If the CG is moved aft, what happens to the tail force required to maintain equilibrium?

• A. Tail force must be increased
• B. Tail force must be decreased
• C. Tail force remains the same
• D. Direction of tail force reverses

Answer: (A)

Equilibrium relies on balancing moments about the center of gravity. The tail’s effect on the aircraft’s pitch is the tail force times the distance from the CG to the tail (the lever arm). When the CG is moved aft, that lever arm becomes shorter, so the tail’s moment for a given tail force is reduced. To maintain the same overall moment balance, the tail force must increase in magnitude while keeping the same direction (in a conventional tailplane, downward force). So the tail must be pushed harder to keep the aircraft in equilibrium.

Decreasing the tail force would reduce the balancing moment, and keeping the tail force the same wouldn’t compensate for the shorter lever arm. Reversing the direction of the tail force would imply a different pitch moment altogether, which isn’t what’s required for maintaining equilibrium as the CG moves aft.