Which statement about rearward center of gravity on an airplane is true?

• A. Produces no change in stability.
• B. Increases stability about the vertical axis.
• C. Reduces stability about the vertical axis.
• D. Has no effect on stall speed.

Answer: (A)

The key idea here is directional stability, or stability about the vertical axis. The aircraft tends to weathercock into the relative wind, and the gravity-related pendulum effect helps provide a restoring yaw moment when the CG is forward of the vertical plane through the center of gravity. When the CG is moved rearward, that restoring moment is reduced, so the airplane becomes less directionally stable and yaw is more likely or harder to damp. So a rearward center of gravity reduces stability about the vertical axis.

The statement claiming no change in stability is inaccurate because shifting the CG along the longitudinal axis changes how much restoring yaw moment gravity provides. It also doesn’t increase stability about the vertical axis, and while stall speed is mostly governed by weight and wing loading, the main point tested here is the effect on directional stability.