How does tailplane position influence stability and trim in pitch?

• A. Tailplane size only affects drag, not stability.
• B. Tailplane provides restoring force; larger or forward-placed tail increases pitch stability and trim authority.
• C. Tailplane position does not affect stability.
• D. Tailplane only affects yaw stability.

Answer: (B)

The main idea is that the horizontal stabilizer (tailplane) acts as a restoring force in pitch, creating a moment about the aircraft’s center of gravity that tends to bring the nose back toward the trimmed attitude when it’s disturbed. How strong that restoring force is depends on two things: how much lift (or downforce) the tailplane can generate, and how far its line of action is from the CG—the moment arm. A larger tailplane can generate more restoring force for a given angle of attack, increasing the stabilizing effect and making it easier to hold or return to trim. If the tail is placed in a position that increases its leverage—that is, enhances the effectiveness of that tail force—the aircraft becomes more stable in pitch and requires less elevator travel to trim.

So, the tailplane provides a restoring moment, and increasing its size or positioning it to be more effective in producing that moment improves pitch stability and trim authority.