta} = \frac{\partial C_n}{\partial \beta}. \] It indicates the aircraft's static directional stability.

Step 2: Interpret the sign of \(C_{n_\beta}\). If \[ C_{n_\beta}>0, \] a positive sideslip generates a restoring yawing moment that tends to align the aircraft with the relative wind. Hence, the aircraft possesses \[ \boxed{\text{positive static directional stability}.} \] Therefore, \[ \boxed{\text{The aircraft has positive static directional stability}} \] is the correct answer. Thus, \[ \boxed{(B)} \] is the correct answer.