Which best describes the purpose of tailplane incidence and elevator trim in energy-efficient stable flight?

• A. They adjust pitching moment and reduce stick forces.
• B. They control roll stability.
• C. They increase drag during approach.
• D. They have no effect on stability.

Answer: (A)

Tailplane incidence and elevator trim tune the aircraft’s longitudinal balance to achieve stable, efficient flight in pitch. By setting the stabilizer angle, the tail creates a pitching moment (up or down force at the tail) that counters the aircraft’s natural nose-up or nose-down tendencies, so the airplane wants to fly at a chosen pitch with minimal effort from the pilot. Elevator trim fixes that moment, meaning you can hold the desired pitch with little or no continuous elevator input, which reduces stick forces and keeps drag lower since you don’t need large, continuous elevator deflections to maintain attitude. In energy-efficient cruise, this combination of balanced pitch and reduced control effort is key to stable flight with minimal power usage.

The other options don’t fit as well: roll stability is mainly about lateral behavior and ailerons, not pitch trim; increasing drag during approach is not the purpose of tailplane incidence and trim; and they do affect stability, so saying there’s no effect on stability isn’t correct.