Static aeroelastic torsional divergence of a straight wing occurs when:

(A) Aerodynamic pitching moment rate with twist equals or exceeds torsional structural restoring stiffness

(B) Bending natural frequency equals torsional natural frequency

(C) Flight Mach number equals unity

(D) Damping ratio of the Phugoid mode reaches zero

Answer Key:

A

### Solution: Torsional divergence is a static instability that occurs at divergence speed $V_D$ when the aerodynamic twist moment gradient overcomes the structural torsional stiffness. Correct: A.

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